Process for the co-production of n-substituted caprolactam

Abstract

The present application relates to the technical field of caprolactam derivative preparation, and discloses a method for co-producing N-substituted caprolactam, which comprises the following steps: contacting a first cyclohexanone oxime stream with a first catalyst to perform a gas phase rearrangement reaction, and then contacting a stream containing the product of the gas phase rearrangement reaction with a second catalyst to perform an N-alkylation reaction; the first cyclohexanone oxime stream contains cyclohexanone oxime and an alcohol solvent; wherein the first catalyst is a full-silicon MFI molecular sieve catalyst; the second catalyst comprises an MFI molecular sieve and a modified element, the MFI molecular sieve contains a doping element, the doping element has an ion valence of +3 or +4, and the modified element comprises phosphorus and a rare earth element. The method can effectively increase the content of high-value-added N-substituted (methyl or ethyl) caprolactam in the product and improve the total selectivity of caprolactam and N-substituted caprolactam.

Description

Technical Field

[0001] This invention relates to the field of caprolactam derivative preparation technology, and specifically to a method for co-producing N-substituted caprolactam. Background Technology

[0002] N-substituted caprolactams, including N-methylcaprolactam and N-ethylcaprolactam, are a class of high-value-added organic chemicals. N-methylcaprolactam is a novel solvent widely used in the separation of organic compounds and organic synthesis. It can also be used as a diluent, extractant, cleaning agent, degreasing agent, absorbent, and dispersant. N-ethylcaprolactam is an important organic synthesis intermediate and solvent.

[0003] The traditional method for synthesizing N-methylcaprolactam involves using caprolactam as a raw material and iodomethane or dimethyl sulfate as a methylating agent, synthesizing it under strong alkaline conditions of NaOH or a mixed alkaline environment of KOH and K2CO3 with the addition of a phase transfer catalyst. However, this method has harsh reaction conditions and high costs.

[0004] CN103547569A discloses a method for manufacturing N-substituted lactam compounds, which uses lactone and organic amine as raw materials, but the product separation process is complex. CN107056567A discloses a novel method for synthesizing N-substituted amide derivatives, which uses SO / MO type solid superacid catalyst, caprolactone and ammonia (amine) as raw materials in a fixed bed reaction at 180.0-320.0℃. The product is collected by condensation, dehydrated, and desolventized to obtain high-purity caprolactam and N-substituted caprolactam, with a yield of over 95%. However, this method uses a solid superacid as a catalyst, and the presence of water and basic ammonia (amine) under the reaction conditions leads to catalyst deactivation, making it difficult for industrial production applications.

[0005] US20210179562A1 discloses a method for synthesizing N-alkylated caprolactam via reduction. In this method, formaldehyde is added to caprolactam in the presence of a catalyst with a noble metal content of 10% under a reducing atmosphere of H2, and the reaction is carried out at 100°C for 18 hours, yielding 71% N-methylcaprolactam. However, the production cost is high due to the use of hydrogen and a noble metal catalyst during the reaction, and caprolactam may also undergo hydrogenation, reducing the yield of the desired product.

[0006] In addition, N-substituted caprolactam is also a byproduct of the gas-phase Beckmann rearrangement reaction of cyclohexanone oxime. To recover some of the high-value-added impurities, CN109574929A discloses a method for separating and purifying N-methylcaprolactam from the light impurity components of the cyclohexanone oxime gas-phase Beckmann rearrangement product. This method is used to separate impurities from the cyclohexanone oxime gas-phase Beckmann rearrangement product and obtain N-methylcaprolactam through separation and purification. However, due to the high temperature of the gas-phase Beckmann rearrangement reaction, it often contains multiple impurities, resulting in a low content of high-value-added N-methylcaprolactam. Furthermore, the catalyst is a weakly acidic all-silica molecular sieve with insufficient acidic centers, limiting the yield of N-methylcaprolactam and making it economically unfeasible. Summary of the Invention

[0007] The purpose of this invention is to overcome the problems of limited yield and poor economic efficiency of N-methylcaprolactam in the prior art, and to provide a method for co-producing N-substituted caprolactam. This method can improve the overall selectivity of caprolactam and N-substituted caprolactam in the gas-phase Beckmann rearrangement reaction, efficiently co-produce N-substituted caprolactam, and has a simple process route, low cost, and is conducive to industrial application.

