Modified all-silica molecular sieve catalyst for cyclohexanone oxime gas phase rearrangement reaction and preparation method thereof

Abstract

The application provides a modified full-silicon molecular sieve catalyst for a cyclohexanone oxime gas phase rearrangement reaction and a preparation method thereof. The preparation method comprises the following steps: S1, full-silicon molecular sieve is sequentially subjected to impregnation, first drying and first calcination in a solution of a metal salt, so that a calcined molecular sieve is obtained; S2, the calcined molecular sieve is subjected to acid modification treatment by using an organic acid solution, and solid-liquid separation, so that a modified molecular sieve is obtained; and S3, the modified molecular sieve is sequentially subjected to second drying and second calcination, so that a modified full-silicon molecular sieve catalyst is obtained. The modified full-silicon molecular sieve catalyst has the advantages of low cost, high selectivity, good stability, environmental friendliness, long service life and the like when being used for the cyclohexanone oxime gas phase rearrangement reaction, and can realize industrialized production of caprolactam.

Description

Technical Field

[0001] This invention relates to the field of molecular sieve catalyst technology, and more specifically, to a modified all-silica molecular sieve catalyst for the gas-phase rearrangement reaction of cyclohexanone oxime and its preparation method. Background Technology

[0002] Caprolactam is an important chemical raw material for the industrial production of nylon, and the Beckmann rearrangement of cyclohexanone oxime plays a crucial role in caprolactam production. Traditional caprolactam synthesis processes use concentrated sulfuric acid as a catalyst, which easily leads to equipment corrosion and environmental pollution. Therefore, in recent years, more and more technicians have begun to focus on solid acid catalysts such as molecular sieves, and are using them to replace conventional concentrated sulfuric acid catalysts in catalytic applications.

[0003] For example, existing patent applications, including US Patent Application No. USP4061724, Japanese Patent Application No. JP59164617, and Chinese Patent Application No. CN1338427, have successively reported methods for synthesizing silica molecular sieves and revealed their application in the synthesis of caprolactam. However, the gas-phase Beckmann rearrangement process requires high reaction temperatures, leading to easy catalyst deactivation, poor stability, and low selectivity, thus limiting its further application. To address this issue, researchers have used base modification to adjust the composition and number of acidic sites and optimize the morphology of silica molecular sieves to improve their performance in catalyzing the gas-phase Beckmann rearrangement of cyclohexanone oxime. However, the aforementioned existing methods for modifying molecular sieves with bases all suffer from problems such as low selectivity, poor stability, environmental unfriendliness, or low overall lifetime. There is an urgent need to provide a method for preparing modified molecular sieves and their applications to improve these problems. Summary of the Invention

[0004] The main objective of this invention is to provide a modified all-silica molecular sieve catalyst for the gas-phase rearrangement reaction of cyclohexanone oxime and its preparation method, so as to solve the problem that the catalytic effect of silicon molecular sieves on the gas-phase rearrangement reaction of cyclohexanone oxime in the prior art is poor.

[0005] To achieve the above objectives, according to one aspect of the present invention, a method for preparing a modified all-silica molecular sieve catalyst for the gas-phase rearrangement reaction of cyclohexanone oxime is provided. The method includes: step S1, impregnating the all-silica molecular sieve in a solution of a metal salt, performing a first drying, and a first calcination sequentially to obtain a calcined molecular sieve; step S2, subjecting the calcined molecular sieve to acid modification treatment with an organic acid solution and performing solid-liquid separation to obtain a modified molecular sieve; and step S3, subjecting the modified molecular sieve to a second drying and a second calcination sequentially to obtain the modified all-silica molecular sieve catalyst.

[0006] Furthermore, in step S2 above, the content of iron-based metal ions in the modified molecular sieve is 100-500 ppm. The iron-based metal ions refer to any one or more of Fe, Co, and Ni. Preferably, the iron-based metal ions include Fe, Co, and Ni, and the mass ratio of Fe, Co, and Ni is 1:0.005-35:0.001-40.

