Process for the preparation of 2-py in two steps

Abstract

The application belongs to the field of chemical production and particularly relates to a method for preparing 2-PY in two steps, which comprises the following steps: (1) a nucleophilic reaction of gamma-butyrolactone and ammonia to obtain a first reaction solution comprising 4-hydroxybutyramide; (2) heating the first reaction solution to 200-300 DEG C to obtain a second reaction solution; (3) subjecting the second reaction solution to a dehydration condensation reaction to obtain a third reaction solution comprising 2-pyrrolidone; in step (2), the heating time of the first reaction solution to 200 DEG C is less than or equal to 10 min. The application realizes the selectivity of products in different steps, reduces the generation of by-products, and further improves the yield of 2-PY products by two-step reactions and accurate control of process conditions in different steps, so that high-quality 2-PY products with higher purity and lower residue content are obtained.

Description

Technical Field

[0001] This invention belongs to the field of chemical production, and specifically relates to a two-step method for preparing 2-PY. Background Technology

[0002] 2-Pyrrolidone (2-PY) is a five-membered nitrogen-containing heterocyclic compound found in both natural and non-natural products. It possesses significant pharmaceutical and industrial application value, serving as a key intermediate and fine chemical in pharmaceuticals and organic chemistry. Its non-corrosive and chemically stable properties have garnered widespread attention in pharmaceutical chemistry and fine chemicals. Furthermore, with the development of the biodegradable plastics industry, polypyrrolidone (also known as Nylon 4), a traditional polymer material synthesized from 2-PY monomers, has regained attention due to its excellent biodegradability, becoming a promising material with a broad market prospect. However, whether 2-PY is used in pharmaceuticals, fine chemicals, or as a monomer in the synthesis of Nylon-4, high product quality is required. Impurities such as GBL, amides, and heavy components in the product can interfere with downstream polymerization processes, significantly impacting the performance of the final product. Therefore, the quality requirements for 2-PY in this field are becoming increasingly stringent.

[0003] 2-PY has a long history of use, with its synthesis process dating back to the 1930s, discovered by German chemist J. Walter Reppe during his research on acetylene chemicals. Due to the inexpensive and readily available raw materials, the ammonolysis reaction using GBL as a raw material to prepare 2-PY remains the main method for industrial production, and this method has been the most thoroughly studied. However, this method has consistently suffered from common drawbacks such as high energy consumption, low yield, large amounts of residue, and low product quality. Methods for producing 2-PY based on GBL as a raw material mainly fall into two categories.

[0004] The first category involves catalyst-based systems. These reactions are typically gas-phase reactions conducted in fixed-bed reactors containing acidic catalysts at relatively high temperatures. This was an early process for preparing 2-PY. However, it suffers from low conversion rates and complex post-processing, requiring the separation and purification of 2-pyrrolidone from the reaction solvent. This method is now largely obsolete. Examples include the methods described in US3136780A, US4824967, and US5393888A.

[0005] The second category is catalyst-free systems. GBL reacts with liquid ammonia or ammonia water under liquid-phase conditions, which can include one-step or multi-step reactions, and is currently the mainstream preparation process. Although this method is relatively simple, liquid-phase processes usually employ high-temperature and high-pressure conditions, resulting in problems such as low 2-PY product yield, high energy consumption, and numerous side reactions and low product quality due to long residence times. CN104725293A discloses a one-step reaction method for the continuous production of 2-PY through the excess recycling of γ-butyrolactone. CN1258528C discloses a method for the continuous preparation of 2-pyrrolidone by reacting γ-butyrolactone with ammonia in the liquid phase in the presence of water. RO108561B1 discloses a two-step continuous reaction method for the preparation of 2-PY, in which the first step generates hydroxybutyramide, and the second step completes cyclization in a tubular reactor under high temperature and high pressure, thereby solving the problems of easy cavitation and difficult operation when using liquid ammonia as a raw material. WO1999052866A1 discloses a three-stage liquid-phase reaction, but it is actually still a two-step reaction. The third step only achieves the purpose of increasing the conversion rate by using a higher temperature. In this process, GBL and ammonia react in three consecutive stages in the liquid phase: the first stage is carried out at 130-200℃, the second stage at 200-250℃, and the third stage at 250-320℃, with a pressure between 60.80-101.33 bar.

[0006] As mentioned above, none of the published literature, whether employing one-step or multi-step reactions, has adopted corresponding process control strategies for controlling intermediates. In organic synthesis reactions involving the ammonolysis of lactones to generate heterocyclic compounds, high temperature and high pressure are recognized as essential conditions. Therefore, achieving energy-efficient and environmentally friendly synthetic processes under milder conditions, and controlling intermediates during the reaction process based on reaction mechanism studies to improve product yield, suppress byproduct formation, and obtain high-quality products are among the most promising synthetic strategies. Summary of the Invention

[0007] The purpose of this invention is to provide a two-step method for preparing 2-PY.

[0008] A first aspect of the present invention provides a method for preparing 2-pyrrolidone, the method comprising the steps of:

[0009] (1) γ-Butyrolactone and ammonia undergo a nucleophilic reaction to obtain a first reaction solution containing 4-hydroxybutyramide;

[0010] (2) Heat the first reaction solution to 200-300℃ to obtain the second reaction solution;

[0011] (3) The 4-hydroxybutyramide in the second reaction solution undergoes a dehydration condensation reaction to obtain a third reaction solution containing 2-pyrrolidone;

[0012] In step (2), the heating time for heating the first reaction solution to 200°C is ≤10 min.

[0013] In one or more embodiments, in step (1), an ammonia-nitrogen mixture is introduced into the reaction system for heat removal.

[0014] In one or more embodiments, in the ammonia-nitrogen mixture, the total molar ratio of ammonia and nitrogen to the molar ratio of γ-butyrolactone in the feed is 1:(1-1000).

[0015] In one or more embodiments, the molar concentration of ammonia in the ammonia-nitrogen mixture is 0.1-99%.

[0016] In one or more embodiments, the inlet temperature of the ammonia-nitrogen mixture is 10-50°C.

[0017] In one or more embodiments, the inlet pressure of the ammonia-nitrogen mixture is 10-40 bar.

[0018] In one or more embodiments, before proceeding to step (2), the concentration of 4-hydroxybutyramide in the first reaction solution is first adjusted to 50-80 wt%.

[0019] In one or more embodiments, in step (1), the ammonia is derived from concentrated ammonia water or liquid ammonia with a molar concentration ≥40%.

[0020] In one or more embodiments, in step (1), the molar ratio of γ-butyrolactone to ammonia is 1:(1.05-3).