[0008] To achieve the above objectives, the present invention provides a method for the co-production of N-substituted caprolactam, the method comprising: contacting a first cyclohexanone oxime stream with a first catalyst to perform a gas-phase rearrangement reaction, and then contacting a stream containing the product of the gas-phase rearrangement reaction with a second catalyst to perform an N-alkylation reaction; wherein the first cyclohexanone oxime stream contains cyclohexanone oxime and an alcohol solvent;

[0009] The first catalyst is an all-silicon MFI molecular sieve catalyst; the second catalyst includes an MFI molecular sieve and a modifying element, wherein the MFI molecular sieve contains a dopant element having a +3 ion valence state and / or a +4 ion valence state, and the modifying element includes phosphorus and rare earth elements.

[0010] Because the gas-phase Beckmann rearrangement reaction is at a high temperature, it often contains a variety of impurities, resulting in a low content of high-value-added N-methylcaprolactam. At the same time, the catalysts used in the existing gas-phase Beckmann rearrangement reaction are generally weakly acidic all-silica molecular sieves with insufficient acid centers, which also limits the yield of N-methylcaprolactam and leads to poor economic efficiency of co-production.

[0011] The method for co-producing N-substituted caprolactam provided by this invention can increase the content of high-value-added N-substituted (methyl or ethyl) caprolactam in the product, improve the overall selectivity of caprolactam and N-substituted caprolactam, enhance technical and economic efficiency, and maintain catalyst lifetime and regeneration cycle. This method has the advantages of simple process route, ease of industrial production, and environmental friendliness. Detailed Implementation

[0012] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0013] The present invention provides a method for co-producing N-substituted caprolactam, the method comprising: contacting a first cyclohexanone oxime stream with a first catalyst to perform a gas-phase rearrangement reaction, and then contacting a stream containing the product of the gas-phase rearrangement reaction with a second catalyst to perform an N-alkylation reaction; wherein the first cyclohexanone oxime stream contains cyclohexanone oxime and an alcohol solvent;

[0014] The first catalyst is an all-silicon MFI molecular sieve catalyst; the second catalyst includes an MFI molecular sieve and a modifying element, wherein the MFI molecular sieve contains a dopant element having a +3 ion valence state and / or a +4 ion valence state, and the modifying element includes phosphorus and rare earth elements.

[0015] In this invention, both the gas-phase rearrangement reaction and the N-alkylation reaction involve the simultaneous gas-phase Beckmann rearrangement of cyclohexanone oxime and the N-alkylation process. The alcohol solvent serves as both the solvent for the gas-phase Beckmann rearrangement reaction and the alkylating agent. Because the gas-phase Beckmann rearrangement reaction is carried out at high temperatures, it often contains various impurities, resulting in a low content of high-value-added N-methylcaprolactam. Furthermore, the catalysts used in existing gas-phase Beckmann rearrangement reactions are generally weakly acidic all-silica molecular sieves, lacking sufficient acidic centers, which also limits the yield of N-methylcaprolactam, leading to poor co-production economics. In this invention, by combining the first and second catalysts, a high concentration of N-substituted (methyl or ethyl)caprolactam is produced concurrently during the caprolactam generation process. This effectively increases the content of high-value-added N-substituted (methyl or ethyl)caprolactam in the product, improves the overall selectivity of caprolactam and N-substituted caprolactam, and enhances technical and economic efficiency.

[0016] In this invention, the first catalyst and the second catalyst can be respectively packed in two reactors connected in series, or they can be packed in two independent reaction zones of the same reactor. This invention does not have any particular limitation on this, as long as it can achieve the following: the first cyclohexanone oxime stream first contacts the first catalyst, and then the stream containing the product of the gas-phase rearrangement reaction is contacted with the second catalyst.

[0017] According to a preferred embodiment of the present invention, the stream containing the product of the gas-phase rearrangement reaction further includes a second cyclohexanone oxime stream. The composition of the second cyclohexanone oxime stream may be the same as or different from that of the first cyclohexanone oxime stream. Using the above preferred embodiment is advantageous for further improving the selectivity of N-substituted caprolactams.

[0018] Preferably, the mass ratio of the second cyclohexanone oxime stream to the first cyclohexanone oxime stream is 0.1-1:1, for example, it can be a typical but not limiting ratio or a range between the two, such as 0.1:1, 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, etc.

[0019] According to the present invention, preferably, the mass ratio of the second catalyst to the first catalyst is 0.1-1:1. For example, it can be a typical but not limiting ratio or a range between the two, such as 0.1:1, 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, etc.

[0020] According to the present invention, the first catalyst is an all-silica MFI molecular sieve catalyst, preferably silicalite-1 molecular sieve, which does not contain aluminum atoms and will not undergo dealuminization or other phenomena under high temperature conditions. The all-silica MFI molecular sieve catalyst can be commercially available or prepared using any preparation method known in the art.

