Solvent-free method for preparing 5-methyl-2-pyrrolidone through cobalt-doped nitrogen-carbon catalysis

Abstract

The invention relates to the technical field of catalyst preparation and biomass catalytic conversion, in particular to a solvent-free method for preparing 5-methyl-2-pyrrolidone through cobalt-doped nitrogen-carbon catalysis, which comprises the following steps: adding a mixed gas of methyl acetyl propyl ester, a catalyst, ethanol, ammonia gas and hydrogen into a closed high-pressure reactor, and uniformly mixing; after the reaction is finished, the target product 5-methyl-2-pyrrolidone is obtained. According to the invention, a composite structure that cobalt nanoparticles are embedded into a nitrogen-doped carbon skeleton (Co (at) NC-X) is constructed through directional pyrolysis of a ZIF-67 precursor at 500-700 DEG C in a nitrogen atmosphere. The nitrogen-doped carbon carrier enhances the electron conduction capacity and promotes dissociation and adsorption of cobalt active sites to hydrogen, nitrogen atoms on the surface of the nitrogen-doped carbon carrier and carbonyl of methyl levulinate form strong interaction, the nitrogen atoms and ammonia gas generate an imine intermediate firstly, and then the imine intermediate is subjected to catalytic hydrogenation closed loop through cobalt. Gt of Co-coated NC-600 is realized under the conditions that the temperature is 80 DEG C and the pressure is 3MPa; the conversion rate and the yield are 99.9%.

Description

Technical Field

[0001] The present invention relates to the technical field of catalyst preparation and biomass catalytic conversion, and in particular to a method for preparing 5-methyl-2-pyrrolidone in a solvent-free manner using cobalt-doped nitrogen-carbon catalysis. Background Art

[0002] In the synthesis of pyrrolidone compounds, traditional industry mainly relies on fossil-based raw materials. In recent years, researchers have proposed a new route for synthesizing pyrrolidone through reductive amination reaction using bio-based levulinic acid (LA) and its esters as raw materials. Among them, N-methyl-2-pyrrolidone (NMP) is a widely used aprotic polar solvent in industry. It is completely miscible with water and has a wide range of applications. However, due to the potential environmental and health risks of NMP, N-alkyl-5-methyl-2-pyrrolidone is regarded as an ideal alternative with higher sustainability and application potential.

[0003] In the synthesis research of 5-methyl-2-pyrrolidone, although precious metal catalysts (such as Pd, Pt) show excellent catalytic activity, their high cost limits industrial application; and non-precious metal catalysts (such as Ni, Co) with significant cost advantages still have room for improvement in activity and selectivity. The existing technology generally adopts a solvent system (concentration 1%-20%), which not only increases the energy consumption of subsequent separation and purification, but also does not meet the development requirements of green chemistry. It is worth noting that there are still limited research reports on the direct conversion of levulinic acid to prepare 5-methyl-2-pyrrolidone under solvent-free conditions, and this technical gap needs to be filled. Develop efficient non-precious metal catalysts, construct solvent-free reaction systems, establish low-energy consumption processes, promote green economic production, and promote the high-value utilization of biomass.

[0004] Chinese Patent CN1764451A discloses a method for synthesizing 5-methyl-N-substituted-2-pyrrolidone derivatives by reductive amination of levulinic acid with an aromatic cyano compound. While this method can produce a variety of derivatives, it suffers from harsh reaction conditions (75-200°C, 1.3-7.6 MPa), low yield (only 21.8% yield with 5% Pt / C catalysis), and the use of the carcinogenic solvent dioxane, which severely restricts its industrial application. Chinese Patent CN1764638A reports a process for preparing 5-methyl-N-substituted-2-pyrrolidone derivatives from levulinic acid esters and cyano compounds. While this process can synthesize three important products, it must be carried out at 75-225°C and 1.3-7.6 MPa, and the selectivity is only 21.7% when catalyzed by 5% Ru. Chinese Patent CN1764376A develops a route for synthesizing 5-methyl-N-substituted-2-pyrrolidone from levulinic acid with a nitro compound. Despite the innovative reactant selection, the harsh reaction conditions and low yield of 3.3% limit its industrial application value.