[0007] Further, in step S2 above, the mass concentration of the organic acid solution is 1-10 wt%, preferably the mass ratio of the all-silica molecular sieve to the organic acid solution is 1:3-10; preferably the organic acid is selected from any one or more of monocarboxylic acids, dicarboxylic acids, and benzoic acid, preferably the monocarboxylic acid is formic acid and / or acetic acid, preferably the dicarboxylic acid is selected from any one or more of oxalic acid, malonic acid, and succinic acid; further, preferably the organic acid is a mixed acid of dicarboxylic acid and benzoic acid, preferably the mass ratio of dicarboxylic acid to benzoic acid in the mixed acid is 1:0.1-0.5.

[0008] Furthermore, in step S2 above, the temperature of the acid modification treatment is 120-180°C, and the preferred time of the acid modification treatment is 2-12 hours.

[0009] Furthermore, in step S2 above, solid-liquid separation is performed by filtration.

[0010] Further, in step S1 above, a pretreatment process for the all-silica molecular sieve is included before impregnation. The pretreatment process includes: modifying the all-silica molecular sieve with an alkaline solution, wherein the mass concentration of the alkaline solution is 0.1–25 wt%, preferably ammonia or an aqueous solution of an organic amine, preferably selected from any one or more of methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, and ethylenediamine; preferably, the modification treatment temperature is 60–160°C, and the modification treatment time is 0.5–24 h; preferably, the particle size of the all-silica molecular sieve is 50–300 nm; preferably, the specific surface area of ​​the all-silica molecular sieve is ≥400 m². 2 / g; preferably, the total content of metal ions in the all-silica molecular sieve is 0-50ppm, preferably the total content of iron-based metal ions in the all-silica molecular sieve is 0-20ppm; preferably, the all-silica molecular sieve is selected from any one or more of Silicalite-1, MCM-1, MWW-22 or β-molecular sieve.

[0011] Further, in step S1 above, the metal salt is selected from any one or more of soluble iron salt, soluble cobalt salt, and soluble nickel salt. The mass concentration of the metal salt solution is 0.5 to 5 wt% based on the metal ions in the metal salt. Preferably, the mass ratio of the all-silica molecular sieve to the metal salt solution is 1:1.5 to 5. Preferably, the immersion time is 2 to 24 hours.

[0012] Furthermore, in step S1 above, the temperature of the first drying is 60-150°C, and the time of the first drying is preferably 2-24 hours; the temperature of the first calcination is preferably 450-600°C, and the time of the first calcination is preferably 48-480 hours.

[0013] Furthermore, in step S1 above, the temperature of the second drying is 80-120°C, and the time of the second drying is preferably 2-24 hours; the temperature of the second calcination is preferably 300-500°C, and the time of the second calcination is preferably 2-24 hours.

[0014] According to one aspect of the present invention, a modified all-silica molecular sieve catalyst for the gas-phase rearrangement reaction of cyclohexanone oxime is provided, which is prepared by the above-described preparation method.

[0015] By applying the technical solution of this application, after impregnating the all-silica molecular sieve in a metal salt solution, a large number of metal ions adhere to the surface of the all-silica molecular sieve. Under the high temperature conditions of the first calcination, the thermal migration of metal ions is promoted, and some metal ions are eventually embedded into the space-accommodating space formed by silanol groups on the surface of the all-silica molecular sieve. This controls the acid strength of the silanol groups on the surface of the all-silica molecular sieve and optimizes the morphology and structure of the all-silica molecular sieve. The embedded metal ions also help to improve the catalytic effect, thereby improving the efficiency and effect of the modified all-silica molecular sieve catalyst in catalyzing the gas-phase Beckmann rearrangement of cyclohexanone oxime. Subsequently, the metal ions that failed to embed into the space-accommodating space formed by silanol groups after calcination are removed by organic acid treatment, thereby avoiding the negative impact of unembedded residual metal ions on the gas-phase rearrangement of cyclohexanone oxime. In addition, the above modified all-silica molecular sieve catalyst has advantages such as low cost, high selectivity, good stability, environmental friendliness, and long service life in the gas-phase rearrangement reaction of cyclohexanone oxime, and can realize the industrial production of caprolactam. Detailed Implementation

[0016] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0017] As analyzed in the background section of this application, the prior art has the problem of poor catalytic effect of silicon molecular sieves on the gas-phase rearrangement reaction of cyclohexanone oxime. In order to solve this problem, this application provides a modified all-silicon molecular sieve catalyst for the gas-phase rearrangement reaction of cyclohexanone oxime and its preparation method.