[0021] In one or more embodiments, in step (1), the reaction temperature is 10-50°C.

[0022] In one or more embodiments, in step (1), the reaction pressure is 1-30 bar.

[0023] In one or more embodiments, in step (1), the reaction residence time is 5-15 min.

[0024] In one or more embodiments, step (1) is performed in the absence of a catalyst.

[0025] In one or more embodiments, in step (1), the conversion of the nucleophilic γ-butyrolactone is ≥98%.

[0026] In one or more embodiments, in step (1), the selectivity of the nucleophilic 4-hydroxybutyramide is ≥95%.

[0027] In one or more embodiments, in step (2), the heating time for heating the reaction solution to 200°C is ≤5 min.

[0028] In one or more embodiments, in step (3), the reaction temperature is 200-300°C.

[0029] In one or more embodiments, in step (3), the reaction pressure is 60-80 bar.

[0030] In one or more embodiments, in step (3), the conversion rate of 4-hydroxybutyramide in the dehydration condensation reaction is ≥99%.

[0031] In one or more embodiments, in step (3), the yield of 2-pyrrolidone from the dehydration condensation reaction is ≥98%.

[0032] In one or more embodiments, the temperature of the second reaction solution is always maintained in the range of 200-300°C.

[0033] In one or more embodiments, the content of γ-butyrolactone in the first reaction solution is ≤1.00wt%.

[0034] In one or more embodiments, the content of 4-hydroxybutyramide in the first reaction solution is ≥85.00 wt%.

[0035] In one or more embodiments, the ammonia content in the first reaction solution is ≤1.00wt%.

[0036] In one or more embodiments, the content of residue in the first reaction solution is ≤2.00wt%.

[0037] In one or more embodiments, the content of γ-butyrolactone in the third reaction solution is ≤1.00wt%.

[0038] In one or more embodiments, the content of 4-hydroxybutyramide in the third reaction solution is ≤1.00wt%.

[0039] In one or more embodiments, the residue content in the third reaction solution is ≤2.00wt%.

[0040] In one or more embodiments, the content of 4-(N-pyrrolidone)butyramide in the third reaction solution is ≤0.10wt%.

[0041] In one or more embodiments, the method further includes the step of: (4) subjecting the third reaction liquid to a first distillation for removing light component impurities and a second distillation for removing heavy component impurities in sequence to obtain a 2-pyrrolidone product.

[0042] In one or more embodiments, the temperature of the first distillation is 150±5°C.

[0043] In one or more embodiments, the pressure of the first distillation is 400 ± 20 kPa.

[0044] In one or more embodiments, the temperature of the second distillation is 175°C ± 5°C.

[0045] In one or more embodiments, the pressure of the second distillation is 5 kPa ± 1.

[0046] In one or more embodiments, the second distillation further includes a step of recovering 2-pyrrolidone impurities contained in the heavy component impurities.

[0047] In one or more embodiments, the purity of 2-pyrrolidone in the 2-pyrrolidone product is ≥99%.

[0048] In one or more embodiments, the residue content in the 2-pyrrolidone product is ≤0.01wt.

[0049] A second aspect of the invention also provides applications selected from the group consisting of:

[0050] (1) The method described in the first aspect of the present invention, wherein step (1) is performed at 10-50°C, is used to improve the GBL conversion rate and / or 4-HBA selectivity in step (1);

[0051] (2) In the method described in the first aspect of the present invention, the use of ammonia water or liquid ammonia with a molar concentration of ≥40% in improving the GBL conversion rate and / or 4-HBA selectivity in step (1);

[0052] (3) In the method described in the first aspect of the present invention, the application of the ammonia-nitrogen mixture gas introduced in step (1) for heat removal in improving the GBL conversion rate and / or 4-HBA selectivity in step (1);

[0053] (4) In the method described in the first aspect of the present invention, the heating time of heating the first reaction solution to 200°C in step (2) is ≤10 min, which is used to reduce the content of heavy component impurities generated in step (3);

[0054] (5) In the method described in the first aspect of the present invention, the concentration of 4-hydroxybutyramide in the first reaction solution is first adjusted to 50-80 wt% before step (2) in reducing the content of heavy component impurities generated in step (3);

[0055] (6) The use of 2-pyrrolidone obtained by the method described in the first aspect of the present invention as a raw material for polymerization reaction.

[0056] The present invention has the following beneficial effects:

[0057] (1) In the method of the present invention, step (1) is carried out under mild conditions. In step (1), the low-temperature ammonia-nitrogen mixture circulating in the gas phase rapidly absorbs the heat of reaction through the vaporization of a small amount of ammonia, ensuring that the reaction is always carried out at a low temperature. By using the method of step (1) of the present invention, the conversion rate of GBL can be controlled to ≥98%, the selectivity of the reaction to produce 4-hydroxybutyramide (4-HBA) is high, and the occurrence of side reactions is avoided.

[0058] (2) In the method of the present invention, step (1) controls the reaction product to be mainly 4-HBA, and step (2) heats 4-HBA to above 200°C by rapidly raising the temperature, which can avoid the condensation reaction between GBL and 4-HBA during the heating process below 200°C, thereby reducing the generation of heavy component impurity 4-(N-pyrrolidone)butyramide (PBA). Step (2) also reduces the generation of heavy component impurities such as oligomers of 2-PY by precisely controlling the amount of water added, resulting in a low residue content in the final product.

[0059] (3) The present invention achieves selectivity of products in different steps and reduces the generation of by-products by means of two-step reaction and precise control of process conditions in different steps, thereby improving the yield of 2-PY products and obtaining higher quality 2-PY products. Detailed Implementation

[0060] To enable those skilled in the art to understand the features and effects of the present invention, the terms and expressions used in the specification and claims are explained and defined in general below. Unless otherwise specified, all technical and scientific terms used herein have the ordinary meaning understood by those skilled in the art regarding the present invention, and in case of conflict, the definitions in this specification shall prevail.

[0061] The theories or mechanisms described and disclosed herein, whether right or wrong, should not in any way limit the scope of the invention, that is, the contents of the invention can be implemented without being limited by any particular theory or mechanism.

[0062] In this document, the terms “contains,” “includes,” “containing,” and similar terms encompass the meanings of “basically composed of” and “composed of.” For example, when this document discloses “A contains B and C,” “A is basically composed of B and C” and “A is composed of B and C” should be considered as having been disclosed in this document.

[0063] In this document, all features defined in the form of numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are for the sake of brevity and convenience only. Accordingly, descriptions of numerical ranges or percentage ranges should be considered as covering and specifically disclosing all possible sub-ranges and individual numerical values ​​(including integers and fractions) within those ranges.

