Method for preparing caprolactam

Abstract

The invention relates to a method for preparing caprolactam, which is characterized in that cyclohexanone-oxime is subjected to rearrangement reaction in a homogeneous reaction system consisting of SO3, concentrated sulfuric acid and sulfur to generate caprolactam, the molar ratio of the concentrated sulfuric acid to the sulfur is 0.5: 1-1.5: 1, the mass ratio of the sulfur to the cyclohexanone-oxime is 1: 1000-3: 100, the mass ratio of SO3 to the cyclohexanone-oxime is 6: 1-30: 1, the reaction is carried out for 4.0-8.0 hours under the reflux condition, and the caprolactam is obtained. The cyclohexanone-oxime conversion rate is greater than or equal to 99.8%, and the caprolactam selectivity is greater than After the reaction is finished, the SO3 is recovered through atmospheric distillation, the recovery rate is greater than or equal to 99.4%, and the solvent performance is kept stable after the SO3 is recycled for 25 times; after SO3 is recycled, residual reaction mixed liquid is subjected to ammonia water neutralization, benzene extraction, water back extraction, anion / cation exchange and decompression dehydration, a caprolactam product is obtained, the yield of the caprolactam product is larger than or equal to 97.8%, and the purity of the caprolactam product is larger than or equal to 99.8%. Compared with the traditional method, the method has the advantages that polythionic acid generated by in-situ reaction of SO3, concentrated sulfuric acid and sulfur has good catalysis and polymerization inhibition performance on the reaction; sO3 has good dissolving performance on organic matters involved in the reaction; in the reaction process, the reaction temperature can be accurately controlled by utilizing the proper boiling point of SO3; after the reaction, SO3 is easy to recover and can be repeatedly used.

Description

Technical Field

[0001] The present invention belongs to the technical field of chemical engineering and organic synthesis, and particularly relates to a method for preparing caprolactam by catalyzing the rearrangement reaction of cyclohexanone oxime in a homogeneous reaction system composed of SO3, concentrated sulfuric acid and sulfur. Background Art

[0002] As an important monomer of nylon 6, the efficient preparation technology of caprolactam has always been a research hotspot in the chemical industry. Currently, the mainstream method for industrial production of caprolactam is the Beckmann rearrangement method, which produces caprolactam through the rearrangement reaction of cyclohexanone oxime in the presence of an acidic catalyst. In traditional processes, fuming sulfuric acid or concentrated sulfuric acid is often used as a catalyst to produce caprolactam through the rearrangement reaction of cyclohexanone oxime. However, this method has significant drawbacks such as the production of a large amount of cheap ammonium sulfate as a by-product, high difficulty in separation and purification, difficulty in catalyst recovery, and local overheating polymerization, which affect the yield and purity of caprolactam.

[0003] In recent years, researchers have attempted to optimize the reaction system by introducing metal chlorides or solid acid catalysts. Patent CN100506383C utilizes MFI-structured zeolites treated with fluorine-containing compounds for catalysis, and Patent CN104418338B prepares an all-silicon molecular sieve to reduce production energy consumption. While these methods improve catalytic efficiency to some extent, they still face challenges such as rapid catalyst deactivation, poor cyclic stability, and insufficient polymerization inhibition. Patent CN110615752A from Tianjin Binhai Kewei Intellectual Property Agency Co., Ltd. utilizes ionic liquids as the reaction medium, but their high preparation cost and difficulty in recycling limit their large-scale application. Furthermore, precise control of reaction conditions and effective suppression of byproducts remain technical challenges. Patent CN103073497A utilizes concentrated sulfuric acid or fuming sulfuric acid as a catalyst to produce caprolactam in a two-stage series reaction system consisting of a high-gravity reactor and a rearrangement reactor. However, this process requires twice as much concentrated sulfuric acid as cyclohexanone oxime and does not address the issue of large amounts of ammonium sulfate as a byproduct. These unresolved issues indicate that existing methods are difficult to simultaneously meet the requirements of high-efficiency catalysis, environmental protection, and low cost. Therefore, developing a new caprolactam preparation technology to reduce environmental impact and production costs while maintaining high efficiency has important research value and industrial significance.