[0018] In a typical embodiment of this application, a method for preparing a modified all-silica molecular sieve catalyst for the gas-phase rearrangement reaction of cyclohexanone oxime is provided. The preparation method includes: step S1, impregnating the all-silica molecular sieve in a solution of a metal salt, performing a first drying, and a first calcination to obtain a calcined molecular sieve; step S2, performing acid modification treatment on the calcined molecular sieve with an organic acid solution, followed by solid-liquid separation to obtain a modified molecular sieve; and step S3, performing a second drying and a second calcination on the modified molecular sieve to obtain the modified all-silica molecular sieve catalyst.

[0019] After impregnating the all-silica molecular sieve in a metal salt solution, a large number of metal ions adhere to its surface. Under the high temperature of the first calcination, the thermal migration of these metal ions is promoted, and some of them eventually embed into the space-accommodating silanol groups on the surface of the all-silica molecular sieve. This modulates the acid strength of the silanol groups on the surface of the all-silica molecular sieve and optimizes its morphology. The embedded metal ions also contribute to improved catalytic efficiency, thereby enhancing the efficiency and effectiveness of the modified all-silica molecular sieve catalyst in the gas-phase Beckmann rearrangement of cyclohexanone oxime. Subsequently, organic acid treatment removes any metal ions that failed to embed into the space-accommodating silanol groups after calcination, thus avoiding the negative impact of residual unembedded metal ions on the gas-phase rearrangement of cyclohexanone oxime. Furthermore, using the above modified all-silica molecular sieve catalyst for the gas-phase rearrangement of cyclohexanone oxime offers advantages such as low cost, high selectivity, good stability, environmental friendliness, and long service life, enabling the industrial production of caprolactam.

[0020] In one embodiment of this application, in step S2 above, the content of iron-based metal ions in the modified molecular sieve is 100-500 ppm. The iron-based metal ions refer to any one or more of Fe, Co, and Ni. Preferably, the iron-based metal ions include Fe, Co, and Ni, and the mass ratio of Fe, Co, and Ni is 1:0.005-35:0.001-40.

[0021] The optimal content of iron-based metal ions in the modified molecular sieve not only enhances its modification effect on the acid strength of the silanol groups on the surface of the all-silica molecular sieve, but also improves its final catalytic effect on the gas-phase Beckmann rearrangement reaction of cyclohexanone oxime. The optimal mass ratio of Fe, Co, and Ni helps to improve the overall complexation effect between iron-based metal ions and the silanol groups on the surface of the all-silica molecular sieve, thereby effectively controlling the acid strength and morphological structure of the silanol groups on the surface of the all-silica molecular sieve.

[0022] In one embodiment of this application, in step S2 above, the mass concentration of the organic acid solution is 1-10 wt%, and preferably the mass ratio of the all-silica molecular sieve to the organic acid solution is 1:3-10; preferably the organic acid is selected from any one or more of monocarboxylic acids, dicarboxylic acids, and benzoic acid, preferably the monocarboxylic acid is formic acid and / or acetic acid, and preferably the dicarboxylic acid is selected from any one or more of oxalic acid, malonic acid, and succinic acid; further, preferably the organic acid is a mixed acid of dicarboxylic acid and benzoic acid, and preferably the mass ratio of dicarboxylic acid to benzoic acid in the mixed acid is 1:0.1-0.5.

[0023] Controlling the mass concentration of the organic acid solution helps reduce the impact of its inherent acidity on the all-silica molecular sieve. A preferred mass ratio of the all-silica molecular sieve to the organic acid solution helps ensure sufficient contact between the two and improves the removal efficiency of the organic acid solution on the surface of the all-silica molecular sieve for unintercalated metal ions. The preferred types of organic acids, and more preferably a mixture of dicarboxylic acids and benzoic acid, and the preferred mass ratio of dicarboxylic acids to benzoic acid in the mixture, help utilize the polycarboxyl groups of the dicarboxylic acid and the spatial structure of benzoic acid to enhance the complexation effect of the mixed acid on unintercalated metal ions, thereby improving the removal efficiency of the mixed acid on the surface of the all-silica molecular sieve.