[0021] In conventionally used MFI molecular sieve catalysts for the cyclohexanone oxime gas-phase Beckmann rearrangement reaction, to ensure high selectivity for caprolactam products, the metal content is very low, the acid center is a weak acid silanol pit, and there are no other acid centers, resulting in insufficient number of acid centers and limited yield of N-substituted (methyl or ethyl) caprolactam. In this invention, the second catalyst contains MFI molecular sieves and modifying elements. Simultaneously, doping elements with +3 and / or +4 valence states are introduced into the MFI molecular sieve framework to modify the catalyst, which is beneficial for increasing the number of active centers for N-substituted (methyl or ethyl) caprolactam. Combining the second catalyst with an all-silica MFI molecular sieve catalyst can enhance the reaction characteristics of both and increase the content of N-substituted caprolactam in the product.

[0022] In conventional MFI molecular sieve catalysts of the prior art, the metal content is very low in order to ensure high selectivity of caprolactam products. In this invention, the content of the dopant element is relatively high. Preferably, based on the total amount of the MFI molecular sieve, the content of the dopant element is 0.01-3 wt%.

[0023] In a further preferred embodiment, the doping element having a +3 ionic valence state is selected from at least one of B, Al, and Fe, preferably Al.

[0024] Preferably, based on the total amount of the MFI molecular sieve, the content of the dopant element with the +3 valence state is 0.01-0.2 wt%, for example, it can be a typical but not limiting content or a range between the two, such as 0.01 wt%, 0.02 wt%, 0.03 wt%, 0.04 wt%, 0.05 wt%, 0.06 wt%, 0.07 wt%, 0.08 wt%, 0.09 wt%, 0.1 wt%, 0.15 wt%, 0.2 wt%, etc., more preferably 0.02-0.1 wt%.

[0025] In another preferred embodiment, the dopant element having an ionic valence state of +4 is selected from at least one of Ti, Zr and Sn, preferably Ti.

[0026] Preferably, based on the total amount of the MFI molecular sieve, the content of the dopant element with the +4 valence state is 0.1-3 wt%, for example, it can be a typical but not limiting content or a range between the two, such as 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt%, 2 wt%, 2.2 wt%, 2.5 wt%, 2.8 wt%, 3 wt%, etc., preferably 2-3 wt%.

[0027] According to the present invention, preferably, the mass ratio of rare earth elements to phosphorus in the modified elements is 1-10:1, for example, typical but not limiting ratios or ranges between 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, etc. Preferably, the mass ratio of rare earth elements to phosphorus is 1-3:1. In the above preferred cases, the synergistic effect of phosphorus and rare earth elements can be further utilized, which helps to increase the content of N-substituted caprolactam in the product.

[0028] In this invention, the rare earth element refers to the lanthanide elements, and preferably, the rare earth element is selected from La and / or Ce.

[0029] Preferably, in the second catalyst, the content of rare earth elements is 0.01-0.2 wt%, preferably 0.05-0.1 wt%, and the content of phosphorus elements is 0.01-0.1 wt%, preferably 0.01-0.04 wt%.

[0030] In this invention, the trace element content in the catalyst is obtained by ICP testing.

[0031] According to a specific embodiment of the present invention, the method for preparing the second catalyst includes:

[0032] (1) A silicon source, a doped element precursor, an organic template agent, water and an alcohol are mixed to obtain a mixture; wherein the silicon source is an organosilicate, and the molar ratio of the silicon source, organic template agent, alcohol and water in the mixture, calculated as SiO2, is 1:(0.05-0.5):(4-10):(5-100);

[0033] (2) The mixture is subjected to hydrothermal crystallization, then dried and calcined to obtain MFI molecular sieve;

[0034] (3) The MFI molecular sieve is impregnated with a solution containing phosphorus source and rare earth element precursor;

[0035] (4) Contact the product obtained in step (3) with an alkaline buffer solution containing nitrogen compounds.

[0036] In a further preferred embodiment, the molar ratio of silicon source, organic template agent, alcohol and water (calculated as SiO2) in the mixture is 1:(0.2-0.3):(6-8):(10-20).

[0037] Preferably, the silicon source is tetraethyl orthosilicate.

[0038] The organic template agent can be any of the organic template agents commonly used in the art for preparing all-silica-1 molecular sieves. Preferably, the organic template agent is selected from quaternary ammonium base compounds. The quaternary ammonium base compounds can be alkyl quaternary ammonium base compounds containing 1-4 carbon atoms, more preferably tetrapropylammonium hydroxide and / or tetraethylammonium hydroxide, and more preferably tetrapropylammonium hydroxide.

[0039] The present invention has a wide range of choices for the specific types of precursors containing doped elements, which can be selected from soluble compounds containing doped elements. Those skilled in the art can make the selection according to actual needs.