[0004] Based on this, the present invention proposes a method for catalyzing the rearrangement reaction of cyclohexanone oxime to produce caprolactam in a homogeneous reaction system composed of SO3, concentrated sulfuric acid, and sulfur. The method comprises the following steps: a mass ratio of sulfur to cyclohexanone oxime of 1:1000-3:100, a mass ratio of SO3 to cyclohexanone oxime of 6:1-30:1, and a molar ratio of concentrated sulfuric acid to sulfur of 0.5:1-1.5:1. The reaction is carried out under reflux for 4.0-8.0 hours, achieving a cyclohexanone oxime conversion rate of ≥99.8% and a caprolactam selectivity of ≥98.7%. After completion of the reaction, the SO3 is recovered by atmospheric distillation with a recovery rate of ≥99.4%, significantly superior to catalytic systems such as solid acids and ionic liquids. Furthermore, the solvent performance remains stable after 25 cycles. Compared with conventional concentrated sulfuric acid, the method solves the key issues of large amounts of ammonium sulfate by-products and high-temperature caprolactam polymerization, exhibits excellent catalytic and inhibitory properties, and provides a new technical path for caprolactam production. Summary of the Invention

[0005] The purpose of the present invention is to provide an efficient, green and economical method for preparing caprolactam. By using a homogeneous reaction system composed of SO3, concentrated sulfuric acid and sulfur, the traditional concentrated sulfuric acid catalytic process is replaced, thereby solving the defects of the prior art such as large number of by-products, non-recyclable catalysts, low selectivity and serious environmental pollution.

[0006] Based on the above, the present invention relates to a method for preparing caprolactam, characterized in that cyclohexanone oxime is rearranged to produce caprolactam in a homogeneous reaction system composed of SO3, concentrated sulfuric acid and sulfur, comprising the following steps:

[0007] (1) Rearrangement reaction: the mass ratio of sulfur to cyclohexanone oxime is 1:1000-3:100, the mass ratio of SO3 to cyclohexanone oxime is 6:1-30:1, the molar ratio of concentrated sulfuric acid to sulfur is 0.5:1-1.5:1, the reaction is carried out under reflux for 4.0-8.0 hours, the cyclohexanone oxime conversion is ≥99.8%, and the caprolactam selectivity is ≥98.7%;

[0008] (2) SO3 recovery: After the reaction is completed, the reaction mixture is distilled under normal pressure to recover the solvent SO3, which is then directly recycled after cooling and condensation. The SO3 recovery rate is ≥99.4%. The solvent solubility of the recovered SO3 remains stable when it is recycled 25 times;

[0009] (3) Product purification: After SO3 is recovered, the remaining reaction mixture is neutralized with ammonia water, extracted with benzene, back-extracted with water, anion / cation exchanged, and dehydrated under reduced pressure to obtain the caprolactam product with a yield of ≥97.8% and a purity of ≥99.8%.

[0010] The present invention is characterized in that the SO3, concentrated sulfuric acid and sulfur in step (1) react in situ to generate polythionic acid, which has a substantial catalytic effect on the rearrangement reaction of cyclohexanone oxime and has good inhibition performance on the polymerization reaction of the product caprolactam.

[0011] The present invention is characterized in that in step (3), the neutralization treatment adopts an ammonia solution with a mass concentration of 15-30% to adjust the pH to neutral; the benzene extraction removes most of the ammonium sulfate salt; the water back extraction utilizes the good solubility of caprolactam in water and the good solubility of other organic substances in benzene to remove organic substances that are not target products; and the anion / cation exchange resin removes the ammonium sulfate salt remaining in the water extraction phase.

[0012] The present invention solves this technical problem through the following technical solutions:

[0013] The specific technical solution is illustrated by taking dithionic acid generated by the in-situ reaction of SO3, concentrated sulfuric acid and sulfur as an example, wherein this dithionic acid is named H2S2O6, and the naming method of other polythionic acids is similar.

[0014] Sulfur and cyclohexanone oxime in a mass ratio of 1:500, SO3 and cyclohexanone oxime in a mass ratio of 18:1, and concentrated sulfuric acid and sulfur in a molar ratio of 1.3:1 were added to an acid-resistant reactor and stirred to form a homogeneous system. During the stirring process, polythionic acid H2S2O6 and H2S3O6 were generated in situ as catalyst-inhibitors. The mixture was kept under reflux for reaction for 6.0 hours. After the reaction, the reflux apparatus was changed to a distillation apparatus, and SO3 was recovered by atmospheric distillation with a recovery rate of 99.8%. The remaining reaction solution was neutralized with 25% ammonia water to neutrality, extracted three times with an equal volume of benzene (to remove part of the ammonium salt), and the extracts were combined (samples were taken and their composition was determined by gas colorimetry, and the cyclohexanone oxime conversion was 100.0% and the caprolactam selectivity was 99.6% by area normalization method). The extracts were back-extracted three times with an equal volume of deionized water (to remove organic by-products). The extracts were combined and passed sequentially through three ion exchange columns consisting of an anion exchange resin, a cation exchange resin, and an anion / cation exchange resin (to remove the remaining ammonium salt). The resulting ion exchange-treated mixture was then dehydrated to obtain the product caprolactam in a yield of 98.9% and a purity of 99.9%.