[0024] In one embodiment of this application, in step S2 above, the temperature of the acid modification treatment is 120-180°C, and the time of the acid modification treatment is preferably 2-12 hours.

[0025] The control of temperature and time in the above acid modification treatment helps to enhance the interaction between carboxyl groups in the organic acid solution and unintercalated metal ions, thereby more fully removing unintercalated metal ions from the surface of the all-silica molecular sieve.

[0026] In some embodiments of this application, in step S2 above, the solid-liquid separation is filtration, thereby simply and quickly separating most of the excess organic acid solution from the modified molecular sieve.

[0027] In one embodiment of this application, step S1 above includes a pretreatment process for the all-silica molecular sieve before staining. The pretreatment process includes: modifying the all-silica molecular sieve with an alkaline solution, wherein the mass concentration of the alkaline solution is 0.1–25 wt%, preferably ammonia or an aqueous solution of an organic amine, preferably selected from any one or more of methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, and ethylenediamine; preferably, the modification treatment temperature is 60–160°C, and the modification treatment time is 0.5–24 h; preferably, the particle size of the all-silica molecular sieve is 50–300 nm; preferably, the specific surface area of ​​the all-silica molecular sieve is ≥400 m². 2 / g; preferably, the total content of metal ions in the all-silica molecular sieve is 0-50ppm, preferably the total content of iron-based metal ions in the all-silica molecular sieve is 0-20ppm; preferably, the all-silica molecular sieve is selected from any one or more of Silicalite-1, MCM-1, MWW-22 or β-molecular sieve.

[0028] The mass content of silanol groups in an all-silica molecular sieve determines the content of metal ions that can be intercalated within it. Optimal particle size and specific surface area of ​​the all-silica molecular sieve help increase its contact area with metal salts. All-silica molecular sieves with the above-mentioned metal ion and iron-based metal ion contents exhibit superior basic catalytic performance.

[0029] In one embodiment of this application, in step S1 above, the metal salt is selected from any one or more of soluble iron salt, soluble cobalt salt, and soluble nickel salt. The mass concentration of the metal salt solution is 0.5 to 5 wt% based on the metal ions in the metal salt. Preferably, the mass ratio of the all-silica molecular sieve to the metal salt solution is 1:1.5 to 5. Preferably, the immersion time is 2 to 24 hours.

[0030] The preferred mass concentration of the metal salt solution and the mass ratio of the all-silica molecular sieve to the metal salt solution both contribute to sufficient contact between the all-silica molecular sieve and the metal salt solution, and ensure that the metal ions are more uniformly dispersed on the surface of the all-silica molecular sieve. Further optimization of the immersion time helps to ensure the interaction time between the all-silica molecular sieve and the metal salt solution, so that the metal salt adheres to the surface of the all-silica molecular sieve as much as possible.

[0031] In one embodiment of this application, in step S1 above, the temperature of the first drying is 60-150°C, and the time of the first drying is preferably 2-24 hours; the temperature of the first calcination is preferably 450-600°C, and the time of the first calcination is preferably 48-480 hours.

[0032] The preferred conditions for the first drying and first calcination help to further improve the migration effect of metal ions, thereby more fully controlling the acid strength of the silanol groups on the surface of the all-silica molecular sieve.

[0033] In one embodiment of this application, in step S1 above, the temperature of the second drying is 80-120°C, and the time of the second drying is preferably 2-24 hours; the temperature of the second calcination is preferably 300-500°C, and the time of the second calcination is preferably 2-24 hours.

[0034] The preferred conditions for the second drying and second calcination help to further improve the decomposition effect of residual organic acid solution on the modified molecular sieve, thereby more thoroughly removing residual metal ions on the modified molecular sieve.

[0035] In another typical embodiment of this application, a modified all-silica molecular sieve catalyst for the gas-phase rearrangement reaction of cyclohexanone oxime is provided, which is prepared by the above-described preparation method.