[0040] In one specific embodiment, the doping element is Al, and preferably, the precursor of Al is boehmite.

[0041] In some specific embodiments of the present invention, step (1) includes: first, mixing and hydrolyzing pseudoboehmite with a portion of the organic template agent, and then mixing the hydrolyzed product with a silicon source, the remaining portion of the organic template agent, water, and alcohol. The present invention does not specifically limit the specific operation and conditions of the hydrolysis; those skilled in the art can select according to actual needs. Preferably, the hydrolysis temperature is 120-160℃, and the time is 2-5 hours.

[0042] According to the present invention, the amount of the dopant precursor, calculated as a dopant element, is sufficient to ensure that the dopant element content in the obtained second catalyst meets the range defined above, and will not be elaborated further here. Preferably, the mass ratio of the dopant precursor, calculated as a dopant element, to the silicon source, calculated as SiO2, is 0.0001-0.03:1.

[0043] Preferably, the conditions for hydrothermal crystallization include: a temperature of 80-200℃, more preferably 95-170℃; and a time of 0.5-10 days, more preferably 2-5 days.

[0044] Preferably, in step (2), the drying conditions include: a temperature of 100-150°C and a time of 10-24 hours.

[0045] Preferably, in step (2), the calcination conditions include: a temperature of 450-600℃ and a time of 1-10h.

[0046] Preferably, step (2) further includes: optionally washing and separating the product obtained by hydrothermal crystallization, followed by drying and calcination. The present invention does not particularly limit the specific operating methods and conditions for the washing and solid-liquid separation; conventional methods in the art can be used.

[0047] According to the present invention, step (3) involves loading phosphorus and rare earth elements onto the MFI molecular sieve using an impregnation method. The present invention does not impose any particular limitation on the amount of the solution containing the phosphorus source and rare earth element precursors, as long as the above loading process can be achieved; for example, a saturated impregnation method can be used.

[0048] According to the present invention, the specific types of phosphorus source and rare earth element precursor are not particularly limited, as long as phosphorus and rare earth elements can be provided, and conventional soluble compounds containing phosphorus / rare earth elements can be used. The phosphorus source is preferably a phosphate, such as ammonium phosphate. The rare earth element precursor can be at least one of rare earth element nitrates, sulfates, and chlorides.

[0049] The content of phosphorus source and rare earth element precursor in the solution containing phosphorus source and rare earth element precursor shall meet the content of phosphorus element and rare earth element in the second catalyst as specified above, and may be adjusted according to actual needs, which will not be elaborated here.

[0050] Preferably, the impregnation conditions include: a temperature of 80-100℃, more preferably 80-90℃, and a time of 5-10h, more preferably 6-8h.

[0051] Preferably, step (3) further includes drying and calcining the product obtained from the impregnation. The drying temperature is 100-130°C. The present invention does not have a particular limitation on the drying time, as long as residual solvent is removed.

[0052] Preferably, the calcination temperature is 400-500℃ and the calcination time is 4-6 hours.

[0053] In this invention, the product obtained in step (3) is contacted with an alkaline buffer solution containing nitrogen compounds. Preferably, the mass ratio of the product obtained in step (3) to the alkaline buffer solution containing nitrogen compounds is 1:5-20, and more preferably 1:5-10.

[0054] Preferably, the alkaline buffer solution of the nitrogen-containing compound contains an ammonium salt and an alkali, and the solvent in the alkaline buffer solution of the nitrogen-containing compound can be water.

[0055] Preferably, the ammonium salt is provided by an aqueous solution of the ammonium salt, the concentration of which is 5-10 wt%.

[0056] Preferably, the ammonium salt is selected from at least one of ammonium nitrate, ammonium chloride, and ammonium acetate.

[0057] Preferably, the alkali is provided by ammonia water with a concentration of 10-30 wt%.

[0058] Preferably, in the alkaline buffer solution containing the nitrogen compound, the mass ratio of ammonia water to the aqueous solution of ammonium salt is 1-5:1, more preferably 2-3:1.

[0059] Preferably, the contact conditions are: temperature 70-100℃, pressure 0.2-0.3MPa, and contact time 0.5-2h. Preferably, the contact is carried out under stirring conditions.

[0060] Preferably, the preparation method further includes: performing solid-liquid separation, washing, and drying on the contacted product. The solid-liquid separation, washing, and drying can be performed using conventional methods in the art, and the present invention does not impose any particular limitation thereon.

[0061] According to the present invention, preferably, the conditions for the gas-phase rearrangement reaction include: a reaction temperature of 300-500°C, preferably 350-400°C; a reaction pressure of 0.1-0.5 MPa, preferably 0.1-0.3 MPa; and a weight hourly space velocity (WHSV) of the reactants, calculated based on cyclohexanone oxime, of 0.1-16 h⁻¹. -1 Preferably 1-8h -1 .