[0064] Unless otherwise specified, percentages refer to mass percentages and proportions refer to mass ratios in this article.

[0065] In this document, when describing embodiments or examples, it should be understood that it is not intended to limit the invention to those embodiments or examples. Rather, all alternatives, modifications, and equivalents of the methods and materials described herein are covered within the scope defined by the claims.

[0066] For the sake of brevity, not all possible combinations of the technical features in each implementation scheme or embodiment are described herein. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each implementation scheme or embodiment can be combined arbitrarily, and all possible combinations should be considered within the scope of this specification.

[0067] In this article, since the reaction takes place in the liquid phase, "ammonia" refers to any NH3-related chemical substances that may be present in the reaction system other than ammonia gas, such as liquid ammonia, NH3 complexes (e.g., ammonia monohydrate, i.e., NH3·H2O), and ammonium salts (…). All of the above-mentioned ammonia-related compounds are stoichiometric in the form of NH3.

[0068] In this article, "residue" refers to the sum of all unidentified heavy component impurities with high boiling points (≥280℃ at normal pressure) that cannot be identified in the chromatogram during gas chromatography analysis of the product. Residue is generated during the reaction process and may undergo further reactions under high-temperature conditions during distillation or recovery, remaining in the product. High residue content indicates numerous side reactions, resulting in low product yield and purity.

[0069] The inventors unexpectedly discovered that the reaction kinetics of different steps in the synthesis of 2-PY differ significantly, requiring the two steps to be carried out under different process conditions. The selectivity of the intermediate product (4-HBA) has a significant impact on the occurrence of side reactions and the control of impurity content in the final product. The method of this invention includes the following steps in the reaction process for preparing 2-PY:

[0070] (a) GBL (γ-butyrolactone) and ammonia undergo ring-opening to form 4-hydroxybutyramide (4-HBA):

[0071] ;

[0072] Side reactions:

[0073] ;

[0074] (b) 4-HBA dehydrates to produce 2-PY:

[0075] .

[0076] Laboratory tests showed that, due to differences in process condition control, impurities such as 4-(N-pyrrolidone)butyramide (PBA) and 4-hydroxybutyric acid, which are byproducts of step (a), are the main reasons for the low yield and poor product quality of the 2-PY production process.

[0077] The inventors also discovered that the difficulty in the preparation of 2-PY lies in the contradiction between the volatile and difficult-to-condense properties of liquid ammonia and high-concentration ammonia and the direction of reaction kinetics. Maintaining the liquid-phase reaction system requires high-pressure and high-temperature reaction conditions, but high temperature and high pressure inevitably increase the occurrence of side reactions. Changes in pressure and temperature have a significant impact on the selectivity of intermediate products such as 4-HBA, PBA, and 4-hydroxybutyric acid, which may lead to an increase in various byproducts such as heavy component impurities (such as oligomers of 2-PY), significantly reducing the reaction yield and making distillation separation difficult. This is also the main reason for the low reaction yield and poor product quality in current industrial applications. In summary, the inventors found that the existing preparation methods mainly have the following problems:

[0078] In step (a) above, to ensure a high conversion rate of GBL and improve the selectivity of 4-HBA, liquid ammonia or ammonia solution with a high concentration is required, which necessitates a lower temperature to maintain the liquid phase system. However, if the temperature is too low, the reaction rate will decrease significantly, making it uneconomical. If the temperature is too high, the concentration of ammonia in the liquid phase will decrease, resulting in incomplete conversion of GBL. Excessively high temperatures can also cause ammonia vaporization, increasing the side reactions of GBL hydrolysis and reducing the selectivity of 4-HBA.

[0079] In step (b) above, the dehydration cyclization reaction of 4-HBA requires a high temperature and a suitable reaction concentration. When the temperature does not reach above 200°C, the dehydration cyclization reaction of 4-HBA does not proceed. During the heating process, the main side reaction is the formation of PBA. High-temperature reaction conditions usually lead to an increase in side reactions, and PBA will further transform into other impurities, resulting in an increase in heavy component impurities, a lower yield, and an inability to improve product quality.

[0080] Based on this, the inventors provide a method for preparing 2-pyrrolidone. This method uses an ammonia-nitrogen mixture for heat removal and controls the heating time of the reaction solution to 200-300°C, which ensures that the nucleophilic reaction is carried out under relatively mild low-temperature conditions. This results in a high conversion rate of γ-butyrolactone and a high selectivity of 4-hydroxybutyramide after step (a) of the reaction. It also results in a high conversion rate of 4-hydroxybutyramide and a high yield of 2-pyrrolidone after step (b) of the reaction, and a low content of heavy component impurities (such as residues) in the system.

[0081] This invention provides a method for preparing 2-pyrrolidone, the method comprising the steps of:

[0082] (1) γ-Butyrolactone and ammonia undergo a nucleophilic reaction to obtain a first reaction solution containing 4-hydroxybutyramide;

[0083] (2) Heat the first reaction solution to 200-300℃ to obtain the second reaction solution;

[0084] (3) The 4-hydroxybutyramide in the second reaction solution undergoes a dehydration condensation reaction to obtain a third reaction solution containing 2-pyrrolidone;

[0085] In step (2), the heating time for heating the first reaction solution to 200°C is ≤10 min.

[0086] In step (1), the low-temperature, high-concentration ammonia reaction conditions are beneficial for improving the conversion rate of the raw material GBL and the selectivity of the product 4-HBA. Due to the basicity of ammonia and its strong nucleophilicity, the ammonolysis of GBL lactone can proceed spontaneously without a catalyst. GBL undergoes ammonolysis to produce 4-HBA under the action of liquid ammonia or high-concentration ammonia, which can be carried out at low temperatures of 10-50℃. If the reaction temperature is too high, it will intensify the hydrolysis of GBL to produce 4-hydroxybutyric acid. When 4-HBA is at ≥100℃, the reaction shifts in the reverse direction and decomposes into GBL + NH3. Therefore, step (1) should be strictly controlled under low reaction temperature and high ammonia concentration conditions. By removing the heat of reaction in time, step (1) keeps the reaction under low-temperature conditions, converting GBL to 4-HBA with a high conversion rate.