[0036] The above preparation method not only modulates the acid strength of the silanol groups on the surface of the all-silica molecular sieve and optimizes its morphology, thereby improving the efficiency and effectiveness of the modified all-silica molecular sieve catalyst in catalyzing the gas-phase Beckmann rearrangement of cyclohexanone oxime, but also removes metal ions that failed to intercalate into the silanol groups through organic acid treatment. This avoids the negative impact of unintercalated residual metal ions on the gas-phase rearrangement of cyclohexanone oxime, fundamentally improving the catalytic effect of the modified all-silica molecular sieve catalyst on the gas-phase rearrangement of cyclohexanone oxime.

[0037] The beneficial effects of this application will be further illustrated below with reference to the embodiments.

[0038] Example 1

[0039] The average particle size is 50 nm and the specific surface area is 400 m². 2 100.0 g of Silicalite-1, a 1 / g all-silica molecular sieve, was added to an autoclave along with 400 g of 5% ammonia solution and pretreated at 100 °C for 4 h. After washing, the solution was dried at 120 °C. The sieve was then impregnated with 150.0 g of 5% Fe(NO3)3 solution for 2 h, followed by filtration to obtain a filter cake. The filtered all-silica molecular sieve filter cake was subjected to a first drying at 60 °C for 24 h, followed by a first calcination at 450 °C for 480 h in a muffle furnace to obtain a calcined molecular sieve. 300 g of 10% formic acid was mixed with the calcined molecular sieve and placed in a para-polystyrene (PPL) liner. The liner was then placed in a homogeneous reactor for acid modification treatment at 120 °C for 12 h. The treated all-silica molecular sieve was filtered, and the filter cake was washed with ultrapure water until pH = 7 to obtain the modified molecular sieve. The modified molecular sieve was subjected to a second drying at 80℃ for 24 h, followed by a second calcination in a muffle furnace at 300℃ for 24 h to obtain the modified Silicalite-1 catalyst.

[0040] Example 2

[0041] With an average particle size of 200 nm and a specific surface area of ​​400 m², 2100.0 g of all-silica MCM-1 molecular sieve ( / g) was added to an autoclave along with 400 g of 5% (w / w) methylamine aqueous solution and pretreated at 100 °C for 4 h, followed by washing and drying at 120 °C. The sieve was then impregnated with 300.0 g of 0.5% (w / w) FeCl3 solution for 12 h, and filtered to obtain a filter cake. The filtered all-silica molecular sieve filter cake was dried at 120 °C for 12 h, and then calcined in a muffle furnace at 500 °C for 240 h. 300 g of 10% (w / w) oxalic acid was mixed with the calcined all-silica molecular sieve and placed in a PPL liner. The liner was then placed in a homogeneous reactor and treated at 140 °C for 6 h. The treated all-silica molecular sieve was filtered, and the filter cake was washed with ultrapure water until pH = 7. The washed all-silicon molecular sieve was dried at 120℃ for 12 hours, and then calcined in a muffle furnace at 500℃ for 4 hours to obtain the modified all-silicon MCM-1 molecular sieve catalyst.

[0042] Example 3

[0043] The average particle size is 50 nm and the specific surface area is 400 m². 2 100.0 g of all-silica MWW-22 molecular sieve ( / g) was added to an autoclave along with 400 g of 5% dimethylamine aqueous solution and pretreated at 100 °C for 4 h, followed by washing and drying at 120 °C. The sieve was then impregnated with 400.0 g of 5% Ni(NO3)2 solution for 24 h, and filtered to obtain a filter cake. The filtered all-silica molecular sieve filter cake was dried at 100 °C for 24 h, and then calcined in a muffle furnace at 450 °C for 480 h. 300 g of 10% propionic acid was mixed with the calcined all-silica molecular sieve and placed in a PPL liner. The liner was then placed in a homogeneous reactor and treated at 120 °C for 12 h. The treated all-silica molecular sieve was filtered, and the filter cake was washed with ultrapure water until pH = 7. The washed all-silicon molecular sieve was dried at 120℃ for 6 hours, and then calcined in a muffle furnace at 500℃ for 6 hours to obtain the modified all-silicon MWW-22 molecular sieve catalyst.