[0062] Preferably, the conditions for the N-alkylation reaction include: a reaction temperature of 300-500℃, more preferably 300-380℃, and a reaction pressure of 0.1-0.5MPa, more preferably 0.1-0.3MPa.

[0063] In this invention, the conditions of the gas-phase rearrangement reaction and the conditions of the N-alkylation reaction can be the same or different. In order to further increase the content of high-value-added N-substituted (methyl or ethyl) caprolactam in the product and increase the overall selectivity of caprolactam and N-substituted caprolactam, preferably, the temperature of the N-alkylation reaction is 20-100°C lower than the temperature of the gas-phase rearrangement reaction, and preferably, the temperature of the N-alkylation reaction is 30-50°C lower than the temperature of the gas-phase rearrangement reaction.

[0064] According to a specific embodiment of the present invention, the gas-phase rearrangement reaction and the N-alkylation reaction are carried out under an inert atmosphere provided by an inert gas.

[0065] Preferably, the inert gas is selected from at least one of nitrogen, helium, argon and neon, and is preferably nitrogen.

[0066] Preferably, the molar ratio of the inert gas to the first cyclohexanone oxime stream is 10-80:1, more preferably 20-50:1.

[0067] Preferably, the alcohol solvent is methanol and / or ethanol. Preferably, in the first cyclohexanone oxime stream, the mass ratio of cyclohexanone oxime to alcohol is 1:1-5, more preferably 1:2-3.

[0068] In a further preferred embodiment, the method further includes: separating the product obtained from the N-alkylation reaction to obtain an N-substituted caprolactam product.

[0069] The separation can be performed using the method disclosed in CN109574929A for separating and purifying N-methylcaprolactam from the light impurity component of the cyclohexanone oxime gas-phase Beckmann rearrangement product. The separation may also include steps such as dehydration, desolventization, and extraction concentration, and can be performed using any method known in the art; the present invention does not particularly limit this process.

[0070] The present invention will be described in detail below through embodiments.

[0071] Unless otherwise specified, all raw materials used in the following preparation examples and embodiments are commercially available.

[0072] The following preparation examples illustrate the preparation of the first catalyst.

[0073] Preparation Example 1-1

[0074] (1) At room temperature, 440g of tetraethyl orthosilicate (TEOS), 600g of deionized water, ethanol, and 440g of 24.5wt% tetrapropylammonium hydroxide (TPAOH) were poured into a beaker and mixed and stirred for 30min to obtain a mixture with molar content of TPAOH / SiO2 = 0.25, EtOH / SiO2 = 6, and H2O / SiO2 = 16.

[0075] (2) The above mixture was transferred to a stainless steel reactor and crystallized at 115°C for 3 days. After filtration and washing, it was calcined at 550°C for 4 hours to obtain an all-silica MFI molecular sieve. XRD characterization showed that it was consistent with the standard XRD pattern of MFI structure, indicating that the molecular sieve has an MFI crystal structure.

[0076] (3) Add 10g of the product obtained in step (2) and 100g of the above alkaline buffer solution (the concentration of ammonia water is 26 wt%, the concentration of ammonium nitrate aqueous solution is 7.5 wt%, and the weight ratio of ammonia water to ammonium nitrate aqueous solution is 2:1) into a pressurized reactor, stir for 1 hour at 80℃ and 0.23MPa pressure, and then filter, wash and dry to obtain catalyst A1.

[0077] The following preparation examples illustrate the preparation of the second catalyst.

[0078] Preparation Example 2-1

[0079] (1) Mix 0.2g of pseudoboehmite with 240g of 24.5wt% tetrapropylammonium hydroxide and hydrolyze in a pressure vessel at 140℃ for 3h.

[0080] (2) At room temperature, 440g of tetraethyl orthosilicate (abbreviated as TEOS), 600g of deionized water, ethanol, and 200g of 24.5wt% tetrapropylammonium hydroxide (abbreviated as TPAOH) were poured into a beaker and mixed and stirred for 30min. Then the solution in step (1) was added and stirred evenly to obtain a mixture with molar content of TPAOH / SiO2 = 0.25, EtOH / SiO2 = 6, and H2O / SiO2 = 16, wherein the mass ratio of Al / SiO2 was 0.0006:1.

[0081] (3) The above mixture was transferred to a stainless steel reactor and crystallized at 115°C for 3 days. After filtration and washing, it was calcined at 550°C for 4 hours to obtain MFI molecular sieve. Based on the total amount of the MFI molecular sieve, the Al content was 0.06 wt%.