[0087] In step (1), GBL undergoes ammonolysis with high-concentration ammonia as a raw material. High-concentration ammonia is beneficial for improving the conversion rate of GBL. The high-concentration ammonia can be liquid ammonia or ammonia water with a high concentration. In the ammonia water, the molar concentration of ammonia is 40-100% (when the molar concentration is 100%, it is liquid ammonia). In some embodiments, the ammonia for the nucleophilic reaction in step (1) is derived from concentrated ammonia water or liquid ammonia with a molar concentration ≥40%. The molar concentration of ammonia in the concentrated ammonia water can be 40-70% or 70-90%. Using concentrated ammonia water or liquid ammonia with the concentrations defined herein for the reaction in step (1) can improve the conversion rate of the raw material GBL and the selectivity of the product 4-HBA.

[0088] In step (1), the reaction is carried out under low temperature conditions. The reaction temperature of step (1) is 10-50°C, preferably 20-50°C, more preferably 30-40°C or 25-35°C. Controlling step (1) to be carried out at the temperature defined herein can improve the conversion rate of the feedstock GBL and the selectivity of the product 4-HBA.

[0089] The reaction pressure in step (1) can be 1 bar to 30 bar, for example 1 to 10 bar, 3 to 8 bar or 4 to 6 bar.

[0090] The dwell time in step (1) can be 5-15 minutes, for example, 5-10 minutes or 10-15 minutes.

[0091] In step (1), the molar ratio of γ-butyrolactone to ammonia is 1:(1.05-3), for example 1:(1.1-2), 1:(1.1-1.5), 1:(1.1-1.2).

[0092] In some implementations, step (1) is carried out in the absence of a catalyst.

[0093] In some implementations, in step (1), an ammonia-nitrogen mixture is introduced into the reaction system for heat removal. Specifically, step (1) employs a tower reactor, with GBL and high-concentration ammonia fed from the top of the tower. The ammonia-nitrogen mixture enters the reactor through multiple access points located between the trays of the tower reactor to ensure uniform reaction temperature on each tray. These access points are preferably evenly distributed axially from the top of the tower to the bottom. As the ammonia-nitrogen mixture passes through the trays, it reduces the activation energy, rapidly removes heat, and increases heat and mass transfer during the reaction process, enabling the unidirectional conversion of GBL+NH3 to 4-HBA, thereby avoiding an increase in side reactions.

[0094] In some implementations, the total molar number of ammonia and nitrogen in the ammonia-nitrogen mixture is in a molar ratio of 1:(1-1000) to the feed GBL, for example 1:(1-500), 1:(1-100), 1:(10-100), 1:(50-100), 1:(80-100).

[0095] In some embodiments, the molar concentration of ammonia in the ammonia-nitrogen mixture is 0.1-99%, for example 1%, 10%, 20%, 30%, 50%, 60%, 70%, 80%, 95%, 99%, preferably 50-80%, 80-99%, 80-95%, 75-85%.

[0096] In some embodiments, the inlet temperature of the ammonia-nitrogen mixture can be 10-50°C, for example 10°C, 20°C, 30°C, or 40°C, preferably 20-40°C or 25-35°C, and preferably the same as the reaction temperature inside the reactor.

[0097] In some embodiments, the inlet pressure of the ammonia-nitrogen mixture can be 10-40 bar, for example, 10 bar, 15 bar, 20 bar, 25 bar, 30 bar, preferably 10-20 bar, 15-30 bar, 10-15 bar, and more preferably 10 bar higher than the reaction pressure inside the reactor.

[0098] Existing conventional heat removal methods, such as external circulation heat removal and internal cooling heat removal, all result in a certain temperature rise in the reaction system, high energy consumption, and back mixing of the reaction liquid, leading to an increase in the final residue. Compared with other conventional heat removal methods, this invention uses an ammonia-nitrogen mixture for heat removal, utilizing the mechanism of ammonia vaporization for heat removal. This can maximize the temperature uniformity of the reaction system, avoid the occurrence of many side reactions, and improve the purity and yield of the product. In summary, by using an ammonia-nitrogen mixture for heat removal in step (1), and controlling the amount of ammonia-nitrogen mixture relative to GBL, controlling the molar concentration of ammonia in the ammonia-nitrogen mixture, and controlling the inlet temperature and inlet pressure of the ammonia-nitrogen mixture within the range defined herein, the conversion rate of the raw material GBL and the selectivity of the product 4-HBA can be improved.

[0099] In some embodiments, in step (1), the conversion of the nucleophilic γ-butyrolactone is ≥98%, preferably ≥99%, and more preferably ≥99.4%.

[0100] In some embodiments, in step (1), the selectivity of the nucleophilic 4-hydroxybutyramide is ≥95%, preferably ≥96%, ≥97%, ≥98%, or ≥99%.

[0101] In some embodiments, the content of γ-butyrolactone in the first reaction solution is ≤1.00wt%, preferably ≤0.50wt%.

[0102] In some embodiments, the content of 4-hydroxybutyramide in the first reaction solution is ≥85.00wt%, preferably ≥90.00wt%.

[0103] In some embodiments, the ammonia content in the first reaction solution is ≤1.00wt%, preferably ≤0.90wt%.

[0104] In some embodiments, the residue content in the first reaction solution is ≤2.00wt%, preferably ≤1.00wt%, and more preferably ≤0.70wt%.

[0105] In contrast to step (1), step (3) requires high-temperature, short-residence-time reaction conditions, and the concentration of 4-HBA in the reaction solution cannot be too high. At 150-170°C, 4-HBA will begin to undergo a condensation reaction with GBL to generate PBA, a non-degradable heavy component. The presence of PBA will reduce the yield of 2-PY and affect product quality. When 4-HBA reaches above 200°C, it will undergo a dehydration reaction to generate 2-PY, which is a slightly exothermic reaction. Therefore, during the process of heating from low temperature to above 200°C, the residence time in the 150-170°C range is the main factor affecting the PBA content. To reduce the PBA content and increase the yield of 2-PY, the heating time needs to be shortened to reach above 200°C with the shortest possible residence time. Therefore, in step (2), the heating time of the reaction solution to 200°C can be further controlled to ≤5 min, preferably ≤3 min, ≤2 min, or ≤1 min. By controlling the heating time of the first reaction solution to 200-300℃ in step (2) within the range defined herein, the content of heavy component impurities (especially PBA) in step (3) can be effectively reduced, thereby increasing the yield of 2-pyrrolidone.

[0106] In step (2), a single-pass heater can be used to heat the first reaction solution. Exemplary single-pass heaters include self-rising film evaporators, falling film evaporators, electric heaters, etc. After being rapidly heated to above 200°C by this heater, the solution enters the reactor, where the dehydration and cyclization reaction of 4-HBA is completed.