[0044] Example 4

[0045] The average particle size is 50 nm and the specific surface area is 400 m². 2100.0 g of all-silica β-molecular sieve ( / g) was added to an autoclave along with 400 g of 5% ethylamine aqueous solution and pretreated at 100 °C for 4 h, followed by washing and drying at 120 °C. The sieve was then impregnated with 300.0 g of 5% Co(NO3)2 solution for 10 h, and filtered to obtain a filter cake. The filtered all-silica molecular sieve filter cake was dried at 150 °C for 2 h, and then calcined in a muffle furnace at 500 °C for 480 h. 300 g of 10% succinic acid was mixed with the calcined all-silica molecular sieve and placed in a PPL liner. The liner was then placed in a homogeneous reactor and treated at 120 °C for 12 h. The treated all-silica molecular sieve was filtered, and the filter cake was washed with ultrapure water until pH = 7. The washed all-silicon molecular sieve was dried at 80°C for 24 hours, and then calcined in a muffle furnace at 300°C for 24 hours to obtain the modified all-silicon β-molecular sieve catalyst.

[0046] Example 5

[0047] The average particle size is 50 nm and the specific surface area is 400 m². 2 100.0 g of Silicalite-1, a 5% (w / g) all-silica molecular sieve, was added to an autoclave along with 400 g of a 5% (w / w) ethylenediamine aqueous solution. The mixture was pretreated at 100 °C for 4 h, followed by washing and drying at 120 °C. The sieve was then impregnated with a 0.5% (w / w) equimolar mixed solution of Fe2(SO4)3 and Ni(NO3)2 for 2 h, and filtered to obtain a filter cake. The filtered all-silica molecular sieve filter cake was dried at 60 °C for 24 h, and then calcined in a muffle furnace at 500 °C for 480 h. 300 g of 10% (w / w) formic acid was mixed with the calcined all-silica molecular sieve and placed in a PPL liner. The liner was then placed in a homogeneous reactor and treated at 120 °C for 12 h. The treated all-silica molecular sieve was filtered, and the filter cake was washed with ultrapure water until pH = 7. The washed all-silica molecular sieve was dried at 120℃ for 2 hours, and then calcined in a muffle furnace at 300℃ for 24 hours to obtain the modified all-silica molecular sieve Silicalite-1 catalyst.

[0048] Based on Example 1, the treatment conditions for the all-silica molecular sieve in step S1 are changed as follows:

[0049] Table 1

[0050]

[0051] Based on Example 1, the organic acid treatment conditions of the calcined molecular sieve in step S2 were changed as follows:

[0052] Table 2

[0053]

[0054]

[0055] In Example 23, the mass ratio of succinic acid to formic acid was 1:0.3. In Examples 25, 26, and 27, the mass ratios of succinic acid to benzoic acid were 1:0.1, 1:0.5, and 1:0.6, respectively.

[0056] Example 28

[0057] The difference from Example 1 is that the modification process of the all-silica molecular sieve Silicalite-1 is to pretreat it with 0.1% ammonia water at 160°C for 0.5 h, and finally obtain the modified all-silica molecular sieve catalyst.

[0058] Example 29

[0059] The difference from Example 1 is that the modification process of the all-silica molecular sieve Silicalite-1 involves pretreatment with 25% ammonia water at 60°C for 24 hours to finally obtain the modified all-silica molecular sieve catalyst.

[0060] Example 30

[0061] The difference from Example 1 is that the modification process of the all-silica molecular sieve Silicalite-1 involves pretreatment with 5% ammonia water at 180°C for 0.2 h to finally obtain the modified all-silica molecular sieve catalyst.

[0062] Comparative Example 1

[0063] The average particle size is 50 nm and the specific surface area is 400 m². 2 100.0 g of Silicalite-1, a fully silicate molecular sieve, was added to an autoclave along with 400 g of 5% ammonia solution and pretreated at 100 °C for 4 h. After washing, the mixture was dried at 120 °C. The sieve was then calcined in a muffle furnace at 450 °C for 480 h. 300 g of 10% formic acid was mixed with the calcined Silicalite-1 molecular sieve and placed in a PPL liner. The liner was then placed in a homogeneous reactor and treated at 120 °C for 12 h. The treated Silicalite-1 molecular sieve was filtered, and the filter cake was washed with ultrapure water until the pH reached 7. The washed Silicalite-1 molecular sieve was dried at 80 °C for 24 h and then calcined in a muffle furnace at 300 °C for 24 h to obtain the modified Silicalite-1 molecular sieve catalyst.