[0015] Compared with the traditional method, the characteristics of the present invention are:

[0016] 1. The homogeneous system is highly efficient in catalysis. The homogeneous reaction system is formed by in-situ generation of polythionic acid from sulfur, SO3, and concentrated sulfuric acid. It has the dual functions of catalyzing the rearrangement of cyclohexanone oxime and inhibiting the polymerization of caprolactam. The selectivity is ≥97.5%, completely avoiding the formation of ammonium sulfate by-products in traditional processes.

[0017] 2. The solvent is efficiently recycled, with SO3 as the solvent and reaction medium, and recovered by atmospheric distillation (recovery rate ≥ 99.6%). The performance is stable after 25 cycles, solving the problem of difficult treatment and high cost of waste acid in traditional sulfuric acid method.

[0018] 3. Mild reaction conditions, using the appropriate boiling point of SO3 to achieve mild reflux reaction, avoiding thermal decomposition or polymerization of caprolactam caused by high temperature and shortening the reaction time to 4.0-8.0h.

[0019] 4. The multi-stage refining process improves product quality. Through ammonia neutralization, benzene extraction, water back-extraction, anion / cation exchange and vacuum dehydration, pH control and impurity removal are carried out, significantly improving product quality, achieving a yield of ≥96.0% and a purity of ≥99.5%. It is both highly efficient and environmentally friendly, providing a reliable solution for green and efficient production. Specific implementation methods

[0020] The method of the present invention is further described below with reference to the embodiments, which are not intended to limit the present invention.

[0021] Example 1: Sulfur to cyclohexanone oxime in a mass ratio of 1:500, SO₃ to cyclohexanone oxime in a mass ratio of 18:1, and concentrated sulfuric acid to sulfur in a molar ratio of 1.3:1 were added to an acid-resistant reactor and stirred to form a homogeneous system. Polythionic acid (H₂S₂O₆) and H₂S₃O₆ were generated in situ as catalysts and polymerization inhibitors. The mixture was maintained under reflux for 6.0 hours. After the reaction, the reflux apparatus was replaced with a distillation apparatus, and SO₃ was recovered by atmospheric distillation with a recovery rate of 99.8%. The remaining reaction solution was neutralized with 25% ammonia water to neutrality and extracted three times with an equal volume of benzene. The combined extracts showed a cyclohexanone oxime conversion of 100.0% and a caprolactam selectivity of 99.6%. The product was then back-extracted three times with an equal volume of deionized water. The combined extracts were passed through anion, cation, and mixed ion exchange resin columns to remove sulfate. Finally, the product was dehydrated under reduced pressure to obtain the caprolactam product with a yield of 98.9% and a purity of 99.9%.

[0022] Comparative Example 1: SO3 and cyclohexanone oxime mass ratio of 18:1, concentrated sulfuric acid and cyclohexanone oxime molar ratio of 2.0:1 were added to an acid-resistant reactor, and the mixture was kept under reflux for 6.0 hours. After the reaction, the reflux device was replaced with a distillation device, and SO3 was recovered by atmospheric distillation with a recovery rate of 97.0%. The remaining reaction liquid was neutralized with 25% ammonia water to neutrality, extracted three times with an equal volume of benzene, and the extracts were combined. The cyclohexanone oxime conversion rate was 95.2%, the caprolactam selectivity was 89.7%, and the reaction was back-extracted three times with an equal volume of deionized water.

[0023] The extracts were combined and passed through anion, cation and mixed ion exchange resin columns to remove sulfate, and finally dehydrated under reduced pressure to obtain caprolactam product with a yield of 85.4% and a purity of 96.2%.