[0107] The inventors also discovered through research that the effect of water in step (3) on the reaction is bidirectional. On the one hand, water is a byproduct and will affect the reaction equilibrium, so the water content must be controlled; on the other hand, the intramolecular cyclization dehydration reaction of 4-HBA needs to be controlled at a certain concentration to avoid the concentration of the generated 2-PY in the reaction system being too high. Otherwise, the generated 2-PY is prone to polymerize to form 2-PY oligomers, resulting in an increase in heavy component impurities in the product. In order to control the concentration of the reaction solution and reduce the generation of heavy component impurities such as 2-PY oligomers, the 4-HBA feed concentration is controlled within a certain range by adding a certain amount of deionized water without affecting the reaction efficiency. For example, before carrying out step (2), the concentration of 4-hydroxybutyramide in the first reaction solution is first adjusted to 50-80 wt%, such as 60 wt%, 70 wt%, 80 wt%, preferably 60-80 wt%, 65-75 wt%. Preferably, the concentration of 4-hydroxybutyramide in the first reaction solution is adjusted to 50-80 wt% by adding water, for example, by adding a certain amount of water to the first reaction solution after the reaction in step (1). Before proceeding to step (2), controlling the concentration of 4-hydroxybutyramide in the first reaction solution within the range defined herein can effectively reduce the content of heavy component impurities (especially 2-PY oligomers) in step (3) while improving the conversion rate of 4-hydroxybutyramide, thereby increasing the yield of 2-pyrrolidone.

[0108] In step (3), the reaction temperature is 200-300℃, for example 200-250℃, 200-230℃, or 200-210℃. Controlling the reaction in step (3) within the reaction temperature defined herein can effectively reduce the content of heavy component impurities in step (3), thereby increasing the yield of 2-pyrrolidone. The inventors have found that when the reaction temperature in step (3) exceeds 300℃, the reaction proceeds in the reverse direction, resulting in a decrease in the conversion rate of 4-HBA.

[0109] In step (3), the reaction pressure is 60-80 bar, for example 65 bar, 70 bar, 75 bar, 80 bar, preferably 70-80 bar, 73-78 bar.

[0110] In some embodiments, in step (3), the conversion rate of 4-hydroxybutyramide in the dehydration condensation reaction is ≥99%.

[0111] In some embodiments, in step (3), the yield of 2-pyrrolidone from the dehydration condensation reaction is ≥98%.

[0112] In some embodiments, the content of γ-butyrolactone in the third reaction solution is ≤1.00wt%, preferably ≤0.40wt% or ≤0.30wt%.

[0113] In some embodiments, the content of 4-hydroxybutyramide in the third reaction solution is ≤1.00wt%, preferably ≤0.80wt% or ≤0.40wt%.

[0114] In some embodiments, the residue content in the third reaction solution is ≤2.00wt%, preferably ≤1.50wt%, and more preferably ≤1.00wt%.

[0115] In some embodiments, the content of 4-(N-pyrrolidone)butyramide in the third reaction solution is ≤0.10wt%, preferably ≤0.05wt% or ≤0.02wt%.

[0116] In some embodiments, the content of 2-pyrrolidone in the third reaction solution is ≥57.00 wt%.

[0117] In the method, since the heat loss of the second reaction liquid when it enters the reactor of step (3) from the heater of step (2) is negligible, the temperature of the second reaction liquid is always maintained in the range of 200-300℃.

[0118] Based on the above research results, the method of the present invention is a two-step liquid-phase reaction without catalysis. The purpose is to adopt different control strategies to control the formation of products 4-HBA and PBA in the two-step reaction under mild conditions, so as to minimize the occurrence of side reactions and reduce the formation of heavy component impurities, thereby obtaining high-purity 2-PY products with high yield.

[0119] In some embodiments, the method further includes the step of: (4) subjecting the third reaction liquid to a first distillation for removing light component impurities and a second distillation for removing heavy component impurities in sequence to obtain the 2-pyrrolidone product.

[0120] In some embodiments, the temperature of the first distillation is 150±5℃.

[0121] In some implementations, the pressure of the first distillation is 400 ± 20 kPa.

[0122] In some embodiments, the temperature of the second distillation is 175°C ± 5°C, for example, 175°C ± 2°C.

[0123] In some implementations, the pressure of the second distillation is 5 kPa ± 1.

[0124] The first distillation is used to remove light component impurities from the third reaction solution. These light component impurities may include ammonia, water, and γ-butyrolactone.

[0125] The second distillation is used to remove heavy component impurities from the third reaction solution. These heavy component impurities may include 4-(N-pyrrolidone)butyramide, 4-hydroxybutyric acid, and oligomers of 2-PY. The heavy component impurities separated by the second distillation may also carry a certain amount of 2-pyrrolidone. Therefore, the second distillation process further includes a step of recovering the 2-pyrrolidone mixed in with the heavy component impurities to improve the yield of 2-PY. The recovery method involves recovering the 2-pyrrolidone from the heavy component impurities discharged from the column bottom via a thin-film evaporator. An exemplary recovery pressure may be 3 kPa ± 1. An exemplary recovery temperature may be 160 °C ± 2.

[0126] In the second distillation process, the top of the column typically contains a lower concentration of 2-PY, while the bottom contains heavier impurities, which are discharged. The 2-pyrrolidone product is collected from the middle section of the second distillation column used for the second distillation (mid-collection). The mid-collection can be located at the 2nd to 5th tray counting downwards from the top of the column. For example, when the second distillation column has 56 trays, the 2-PY product can be collected from the 3rd, 4th, or 5th tray.

[0127] In some implementations, the 2-pyrrolidone product obtained from the second distillation column has a purity of ≥99%, for example, ≥99.5% or ≥99.8%.

[0128] In some implementations, the residue content of the 2-pyrrolidone product obtained from the second distillation column is ≤0.01wt%, for example ≤0.006wt%.

[0129] In the method for preparing 2-pyrrolidone of the present invention, the present invention also provides the application of carrying out step (1) at 10-50°C in improving the GBL conversion and / or 4-HBA selectivity in step (1); preferably, step (1) is controlled to be carried out at 10-50°C by removing the heat of reaction during the reaction process in step (1); preferably, the heat of reaction is removed by introducing an ammonia-nitrogen mixture into the reaction system in step (1). The ammonia-nitrogen mixture is defined as described in any embodiment herein.

[0130] In the method for preparing 2-pyrrolidone of the present invention, the present invention provides the application of using ammonia water or liquid ammonia with a molar concentration of ≥40% in improving the GBL conversion rate and / or 4-HBA selectivity in step (1).