[0064] Comparative Example 2

[0065] The average particle size is 50 nm and the specific surface area is 400 m². 2100.0 g of Silicalite-1, a fully silicate molecular sieve, was added to an autoclave along with 400 g of 5% ammonia solution and pretreated at 100 °C for 4 h. After washing, the solution was dried at 120 °C. The sieve was then impregnated with 150.0 g of 5% Fe(NO3)3 solution for 2 h, followed by filtration to obtain a filter cake. The filtered filter cake was dried at 60 °C for 24 h, then calcined in a muffle furnace at 450 °C for 12 h. 300 g of 10% formic acid was mixed with the calcined Silicalite-1 and placed in a PPL liner. The liner was then placed in a homogeneous reactor and treated at 120 °C for 12 h. The calcined Silicalite-1 was filtered, and the filter cake was washed with ultrapure water until pH = 7. The washed Silicalite-1 was dried at 80 °C for 24 h to obtain the modified Silicalite-1 catalyst.

[0066] Comparative Example 3

[0067] The average particle size is 50 nm and the specific surface area is 400 m². 2 100.0 g of Silicalite-1, a fully silicate molecular sieve, was added to an autoclave along with 400 g of 5% ammonia solution. The mixture was pretreated at 100 °C for 4 h, followed by washing and drying at 120 °C. The solution was then impregnated with 150.0 g of 5% Fe(NO3)3 solution for 2 h, and filtered to obtain a filter cake. The filtered filter cake was dried at 60 °C for 24 h, and then calcined in a muffle furnace at 450 °C for 480 h to obtain the modified Silicalite-1 catalyst.

[0068] The concentrations of Fe, Co, and Ni metal ions in the modified all-silica molecular sieve catalysts of the above examples and comparative examples were characterized by ICP, and the results are shown in 3.

[0069] An ethanol solution containing 20% ​​cyclohexanone oxime was used, with nitrogen as the carrier gas, and the modified all-silica molecular sieve catalysts of the above examples and comparative examples were used as catalysts, with a space velocity of 2.0 h⁻¹. -1 Cyclohexanone oxime / ethanol solution was mixed with ammonia and passed through a 360°C bed to undergo a rearrangement reaction to generate caprolactam. The conversion rate of cyclohexanone oxime, the selectivity of caprolactam, and the stable operating time are shown in Table 3.

[0070] Table 3

[0071]

[0072]

[0073] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0074] After impregnating the all-silica molecular sieve in a metal salt solution, a large number of metal ions adhere to its surface. Under the high temperature of the first calcination, the thermal migration of these metal ions is promoted, and some of them eventually embed into the space-accommodating silanol groups on the surface of the all-silica molecular sieve. This modulates the acid strength of the silanol groups on the surface of the all-silica molecular sieve and optimizes its morphology. The embedded metal ions also contribute to improved catalytic efficiency, thereby enhancing the efficiency and effectiveness of the modified all-silica molecular sieve catalyst in the gas-phase Beckmann rearrangement of cyclohexanone oxime. Subsequently, organic acid treatment removes any metal ions that failed to embed into the space-accommodating silanol groups after calcination, thus avoiding the negative impact of residual unembedded metal ions on the gas-phase rearrangement of cyclohexanone oxime. Furthermore, using the above modified all-silica molecular sieve catalyst for the gas-phase rearrangement of cyclohexanone oxime offers advantages such as low cost, high selectivity, good stability, environmental friendliness, and long service life, enabling the industrial production of caprolactam.

[0075] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a modified all-silica molecular sieve catalyst for the gas-phase rearrangement reaction of cyclohexanone oxime, characterized in that, The preparation method includes: Step S1: The all-silica molecular sieve is sequentially impregnated in a solution of metal salt, subjected to a first drying, and then calcined to obtain a calcined molecular sieve; the metal salt is selected from any one or more of soluble iron salt, soluble cobalt salt, and soluble nickel salt; based on the metal ions in the metal salt, the mass concentration of the metal salt solution is 0.5~5wt%, and the mass ratio of the all-silica molecular sieve to the metal salt solution is 1:1.5~5; Step S2 involves acid-modifying the calcined molecular sieve with an organic acid solution, followed by solid-liquid separation to obtain the modified molecular sieve. The mass ratio of the all-silica molecular sieve to the organic acid solution is 1:3~10. The acid-modification temperature is 120~180℃, and the acid-modification time is 2~12 hours. Step S3: The modified molecular sieve is subjected to a second drying and a second calcination in sequence to obtain the modified all-silica molecular sieve catalyst; the temperature of the second calcination is 300~500℃ and the time of the second calcination is 2~24h.