[0082] (4) Dissolve 0.46g of lanthanum nitrate and 0.48g of ammonium phosphate in 100g of deionized water. Vacuum 250g of molecular sieve powder at room temperature for 0.5h, then absorb it into the above solution. Soak it at 85℃ for 6 hours, evaporate the water, dry it at 120℃, and calcine it at 450℃ for 4h.

[0083] (5) Add 10g of the product obtained in step (4) and 100g of alkaline buffer solution (ammonia concentration of 26 wt%, ammonium nitrate aqueous solution concentration of 7.5 wt%, and ammonia to ammonium nitrate aqueous solution weight ratio of 2:1) to a pressurized reactor. Stir at 80℃ and 0.23MPa pressure for 1 hour, then filter, wash, and dry to obtain catalyst S1. The amount of lanthanum is 0.08 wt%, and the mass ratio of lanthanum to phosphorus is 2:1. The catalyst is labeled as S1.

[0084] Preparation Example 2-2

[0085] (1) At room temperature, 440g of tetraethyl orthosilicate (TEOS), 600g of deionized water, ethanol, and 440g of 24.5wt% tetrapropylammonium hydroxide (TPAOH) were poured into a beaker and mixed and stirred for 30min. Then, 17g of tetrabutyl titanate was added and stirred evenly to obtain a mixture with molar content of TPAOH / SiO2 = 0.25, EtOH / SiO2 = 6, and H2O / SiO2 = 16, wherein the mass ratio of Ti / SiO2 was 0.02:1.

[0086] (2) The above mixture was transferred to a stainless steel reactor and crystallized at 170°C for 3 days. After filtration and washing, it was calcined at 560°C for 5 hours to obtain MFI molecular sieve. The content of Ti was 2% based on the total amount of the MFI molecular sieve.

[0087] (3) Dissolve 0.46g of lanthanum nitrate and 0.49g of ammonium phosphate in 100g of deionized water. Vacuum 250g of molecular sieve powder at room temperature for 0.5h, then absorb the above solution, soak at 85℃ for 6 hours, evaporate the water, dry at 120℃, and calcine at 450℃ for 3h.

[0088] (4) Add 10g of the product obtained in step (3) and 100g of alkaline buffer solution (containing 26 wt% ammonia and 7.5 wt% ammonium nitrate, with a weight ratio of 2:1 for ammonia and ammonium nitrate) to a pressurized reactor. Stir at 80℃ and 0.23MPa for 1 hour, then filter, wash, and dry to obtain the catalyst. The amount of lanthanum is 0.08 wt%, and the mass ratio of lanthanum to phosphorus is 2:1. The catalyst is labeled as S2.

[0089] Preparation Examples 2-3

[0090] The method is the same as in Preparation Example 2-1, except that the amount of boehmite used in step (1) is 0.02 g.

[0091] In the prepared MFI molecular sieve, the Al content is 0.006 wt%, based on the total amount of the MFI molecular sieve.

[0092] The catalyst obtained by performing steps (4) and (5) in the same manner is denoted as S3.

[0093] Preparation Examples 2-4

[0094] Following the method in Preparation Example 2-2, except that the amount of lanthanum nitrate used was 0.46 g and the amount of ammonium phosphate was 2 g, the resulting catalyst was designated S4, wherein the contents of lanthanum and phosphorus were 0.08 wt% and 0.16%, respectively, and the mass ratio of lanthanum to phosphorus was 1:2. The catalyst was labeled as S4.

[0095] Comparative Preparation Example 1

[0096] The method was the same as in Preparation Example 2-1, except that step (4) was omitted, and the MFI molecular sieve obtained in step (3) was directly contacted with an alkaline buffer solution. The resulting catalyst was denoted as DS1.

[0097] Comparative Preparation Example 2

[0098] The method was the same as in Preparation Example 2-2, except that step (4) was omitted, and the MFI molecular sieve obtained in step (3) was directly contacted with an alkaline buffer solution. The resulting catalyst was denoted as DS2.

[0099] The following examples illustrate a method for the co-production of N-substituted caprolactam.

[0100] The gas-phase Beckmann rearrangement of cyclohexanone oxime was carried out using two fixed-bed reactors connected in series. The first reactor was the rearrangement reactor and the second was the N-alkylation reactor. The inner diameter of each reactor was 5 mm. The catalyst was loaded according to the combination in Tables 1 and 2. The catalyst bed was filled with 60-mesh quartz sand with a height of 30 mm on top and 60-mesh quartz sand with a height of 60 mm on the bottom.