[0131] In the method for preparing 2-pyrrolidone of the present invention, the present invention provides the application of the ammonia-nitrogen mixture in step (1) for heat removal in improving the GBL conversion and / or 4-HBA selectivity in step (1); the ammonia-nitrogen mixture is as described in any embodiment herein.

[0132] In the method for preparing 2-pyrrolidone of the present invention, the present invention provides the application of controlling the heating time of the first reaction solution to 200°C in step (2) to be ≤10 min in reducing the content of heavy component impurities generated in step (3). The heavy component impurities are defined as described in any embodiment herein, and the heavy component impurities are preferably PBA.

[0133] In the method for preparing 2-pyrrolidone of the present invention, the present invention provides the application of adjusting the concentration of 4-hydroxybutyramide in the first reaction solution to 50-80 wt% before performing step (2) in reducing the content of heavy component impurities generated in step (3). The heavy component impurities are defined as described in any embodiment herein, and are preferably oligomers of 2-PY. Preferably, the application further includes improving the conversion rate of 4-hydroxybutyramide and improving the yield of 2-pyrrolidone.

[0134] This invention provides the application of 2-pyrrolidone prepared by the method of this invention as a raw material for polymerization reactions.

[0135] The present invention also provides a polymerization reaction method, the method comprising the steps of:

[0136] (1) Provides salts of 2-pyrrolidone;

[0137] (2) Nylon 4 is prepared by polymerizing the salt of 2-pyrrolidone.

[0138] In step (1), the salt of 2-pyrrolidone is the potassium salt of 2-pyrrolidone. Step (1) preferably includes the step of preparing the potassium salt of 2-pyrrolidone by reacting 2-pyrrolidone with KOH; preferably, the mass ratio of 2-pyrrolidone to KOH is (10-20):1, such as (10-15):1.

[0139] Preferably, before step (2), the water in the 2-pyrrolidone salt is removed. Water can be removed using methods conventional in the art, such as heating the 2-pyrrolidone salt (e.g., to 60-90°C).

[0140] In step (2), the polymerization is carried out in the presence of an initiator, preferably acetyl chloride. Preferably, the molar ratio of the initiator to the salt of 2-pyrrolidone in step (1) is 1:(40-60), for example 1:(45-50).

[0141] In step (2), the polymerization reaction temperature is 40-80℃, for example 60℃.

[0142] In step (2), the polymerization reaction takes 40-80 minutes, for example, 60 minutes.

[0143] In some implementations, the resulting nylon 4 has a color value ≤8, preferably ≤5. The color value of nylon 4 is tested using a Koncia Minolta CM-5 tester, according to ASTM D1925.

[0144] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following embodiments, unless otherwise specified, are generally performed under conventional conditions or as recommended by the manufacturer. Percentages and parts are by weight unless otherwise stated.

[0145] In this article, the methods for detecting the content of each component are as follows:

[0146] The selectivity of 4-HBA is calculated as: the amount of GBL consumed to generate 4-HBA × 100% / the amount of GBL reactants involved in the reaction.

[0147] The GBL conversion rate is calculated as follows: (Infeed GBL amount - Remaining GBL amount) × 100% / Feed GBL amount.

[0148] The yield of 2-PY is calculated as the actual yield of 2-PY divided by the theoretical yield of 2-PY. The theoretical yield of 2-PY is calculated based on the content of γ-butyrolactone actually added before the start of the reaction in step 1.

[0149] Example 1

[0150] Step 1: γ-Butyrolactone and a 70% concentrated ammonia solution (prepared from liquid ammonia and water) were precisely metered, with a molar ratio of GBL:ammonia of 1:1.05. After passing through a static mixer, the mixture was added from the top of a tower reactor with 10 trays. The reaction temperature was 30℃±2°C, and the reaction pressure was 5 bar±0.1 bar. A low-temperature ammonia-nitrogen mixture was added from the top of the tower to the bottom via five axially distributed inlets (one inlet between every two trays). The ratio of the total molar amount of ammonia and nitrogen in the mixture to the molar amount of GBL was 1:100, and the molar fraction of ammonia in the mixture was 95%. The inlet temperature was maintained at 30±1°C, and the inlet pressure was 15 bar, yielding 4-HBA. The residence time for the reaction in Step 1 was 10 min. After the reaction, the composition and content of the liquid in the bottom of the tower were measured, as shown in Table 1 of Test Example 1.

[0151] Step 2: Add a certain amount of deionized water to the prepared 4-HBA to control the 4-HBA concentration at 70wt%±5. Continuously feed the 4-HBA into the heater and rapidly heat it to 200℃ for 2 minutes.

[0152] Step 3: A dehydration condensation reaction is carried out at a temperature of 205℃±2°C and a pressure of 75±1 bar. After the reaction in Step 3 is completed, the composition and content of the reaction solution are shown in Table 3 of Test Example 1. The heavy component impurities after the reaction in Step 3 mainly include oligomers of PBA, 4-hydroxybutyric acid, and 2-PY, as well as residues of unknown components.

[0153] Step 4: Post-processing of the distillation section includes first and second distillations. The first distillation removes ammonia, water, and small amounts of light components such as GBL from the 2-PY reaction solution, performing preliminary separation. The operating temperature is 150±5℃, and the operating pressure is 400±20 kPa. The light components are collected at the top of the column, while the crude 2-PY from the bottom is sent to the second distillation column. The second distillation is a vacuum column, used to purify the 2-PY after the light components have been removed to obtain the final product. The operating pressure is 5 kPaA±1, and the temperature is 175℃±2. The top of the column contains a lower concentration of 2-PY. The final 2-PY product is collected from the middle of the column (at the third tray; the second distillation column has a total of 56 trays). Heavy component impurities, which contain 2-PY, are discharged from the bottom of the column. These heavy component impurities mainly consist of PBA, 4-hydroxybutyric acid, and oligomers of 2-PY. Therefore, the heavy component impurities in the second distillation column are further recovered by a thin-film evaporator to improve the yield of 2-PY. The recovery operation is carried out at a pressure of 3 kPa ± 1 and a temperature of 160 °C ± 2. The product recovered after the second distillation is subjected to chromatographic analysis. The purity of the 2-PY product and the residue content are shown in Table 4 of Test Example 1.

[0154] Example 2

[0155] The same preparation method as in Example 1 was used, except that in step 2, the time for the feed to be heated to 200°C by the heater was controlled to be 1.0 min.

[0156] After the reactions in steps 1 and 3 are completed, the composition and content of the reaction solution are shown in Tables 1 and 3 of Test Example 1. After secondary distillation, the purity of the 2-PY product and the content of the residue are shown in Table 4 of Test Example 1.