[0024] Comparative Example 2: Cyclohexanone oxime and concentrated sulfuric acid were added to a reactor at a molar ratio of 2:1, and stirred at 100°C for 8.0 hours. After the reaction was completed, the mixture was cooled to room temperature and neutralized with 30% ammonia water to generate a large amount of sulfur.

[0025] After ammonium sulfate precipitation and filtration to remove ammonium sulfate, the filtrate was extracted three times with an equal volume of benzene. The extracts were combined, and the cyclohexanone oxime conversion was 63.5%, and the caprolactam selectivity was 78.2%. The product was back-extracted three times with an equal volume of deionized water. The extracts were combined and passed through an anion / cation exchange resin column to remove residual ammonium sulfate. Finally, it was dehydrated under reduced pressure to obtain the caprolactam product with a product yield of 49.3% and a purity of 65.8%.

[0026] Comparative Example 3: Fuming sulfuric acid (containing 20% ​​free SO3) and cyclohexanone oxime were added to an acid-resistant reactor at a mass ratio of 18:1, stirred to form a heterogeneous system, and reacted at 80°C for 8.0h. After the reaction was completed, the mixture was cooled to room temperature and 25%

[0027] Ammonia was neutralized to neutrality, generating a large amount of ammonium sulfate precipitate. After filtering to remove the ammonium sulfate, the filtrate was extracted three times with an equal volume of benzene. The combined extracts revealed a cyclohexanone oxime conversion of 95.2% and a caprolactam selectivity of 86.4%. The product was then back-extracted three times with an equal volume of deionized water. The combined extracts were then passed through an anion / cation exchange resin column to remove residual ammonium sulfate. Finally, the product was dehydrated under reduced pressure to obtain the caprolactam product with a yield of 82.1% and a purity of 94.7%.

[0028] Example 2: Sulfur and cyclohexanone oxime mass ratio of 1:100, SO3 and cyclohexanone oxime mass ratio of 10:1, concentrated sulfuric acid and sulfur molar ratio of 1.3:1 were added to an acid-resistant reactor and stirred to form a homogeneous system. H2S2O6 and H2S3O6 were generated in situ as catalysts and inhibitors. After the mixture was kept under reflux for 4.0 hours, the reflux apparatus was replaced with a distillation apparatus, and SO3 was recovered by atmospheric distillation.

[0029] The remaining reaction solution was neutralized with 25% ammonia water to neutrality, extracted three times with an equal volume of benzene, and the combined extracts showed a cyclohexanone oxime conversion of 99.9% and a caprolactam selectivity of 99.1%. The product was back-extracted three times with an equal volume of deionized water, and the combined extracts were passed through anion, cation, and mixed ion exchange resin columns to remove sulfate. Finally, the product was dehydrated under reduced pressure to obtain the caprolactam product with a product yield of 98.3% and a purity of 99.8%.

[0030] Example 3: Sulfur to cyclohexanone oxime in a mass ratio of 3:100, SO₃ to cyclohexanone oxime in a mass ratio of 18:1, and concentrated sulfuric acid to sulfur in a molar ratio of 1.5:1 were added to an acid-resistant reactor and stirred to form a homogeneous system. H₂S₂O₆ and H₂S₃O₆ were generated in situ as catalysts and polymerization inhibitors. The mixture was maintained at reflux for 6.0 hours. The reflux apparatus was then replaced with a distillation apparatus, and SO₃ was recovered by atmospheric distillation with a recovery rate of 99.8%. The remaining reaction solution was neutralized with 30% ammonia water to neutrality and extracted three times with an equal volume of benzene. The combined extracts revealed a cyclohexanone oxime conversion of 99.9% and a caprolactam selectivity of 98.8%. The product was then back-extracted three times with an equal volume of deionized water. The combined extracts were passed through anion, cation, and mixed ion exchange resin columns to remove sulfate. Finally, the product was dehydrated under reduced pressure to obtain the caprolactam product with a yield of 97.9% and a purity of 99.8%.