[0101] In the presence of nitrogen (flow rate 1500 mL / min), a first cyclohexanone oxime stream (cyclohexanone oxime to alcohol mass ratio 1:2) was introduced into the reaction system through the inlet of the rearrangement reactor. The product of the gas-phase rearrangement reaction was then introduced into the N-alkylation reactor along with a second cyclohexanone oxime stream (cyclohexanone oxime to alcohol mass ratio 1:2). Examples of implementations using methanol as the reaction solvent are shown in Table 1, and examples using ethanol as the reaction solvent are shown in Table 2. The reaction products were cooled with an ice-water mixture and then collected in a collection bottle for gas-liquid separation. Product composition analysis was performed after 24 hours of reaction.

[0102] Table 3 shows the changes in the selectivity of N-alkylcaprolactam during the reaction process in Example 1 and Comparative Examples 1 and 2.

[0103] The reaction products were quantitatively analyzed using an Agilent 6890 gas chromatograph (flame ionization detector, PEG20M capillary column, 50m column length). The contents of cyclohexanone oxime, caprolactam, and N-substituted (methyl or ethyl) caprolactam were calculated using the area normalization method after the reaction; the solvent was not included in the integration.

[0104] The conversion rate of cyclohexanone oxime and the selectivity of caprolactam and N-substituted (methyl or ethyl) caprolactam can be calculated using the following formulas.

[0105] Cyclohexanone oxime conversion (mol%) = (100 - cyclohexanone oxime molar percentage in reaction product) / 100 × 100%;

[0106] Caprolactam selectivity (or N-substituted (methyl or ethyl) caprolactam) (mol%) = molar percentage of caprolactam (or N-substituted (methyl or ethyl) caprolactam) in the reaction product / (100 - molar percentage of cyclohexanone oxime in the reaction product) × 100%.

[0107] Table 1 shows the reaction results using methanol as the solvent.

[0108]

[0109]

[0110] *Total selectivity refers to the total selectivity of caprolactam + N-methylcaprolactam.

[0111] Table 2 shows the reaction results using ethanol as the solvent.

[0112]

[0113]

[0114] The reaction product in Example 1 was separated according to the method in CN109574929A. The N-methylcaprolactam purity in the aqueous phase after extraction was 97.9 wt%. The aqueous solution was then evaporated under reduced pressure to remove water (evaporation conditions: temperature 75℃, pressure 10 kPa), finally yielding an N-methylcaprolactam product with a purity of 98.4 wt%. The increased N-methylcaprolactam content in the reaction product led to a corresponding increase in purity after subsequent separation processes. The reaction products in Example 6 and Comparative Example 5 were separated using the same method, yielding N-ethylcaprolactam purities of 98.6 wt% and 97.0 wt%, respectively. This demonstrates that increasing the N-ethylcaprolactam content in the reaction product further improves the purity of the separated N-ethylcaprolactam.

[0115] As shown in Table 1, the method provided by this invention improves the conversion rate of cyclohexanone oxime and significantly increases the content of high-value-added N-methylcaprolactam from 0.4% to approximately 2-2.2%. Furthermore, the overall selectivity of N-methylcaprolactam + caprolactam is slightly improved, increasing the yield of cyclohexanone oxime to the valuable target product while enhancing the product's economic viability. As shown in Table 2, the increased N-ethylcaprolactam content contributes to improved product economics.

[0116] Table 3. Selectivity of N-methylcaprolactam at different reaction times, mol%.

[0117]

[0118]

[0119] The changes in N-methylcaprolactam content at different reaction times (0-24 h) were investigated using the reaction methods described in Examples 1, 1, and 2. Table 3 shows that, in Example 1, the N-methylcaprolactam content increased to 1.5 mol% after 2 h of reaction, and the selectivity subsequently stabilized at around 2 mol%. However, in Comparative Examples 1 and 2, the catalyst exhibited the highest selectivity for N-alkylation reaction in the initial stage of the reaction, reaching 1.14 mol%. During the reaction, the content of the byproduct N-methyl CPL gradually decreased with increasing reaction time, and its selectivity gradually dropped to 0.4-0.55 mol% after 24 h. This indicates that the N-methylcaprolactam content is unstable during the reaction, and the low content leads to high separation costs and poor techno-economic efficiency.

[0120] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for co-producing N-substituted caprolactam, the method comprising: A first cyclohexanone oxime stream is contacted with a first catalyst to undergo a gas-phase rearrangement reaction, and then a stream containing the product of the gas-phase rearrangement reaction is contacted with a second catalyst to undergo an N-alkylation reaction; the first cyclohexanone oxime stream contains cyclohexanone oxime and an alcohol solvent; the stream containing the product of the gas-phase rearrangement reaction further includes a second cyclohexanone oxime stream; the mass ratio of the second cyclohexanone oxime stream to the first cyclohexanone oxime stream is 0.1-1:1; The conditions for the gas-phase rearrangement reaction include: a reaction temperature of 300-500℃ and a reaction pressure of 0.1-0.5 MPa; The conditions for the N-alkylation reaction include: a reaction temperature of 300-500℃ and a reaction pressure of 0.1-0.5MPa; The first catalyst is silicalite.