2. The preparation method according to claim 1, characterized in that, In step S2, the content of iron-based metal ions in the modified molecular sieve is 100~500ppm, and the iron-based metal ions refer to any one or more of Fe, Co, and Ni.

3. The preparation method according to claim 2, characterized in that, The iron-based metal ions include Fe, Co, and Ni, and the mass ratio of Fe, Co, and Ni is 1:0.005~35:0.001~40.

4. The preparation method according to claim 1, characterized in that, In step S2, the mass concentration of the organic acid solution is 1~10wt%.

5. The preparation method according to claim 1, characterized in that, In step S2, the organic acid is selected from any one or more monocarboxylic acids and dicarboxylic acids.

6. The preparation method according to claim 5, characterized in that, The monocarboxylic acid is formic acid and / or acetic acid.

7. The preparation method according to claim 5, characterized in that, The dicarboxylic acid is selected from any one or more of oxalic acid, malonic acid, and succinic acid.

8. The preparation method according to claim 5, characterized in that, The organic acid is a mixture of the dicarboxylic acid and benzoic acid.

9. The preparation method according to claim 8, characterized in that, The mass ratio of the dicarboxylic acid and the benzoic acid in the mixed acid is 1:0.1~0.

5.

10. The preparation method according to claim 1, characterized in that, In step S2, the solid-liquid separation is filtration.

11. The preparation method according to any one of claims 1 to 10, characterized in that, In step S1, a pretreatment process for the all-silica molecular sieve is included before the impregnation. The pretreatment process includes: The all-silica molecular sieve is modified using an alkaline solution with a mass concentration of 0.1~25wt%.

12. The preparation method according to claim 11, characterized in that, The alkaline solution is an aqueous solution of ammonia or organic amines.

13. The preparation method according to claim 12, characterized in that, The organic amine aqueous solution is selected from any one or more of methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, and ethylenediamine.

14. The preparation method according to claim 11, characterized in that, The modification treatment temperature is 60~160℃.

15. The preparation method according to claim 11, characterized in that, The modification treatment takes 0.5 to 24 hours.

16. The preparation method according to claim 1, characterized in that, The particle size of the all-silica molecular sieve is 50~300nm.

17. The preparation method according to claim 1, characterized in that, The specific surface area of ​​the all-silica molecular sieve is ≥400m². 2 / g.

18. The preparation method according to claim 1, characterized in that, The total content of metal ions in the all-silica molecular sieve is 0~50ppm.

19. The preparation method according to claim 1, characterized in that, The total content of iron-based metal ions in the all-silica molecular sieve is 0~20ppm.

20. The preparation method according to claim 1, characterized in that, The all-silicon molecular sieve is selected from any one or more of Silicalite-1, MCM-1, MWW-22, or β-molecular sieves.

21. The preparation method according to claim 1, characterized in that, In step S1, the soaking time is 2 to 24 hours.

22. The preparation method according to claim 1, characterized in that, In step S1, the temperature of the first drying process is 60~150℃.

23. The preparation method according to claim 1, characterized in that, In step S1, the first drying time is 2 to 24 hours.

24. The preparation method according to claim 1, characterized in that, In step S1, the temperature of the first roasting is 450~600℃.

25. The preparation method according to claim 1, characterized in that, In step S1, the first calcination time is 48~480h.

26. The preparation method according to claim 1, characterized in that, The second drying temperature is 80~120℃.

27. The preparation method according to claim 1, characterized in that, The second drying time is 2 to 24 hours.

28. A modified all-silica molecular sieve catalyst for the gas-phase rearrangement reaction of cyclohexanone oxime, characterized in that, The modified all-silica molecular sieve catalyst for the gas-phase rearrangement reaction of cyclohexanone oxime is prepared by the preparation method described in any one of claims 1 to 27.

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