[0031] Example 4: A sulfur-to-cyclohexanone oxime mass ratio of 1:500, a SO₃-to-cyclohexanone oxime mass ratio of 16:1, and a concentrated sulfuric acid-to-sulfur molar ratio of 1.1 were added to an acid-resistant reactor and stirred to form a homogeneous system. H₂S₂O₆ and H₂S₃O₆ were generated in situ as catalysts and polymerization inhibitors. The mixture was maintained at reflux for 6.0 hours. The reflux apparatus was then replaced with a distillation apparatus, and the SO₃ was recovered by atmospheric distillation with a recovery rate of 99.6%. The remaining reaction solution was neutralized with 30% ammonia water to neutrality and extracted three times with an equal volume of benzene. The combined extracts revealed a cyclohexanone oxime conversion of 99.8% and a caprolactam selectivity of 99.2%. The product was then back-extracted three times with an equal volume of deionized water. The combined extracts were passed through anion, cation, and mixed ion exchange resin columns to remove sulfate. Finally, the product was dehydrated under reduced pressure to obtain the caprolactam product with a yield of 98.0% and a purity of 99.8%.

[0032] Example 5: Sulfur and cyclohexanone oxime in a mass ratio of 3:100, SO₃ and cyclohexanone oxime in a mass ratio of 30:1, and concentrated sulfuric acid and sulfur in a molar ratio of 1.2:1 were added to an acid-resistant reactor and stirred to form a homogeneous system. H₂S₃O₆ and H₂S₄O₆ were generated in situ as catalysts and polymerization inhibitors. The mixture was maintained at reflux for 5.0 hours. The reflux apparatus was then replaced with a distillation apparatus, and SO₃ was recovered by atmospheric distillation with a recovery rate of 99.4%. The remaining reaction solution was neutralized with 18% ammonia water to neutrality and extracted three times with an equal volume of benzene. The combined extracts revealed a cyclohexanone oxime conversion of 99.8% and a caprolactam selectivity of 98.8%. The product was then back-extracted three times with an equal volume of deionized water. The combined extracts were passed through anion, cation, and mixed ion exchange resin columns to remove sulfate. Finally, the product was dehydrated under reduced pressure to obtain the caprolactam product with a yield of 96.3% and a purity of 99.9%.

[0033] Example 6: Sulfur and cyclohexanone oxime in a mass ratio of 9:1000, SO₃ and cyclohexanone oxime in a mass ratio of 18:1, and concentrated sulfuric acid and sulfur in a molar ratio of 0.9:1 were added to an acid-resistant reactor and stirred to form a homogeneous system. H₂S₃O₆ and H₂S₄O₆ were generated in situ as catalysts and polymerization inhibitors. The mixture was maintained at reflux for 4.0 hours. The reflux apparatus was then replaced with a distillation apparatus, and SO₃ was recovered by atmospheric distillation with a recovery rate of 99.5%. The remaining reaction solution was neutralized with 15% ammonia water to neutrality and extracted three times with an equal volume of benzene. The combined extracts revealed a cyclohexanone oxime conversion of 99.9% and a caprolactam selectivity of 98.7%. The product was then back-extracted three times with an equal volume of deionized water. The combined extracts were passed through anion, cation, and mixed ion exchange resin columns to remove sulfate, and finally dehydrated under reduced pressure to obtain the caprolactam product with a yield of 98.2% and a purity of 99.8%.

[0034] Example 7: Sulfur and cyclohexanone oxime were added to an acid-resistant reactor in a mass ratio of 9:1000, SO₃ and cyclohexanone oxime in a mass ratio of 6:1, and a molar ratio of concentrated sulfuric acid and sulfur of 0.5:1. The mixture was stirred to form a homogeneous system, generating H₂S₃O₆ and H₂S₄O₆ in situ as catalysts and polymerization inhibitors. The mixture was maintained at reflux for 6.0 hours, and then the reflux apparatus was replaced with a distillation apparatus. SO₃ was recovered by atmospheric distillation with a recovery rate of 99.8%. The remaining reaction solution was neutralized with 25% ammonia water to neutrality and extracted three times with an equal volume of benzene. The combined extracts revealed a cyclohexanone oxime conversion rate of 99.8% and a caprolactam selectivity of 98.9%. The product was then back-extracted three times with an equal volume of deionized water. The combined extracts were passed through anion, cation, and mixed ion exchange resin columns to remove sulfate, and finally dehydrated under reduced pressure to obtain the caprolactam product with a yield of 97.9% and a purity of 99.8%.