[0157] Comparative Example 1

[0158] The same preparation method as in Example 1 was used, the only difference being that the reaction temperature and the inlet temperature of the ammonia-nitrogen mixture in step 1 were controlled at 90°C. The relatively high temperature resulted in a lower 4-HBA content in step 1. Because the 4-HBA concentration in step 1 was too low, no additional water was added to adjust the concentration. At this reaction temperature (90°C), a large amount of ammonia in the liquid phase vaporized, and the higher reaction temperature led to a significant increase in residue.

[0159] After the reactions in steps 1 and 3 are completed, the composition and content of the reaction solution are shown in Tables 1 and 3 of Test Example 1. After secondary distillation, the purity of the 2-PY product and the content of the residue are shown in Table 4 of Test Example 1.

[0160] Comparative Example 2

[0161] The same preparation method as in Example 1 was used, the only difference being that the ammonia-nitrogen mixture was not used for heat removal, and the tower reactor was used only as a conventional adiabatic reactor. Due to the lack of heat removal measures, the temperature gradually increased as the reaction proceeded, and the vaporization of ammonia gradually increased, eventually reaching 81°C at the reactor outlet.

[0162] After the reactions in steps 1 and 3 are completed, the composition and content of the reaction solution are shown in Tables 1 and 3 of Test Example 1. After secondary distillation, the purity of the 2-PY product and the content of the residue are shown in Table 4 of Test Example 1.

[0163] Comparative Example 3

[0164] The same preparation method as in Example 1 was used, except that in step 2, the residence time of the feed at 200°C was 20 minutes.

[0165] After the reaction in step 3 was completed, the composition and content of the reaction solution were shown in Table 3 of Test Example 1. After secondary distillation, the purity of the 2-PY product and the content of the residue were shown in Table 4 of Test Example 1.

[0166] Comparative Example 4 (excessive water content)

[0167] The same preparation method as in Example 1 was used. The only difference was that in step 2, the concentration of 4-HBA was adjusted to 30 wt% by adding water. After the concentration of 4-HBA was reduced, the residue content after the reaction in step 3 was lower, but the conversion rate of 4-HBA was significantly reduced.

[0168] After the reaction in step 3 was completed, the composition and content of the reaction solution were shown in Table 3 of Test Example 1. After secondary distillation, the purity of the 2-PY product and the content of the residue were shown in Table 4 of Test Example 1.

[0169] Comparative Example 5 (Water content too low)

[0170] The same preparation method as in Example 1 was used. The only difference was that the water addition in step 2 was zero, so the concentration of 4-HBA in the feed in step 3 was 91 wt%. After the reaction in step 3 was completed, 4-HBA achieved a high conversion rate, but the residue content increased.

[0171] After the reaction in step 3 was completed, the composition and content of the reaction solution were shown in Table 3 of Test Example 1. After secondary distillation, the purity of the 2-PY product and the content of the residue were shown in Table 4 of Test Example 1.

[0172] Test Example 1

[0173] In this test, the contents of GBL, 4-HBA, 2-PY, heavy component impurities, and light component impurities other than water were analyzed by chromatography. The instrument model was Thermo Fisher Scientific Trace 1310 dual injection system, and the chromatographic column model was DB-5ms.

[0174] The water content was tested using the Karl Fischer method, and the instrument used was a Mettler volumetric moisture analyzer V30.

[0175] Color numbers were tested using a Lovbond PFXi-195 PH spectrophotometer.

[0176] The ammonia content was analyzed by titration with 0.5 mol / L HCl standard solution. A Mettler Toledo s220 titrator was used.

[0177] The components and their contents in the reaction solution after step 1 are shown in Table 1 below. The conversion rate of γ-butyrolactone and the selectivity of 4-hydroxybutyramide are shown in Table a.

[0178] Table 1

[0179]

[0180] Table a

[0181]

[0182] The analysis results of the reaction solution in step 1 from Tables 1 and a show that in Examples 1 and 2, the conversion rate of GBL and the content of 4-HBA were relatively high, while the residue content was low, indicating that the reaction conditions of low temperature and high concentration of ammonia were favorable for the formation of 4-HBA. The experimental results of Comparative Example 1 show that when the reaction temperature was increased to 90℃, a large amount of ammonia in the reaction solution was vaporized, the molar ratio of GBL to NH3 in the liquid phase reaction was approximately 2:1, the conversion rate decreased significantly, and the amount of residue was much greater than in Examples 1 and 2. Similarly, because no heat removal measures were taken in Comparative Example 2, the reaction temperature of the reaction solution increased significantly. In the lower tray of the tower reactor, the ammonia in the liquid phase was severely vaporized, affecting the reaction effect. The ammonia content (calculated by the mass of NH3) and the temperature of the reaction solution in different layers (1#, 2#, 5#) of the tower from the top of Example 1 and Comparative Example 2 are shown in Table 2 below.

[0183] Table 2

[0184]

[0185] The reaction analysis of ammonia nitrogen removal in Table 2 also shows that heat removal via stripping of the ammonia nitrogen mixture stabilizes the temperature of each tray within the reaction temperature range. In contrast, in Comparative Example 2, without heat removal via the ammonia nitrogen mixture, the temperature rises rapidly as the reaction proceeds. With the increase in temperature, the ammonia vaporization rate increases, and the ammonia concentration in the liquid phase of the tray decreases. The ammonia concentration in the liquid phase at the second and fifth trays is already lower than in Example 1. Combined with the lower GBL conversion rate in Comparative Example 2 shown in Table 1, this indicates that the rapid decrease in ammonia is not due to the progress of the reaction, but rather to ammonia volatilization. After the ammonia concentration decreases, the reaction rate also decreases, and the temperature exhibits a phenomenon of first rising and then falling, eventually reaching equilibrium at a lower conversion rate.

[0186] The components and their contents in the reaction solution after step 3 are shown in Table 3. The conversion rate of 4-hydroxybutyramide and the yield of 2-PY are shown in Table b.

[0187] Table 3

[0188]

[0189] Table b

[0190]

[0191] The analysis results of the reaction solution in step 3 from Tables 3 and b show that in Examples 1 and 2, by controlling the heating time to 200℃ to within 10 minutes (specifically within 2 minutes), the formation of PBA was effectively reduced, thereby reducing the content of heavy component impurities. Therefore, the residue content in the reaction solution was low. From the results of Comparative Example 3, after heating to 200℃ for more than 10 minutes, the residue content in the reaction solution increased significantly, far exceeding that of Examples 1 and 2. This indicates that controlling the heating time to 200℃ is effective in reducing the content of heavy component impurities in the reaction solution after the reaction in step 3. From the results of Comparative Examples 4 and 5, it can be seen that when the amount of water added is large, the concentration of 4-HBA in the feed of step 3 is low, which can suppress side reactions and reduce the content of heavy component impurities, but the reaction rate decreases significantly, and the conversion rate of 4-HBA decreases significantly. When the concentration of 4-HBA is too high, the reaction rate increases, but this leads to more side reactions, a sharp increase in the content of heavy component impurities, and a significant increase in the residue content of the reaction solution.