1. Molecular sieve; The second catalyst comprises an MFI molecular sieve and a modifying element. The MFI molecular sieve contains a dopant element having a +3 valence state and / or a +4 valence state. The modifying element comprises phosphorus and rare earth elements. The MFI molecular sieve is a silicalite-1 type molecular sieve. The dopant element having a +3 valence state is selected from at least one of B, Al, and Fe. The dopant element having a +4 valence state is selected from at least one of Ti, Zr, and Sn. The rare earth element is selected from La and / or Ce.

2. The method according to claim 1, wherein, The mass ratio of the second catalyst to the first catalyst is 0.1-1:

1.

3. The method according to claim 1, wherein, Based on the total amount of the MFI molecular sieve, the content of the doping element is 0.01-3 wt%.

4. The method according to claim 1, wherein, The doping element with a +3 valence state is Al.

5. The method according to claim 1, wherein, Based on the total amount of the MFI molecular sieve, the content of the dopant element with a +3 valence state is 0.01-0.2 wt%.

6. The method according to claim 5, wherein, Based on the total amount of the MFI molecular sieve, the content of the dopant element with a +3 valence state is 0.02-0.1 wt%.

7. The method according to claim 1, wherein, The doping element with a +4 valence state is Ti.

8. The method according to claim 1, wherein, The content of the dopant element with a +4 valence state is 0.1-3 wt%.

9. The method according to claim 8, wherein, The content of the dopant element with a +4 valence state is 2-3 wt%.

10. The method according to claim 1, wherein, In the modified elements, the mass ratio of the rare earth elements to phosphorus is 1-10:

1.

11. The method according to claim 10, wherein, In the modified elements, the mass ratio of the rare earth elements to phosphorus is 1-3:

1.

12. The method according to claim 1, wherein, In the second catalyst, the content of rare earth elements is 0.01-0.2 wt% and the content of phosphorus is 0.01-0.1 wt%.

13. The method according to claim 12, wherein, In the second catalyst, the content of rare earth elements is 0.05-0.1 wt% and the content of phosphorus is 0.01-0.04 wt%.

14. The method according to any one of claims 1-13, wherein, The preparation method of the second catalyst includes: (1) A mixture is prepared by mixing a silicon source, a doped element precursor, an organic template agent, water and an alcohol; wherein the silicon source is an organosilicate, and the molar ratio of the silicon source, organic template agent, alcohol and water in the mixture, calculated as SiO2, is 1:(0.05-0.5):(4-10):(5-100). (2) The mixture is subjected to hydrothermal crystallization, followed by drying and calcination to obtain MFI molecular sieve; (3) The MFI molecular sieve is impregnated with a solution containing a phosphorus source and rare earth element precursors; (4) Contact the product obtained in step (3) with an alkaline buffer solution containing nitrogen compounds.

15. The method according to claim 14, wherein, The mass ratio of the dopant precursor (based on the amount of dopant element) to the silicon source (based on SiO2) is 0.0001-0.03:

1. And / or, the conditions for hydrothermal crystallization include: a temperature of 80-200°C and a time of 0.5-10 days.

16. The method according to claim 1, wherein, The conditions for the gas-phase rearrangement reaction include: a reaction temperature of 350-400℃, a reaction pressure of 0.1-0.3 MPa, and a weight hourly space velocity (WHSV) of the reactants (based on cyclohexanone oxime) of 0.1-16 h⁻¹. -1 ; And / or, the conditions for the N-alkylation reaction include: a reaction temperature of 300-380°C and a reaction pressure of 0.1-0.3 MPa.

17. The method according to claim 16, wherein, The conditions for the gas-phase rearrangement reaction include: a weight hourly space velocity (WHSV) of 1-8 h⁻¹ for the reactants, calculated based on cyclohexanone oxime. -1 .

18. The method according to claim 1, wherein, The temperature of the N-alkylation reaction is 20-100°C lower than the temperature of the gas-phase rearrangement reaction; And / or, the gas-phase rearrangement reaction and the N-alkylation reaction are carried out under an inert atmosphere provided by an inert gas.

19. The method according to claim 18, wherein, The N-alkylation reaction is carried out at a temperature 30-50°C lower than the gas-phase rearrangement reaction.

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