[0035] Example 8: The mass ratio of sulfur to cyclohexanone oxime is 1:500, the mass ratio of SO3 to cyclohexanone oxime is 10:1, and the mass ratio of concentrated sulfuric acid to

[0036] Sulfur was added to an acid-resistant reactor at a molar ratio of 0.75:1 and stirred to form a homogeneous system. H2S3O6 and H2S4O6 were generated in situ as catalysts and polymerization inhibitors. The mixture was maintained at reflux for 8.0 hours. The reflux unit was then replaced with a distillation unit, and SO3 was recovered by atmospheric distillation with a recovery rate of 99.8%. The remaining reaction liquid was neutralized with 30% ammonia water to neutrality and extracted three times with an equal volume of benzene. The combined extracts revealed a cyclohexanone oxime conversion rate of 99.8% and a caprolactam selectivity of 98.8%. The product was then back-extracted three times with an equal volume of deionized water. The combined extracts were passed through anion, cation, and mixed ion exchange resin columns to remove sulfate. Finally, the product was dehydrated under reduced pressure to obtain the caprolactam product with a yield of 98.3% and a purity of 99.9%.

[0037] Example 9: A sulfur-to-cyclohexanone oxime mass ratio of 1:1000, a SO₃-to-cyclohexanone oxime mass ratio of 16:1, and a concentrated sulfuric acid-to-sulfur molar ratio of 0.6:1 were added to an acid-resistant reactor and stirred to form a homogeneous system. H₂S₃O₆ and H₂S₄O₆ were generated in situ as catalysts and polymerization inhibitors. The mixture was reacted under reflux for 7.0 hours. The reflux apparatus was then replaced with a distillation apparatus, and the SO₃ was recovered by atmospheric distillation with a recovery rate of 99.7%. The remaining reaction liquid was neutralized with 28% ammonia water to neutrality and extracted three times with an equal volume of benzene. The combined extracts revealed a cyclohexanone oxime conversion of 99.9% and a caprolactam selectivity of 98.7%. The product was then back-extracted three times with an equal volume of deionized water. The combined extracts were passed through anion, cation, and mixed ion exchange resin columns to remove sulfate. Finally, the product was dehydrated under reduced pressure to obtain the caprolactam product with a yield of 97.8% and a purity of 99.9%.

[0038] Example 10: Sulfur and cyclohexanone oxime in a mass ratio of 9:1000, SO₃ and cyclohexanone oxime in a mass ratio of 20:1, and concentrated sulfuric acid and sulfur in a molar ratio of 1:1 were added to an acid-resistant reactor and stirred to form a homogeneous system. H₂S₂O₆ and H₂S₃O₆ were generated in situ as catalysts and polymerization inhibitors. The mixture was reacted under reflux for 8.0 hours. The reflux apparatus was then replaced with a distillation apparatus, and SO₃ was recovered by atmospheric distillation with a recovery rate of 99.6%. The remaining reaction solution was neutralized with 25% ammonia water to neutrality and extracted three times with an equal volume of benzene. The combined extracts revealed a cyclohexanone oxime conversion of 99.8% and a caprolactam selectivity of 99.3%. The product was then back-extracted three times with an equal volume of deionized water. The combined extracts were passed through anion, cation, and mixed ion exchange resin columns to remove sulfate, and finally dehydrated under reduced pressure to obtain the caprolactam product with a yield of 98.8% and a purity of 99.8%.

[0039] Example 11: Take the SO3 recovered in Example 1 and add it to an acid-resistant reactor. According to the mass of the SO3 obtained, the mass ratio of sulfur to cyclohexanone oxime is 1:500, the mass ratio of SO3 to cyclohexanone oxime is 18:1, and the molar ratio of concentrated sulfuric acid to sulfur is 1.3:1. Stir to form a homogeneous system, generate H2S2O6 and H2S3O6 in situ as the main catalyst, heat to reflux temperature and stir the reaction for 6.0 hours. After the reaction is completed, the reflux device is changed to a distillation device, and SO3 is recovered by distillation at normal pressure. The recovery rate is 99.8%, and the performance is stable after 25 cycles. The remaining reaction solution was neutralized with 25% ammonia water to neutrality, extracted three times with an equal volume of benzene, and the extracts were combined. The cyclohexanone oxime conversion was 100.0%, and the caprolactam selectivity was 99.6%. The product was back-extracted three times with an equal volume of deionized water. The extracts were combined and passed through anion, cation, and mixed ion exchange resin columns to remove sulfate. Finally, the product was dehydrated under reduced pressure to obtain the caprolactam product with a product yield of 98.9% and a purity of 99.9%. According to the above operation, the recovered SO3 was applied 6 times, and the conversion rate, selectivity, yield and purity were 99.9%, 99.6%, 98.9% and 99.9% respectively; the recovered SO3 was applied 10 times, and the conversion rate, selectivity, yield and purity were 99.9%, 99.5%, 98.8% and 99.8% respectively; the recovered SO3 was applied 15 times, and the conversion rate, selectivity, yield and purity were 99.9%, 99.3%, 98.7% and 99.8% respectively; the recovered SO3 was applied 25 times, and the conversion rate, selectivity, yield and purity were 99.8%, 99.2%, 98.5% and 99.7% respectively.