[0192] The purity and residue content of the 2-PY product obtained from the second distillation column after two distillations in each embodiment and comparative example are shown in Table 4.

[0193] Table 4

[0194]

[0195] Test Example 2

[0196] 200g of 2-PY prepared in Examples 1-2 and Comparative Examples 1-5 were weighed and placed in a three-necked flask. 15g of KOH was added to prepare potassium pyrrolidone. The mixture was kept at 90°C for 1 hour under stirring, and the water was removed by vacuum evaporation. The mixture was cooled to 30°C, and 4g of acetyl chloride, the initiator, was added. After a vacuum reaction at 60°C for 60 minutes, the product began to solidify and precipitate. A white solid product was obtained after 24 hours. The separated white solid product was washed with acetone and water, filtered through filter paper, and washed again with water. It was dried in a vacuum oven for 12 hours to obtain pure nylon 4. The obtained sample was subjected to colorimetric testing using a Koncia Minolta CM-5 according to ASTM D1925. The test results are shown in Table 5 below.

[0197] Table 5

[0198]

Claims

1. A process for the preparation of 2-pyrrolidone, characterized in that, The method comprises the steps of: (1) a nucleophilic reaction of gamma-butyrolactone and ammonia to obtain a first reaction solution comprising 4-hydroxybutyric amide; (2) heating the first reaction solution to 200-300°C to obtain a second reaction solution; (3) dehydrating and condensing 4-hydroxybutyric amide in the second reaction solution to obtain a third reaction solution comprising 2-pyrrolidone; In step (2), the heating time of the first reaction solution to 200°C is ≤10 min; In step (1), ammonia-nitrogen mixed gas is introduced into the reaction system for heat removal; In the ammonia-nitrogen mixed gas, the molar ratio of the total moles of ammonia and nitrogen to the moles of fed gamma-butyrolactone is 1:(1-1000); In the ammonia-nitrogen mixed gas, the molar concentration of ammonia is 0.1-99%; The inlet temperature of the ammonia-nitrogen mixed gas is 10-50°C; The inlet pressure of the ammonia-nitrogen mixed gas is 10-40 bar; In step (1), the reaction temperature is 10-50°C; In step (1), the ammonia is derived from concentrated ammonia water with a molar concentration ≥40% or liquid ammonia; Before step (2) is performed, the concentration of 4-hydroxybutyric amide in the first reaction solution is first adjusted to 50-80 wt%.

2. The method of claim 1, wherein, The method has one or more of the following characteristics: In step (1), the molar ratio of gamma-butyrolactone to ammonia is 1:(1.05-3); In step (1), the reaction pressure is 1-30 bar; In step (1), the reaction residence time is 5-15 min; Step (1) is performed in the absence of a catalyst; In step (1), the conversion rate of gamma-butyrolactone in the nucleophilic reaction is ≥98%; In step (1), the selectivity of 4-hydroxybutyric amide in the nucleophilic reaction is ≥95%; In step (2), the heating time of the reaction solution to 200°C is ≤5 min; In step (3), the reaction temperature is 200-300°C; In step (3), the reaction pressure is 60-80 bar; In step (3), the conversion rate of 4-hydroxybutyric amide in the dehydrating and condensing reaction is ≥99%; In step (3), the yield of 2-pyrrolidone in the dehydrating and condensing reaction is ≥98%; In the method, the temperature of the second reaction solution is always maintained within the range of 200-300°C.

3. The method of claim 1, wherein, The method has one or more of the following characteristics: In the first reaction solution, the content of gamma-butyrolactone is ≤1.00 wt%; In the first reaction solution, the content of 4-hydroxybutyric amide is ≥85.00 wt%; In the first reaction solution, the content of ammonia is ≤1.00 wt%; In the first reaction solution, the content of residue is ≤2.00 wt%; In the third reaction solution, the content of gamma-butyrolactone is ≤1.00 wt%; In the third reaction solution, the content of 4-hydroxybutyric amide is ≤1.00 wt%; In the third reaction solution, the content of residue is ≤2.00 wt%; In the third reaction solution, the content of 4-(N-pyrrolidinyl) butyric amide is ≤0.10 wt%.

4. The method of claim 1, wherein, The method further comprises the step of: (4) sequentially performing first rectification for removing light component impurities and second rectification for removing heavy component impurities on the third reaction solution to obtain 2-pyrrolidone product.

5. The method of claim 4, wherein, The method comprises the steps of: The temperature of the first rectification is 150±5℃; The pressure of the first rectification is 400±20 kPa; The temperature of the second rectification is 175℃±5; The pressure of the second rectification is 5kPaA±1; The method further comprises a step of recovering 2-pyrrolidone mixed in the heavy component impurities after the second rectification.

6. The method of claim 5, wherein, The purity of 2-pyrrolidone in the 2-pyrrolidone product is ≥99%; and / or, the content of residue in the 2-pyrrolidone product is ≤0.01wt%.

7. Use selected from the group consisting of: (1) use in the method of any one of claims 1-6, wherein step (1) is carried out at a temperature of 10-50℃ to improve the conversion of γ-butyrolactone and / or the selectivity of 4-hydroxybutyramide in step (1); (2) use in the method of any one of claims 1-6, wherein ammonia water with a molar concentration of ≥40% or liquid ammonia is used to improve the conversion of γ-butyrolactone and / or the selectivity of 4-hydroxybutyramide in step (1); (3) use in the method of any one of claims 1-6, wherein the introduction of ammonia-nitrogen mixed gas in step (1) for heat removal improves the conversion of γ-butyrolactone and / or the selectivity of 4-hydroxybutyramide in step (1); (4) use in the method of any one of claims 1-6, wherein the heating time of the first reaction liquid to 200℃ in step (2) is controlled to be ≤10 min to reduce the content of heavy component impurities generated in step (3); (5) use in the method of any one of claims 1-6, wherein before step (2) is carried out, the concentration of 4-hydroxybutyramide in the first reaction liquid is first adjusted to 50-80wt% to reduce the content of heavy component impurities generated in step (3).

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