[0040] Example 12: Take the SO3 recovered in Example 2 and add it to an acid-resistant reactor. According to the mass of the SO3 obtained, the mass ratio of sulfur to cyclohexanone oxime is 1:100, the mass ratio of SO3 to cyclohexanone oxime is 10:1, and the molar ratio of concentrated sulfuric acid to sulfur is 1.3:1. Stir to form a homogeneous system, generate H2S2O6 and H2S3O6 in situ as the main catalyst, heat to reflux temperature and stir the reaction for 4.0 hours. After the reaction is completed, the reflux device is changed to a distillation device, and SO3 is recovered by distillation at normal pressure. The recovery rate is 99.6%, and the performance is stable after 23 cycles. The remaining reaction solution was neutralized with 25% ammonia water to neutrality, extracted three times with an equal volume of benzene, and the extracts were combined. The cyclohexanone oxime conversion rate was 99.9%, and the caprolactam selectivity was 99.1%. The solution was back-extracted three times with an equal volume of deionized water. The extracts were combined and passed through anion, cation, and mixed ion exchange resin columns in sequence to remove sulfate. Finally, the solution was dehydrated under reduced pressure to obtain the caprolactam product with a product yield of 98.3% and a purity of 99.8%. According to the above operation, the recovered SO3 was applied 6 times, and the conversion rate, selectivity, yield and purity were 99.9%, 99.0%, 98.3% and 99.8% respectively; the recovered SO3 was applied 10 times, and the conversion rate, selectivity, yield and purity were 99.9%, 98.9%, 98.2% and 99.7% respectively; the recovered SO3 was applied 15 times, and the conversion rate, selectivity, yield and purity were 99.9%, 98.6%, 98.0% and 99.6% respectively; the recovered SO3 was applied 25 times, and the conversion rate, selectivity, yield and purity were 99.8%, 98.5%, 97.9% and 99.6% respectively.

Claims

1. A method for preparing caprolactam, characterized in that The rearrangement reaction of cyclohexanone oxime to produce caprolactam in a homogeneous reaction system composed of SO3, concentrated sulfuric acid and sulfur includes the following steps: (1) Rearrangement reaction: the mass ratio of sulfur to cyclohexanone oxime is 1:1000-3:100, the mass ratio of SO3 to cyclohexanone oxime is 6:1-30:1, the molar ratio of concentrated sulfuric acid to sulfur is 0.5:1-1.5:1, the reaction is carried out under reflux for 4.0-8.0 hours, the cyclohexanone oxime conversion is ≥99.8%, and the caprolactam selectivity is ≥98.7%; (2) SO3 recovery: After the reaction is completed, the reaction mixture is distilled under normal pressure to recover the solvent SO3, which is then directly recycled after cooling and condensation. The SO3 recovery rate is ≥99.4%. The solvent solubility of the recovered SO3 remains stable when it is recycled 25 times; (3) Product purification: After SO3 is recovered, the remaining reaction mixture is neutralized with ammonia water, extracted with benzene, back-extracted with water, anion / cation exchanged, and dehydrated under reduced pressure to obtain the caprolactam product with a yield of ≥97.8% and a purity of ≥99.8%.

2. The method according to claim 1, characterized in that In step (1), the SO3, concentrated sulfuric acid and sulfur react in situ to generate polythionic acid, which has a substantial catalytic effect on the rearrangement reaction of cyclohexanone oxime and has good inhibition performance on the oligomerization reaction of the product caprolactam.

3. The method according to claim 1, characterized in that In step (3), the neutralization treatment uses an ammonia solution with a mass concentration of 15-30% to adjust the pH to neutral; the benzene extraction removes most of the ammonium sulfate salts; the water back extraction utilizes the good solubility of caprolactam in water and the good solubility of other organic substances in benzene to remove organic substances other than target products; and the anion / cation exchange resin removes the ammonium sulfate salts remaining in the water extraction phase.