[0005] The above-mentioned synthesis methods generally have the following problems, which limit their industrial application: 1. Strong dependence on organic solvents: Some processes require the use of large amounts of organic solvents, which not only increases costs but also brings environmental and recycling problems; 2. Risk of high-pressure hydrogen: Some catalytic hydrogenation routes require high-pressure hydrogen, which poses a safety hazard and has high requirements for equipment; 3. High raw material costs: Some methods rely on precious metal catalysts (such as Pd, Pt, Ru) or high-purity precursors, which are less economical; 4. Insufficient yield and selectivity: Some routes have many side reactions and difficult product separation, resulting in unsatisfactory yield or optical selectivity. These problems limit the practicality of existing methods in large-scale production, and there is an urgent need to develop greener, safer, and more efficient synthesis strategies. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for preparing 5-methyl-2-pyrrolidone in a solvent-free manner using a cobalt-doped nitrogen-carbon catalyst, which uses a cobalt-based non-precious metal catalyst as the core to achieve efficient conversion of levulinic acid and methyl ester in a nitrogen atmosphere.

[0007] In order to achieve the above technical objectives and the above technical effects, the present invention is implemented through the following technical solutions:

[0008] A method for preparing 5-methyl-2-pyrrolidone using cobalt-doped nitrogen-carbon catalysis comprises adding a mixture of methyl acetyl propyl ester, a catalyst, ethanol, ammonia and hydrogen into a closed high-pressure reactor and mixing them evenly; after completion of the reaction, the target product 5-methyl-2-pyrrolidone is obtained.

[0009] Furthermore, the method specifically includes the following steps:

[0010] S1: Add methyl levulinate and cobalt-doped nitrogen-carbon catalyst (Co@NC-X) into the autoclave;

[0011] S2: Introduce a mixture of ammonia and hydrogen, control the hydrogen pressure at 2-4 MPa, and control the ammonia pressure at 0.2-0.8 MPa;

[0012] S3: reacting at 60-100° C. for 1-12 hours, and separating to obtain 5-methyl-2-pyrrolidone after the reaction is completed;

[0013] The mass ratio of the catalyst to methyl levulinate is 10:1, and the substrate concentration is 20-100%.

[0014] Furthermore, the hydrogen pressure is 3 MPa and the reaction time is 6 hours.

[0015] Furthermore, the preparation method of the cobalt-doped nitrogen-carbon catalyst in step S1 includes the following sub-steps:

[0016] S1.1: Dissolve Co(NO₃)₂·6H₂O (6.4 g, 22 mmol) and 2-methylimidazole (5.5 g, 67 mmol) in 150 mL of methanol. Slowly add the metal salt solution to the 2-methylimidazole solution and stir at room temperature for 6 h. Recover the resulting precipitate by centrifugation at 9000 ppm for 5 min, wash it three times with methanol, and dry it in a vacuum oven at 50°C for 24 h to obtain ZIF-67.

[0017] S1.2: After drying, ZIF-67 was ground and then thermally activated in a tube furnace at a temperature range of 500-600 °C for 3 h to obtain Co@NC-X, where "X" represents the pyrolysis temperature. The pyrolysis was carried out under N2 flow and the heating rate was 5 °C / min. -1 .

[0018] Furthermore, the pyrolysis temperature in step S1.2 is 600°C.

[0019] Beneficial effects of the present invention:

[0020] The solvent-free reductive amination of levulinic acid to 5-MP meets the requirements of green chemistry development. However, the relatively harsh reaction conditions, poor catalyst stability, and low reaction efficiency currently limit the development of solvent-free systems. This invention addresses the problems of high hydrogen pressure caused by the addition of an external hydrogen source and solvent volatilization and recovery caused by the use of large amounts of organic solvent. It also addresses the problems of low product selectivity and difficulty in product separation and purification.

[0021] The present invention constructs a composite structure of cobalt nanoparticles embedded in a nitrogen-doped carbon skeleton (Co@NC-X) by directional pyrolysis of a ZIF-67 precursor at 500-700°C in a nitrogen atmosphere. The nitrogen-doped carbon support not only enhances the electron conductivity and promotes the dissociative adsorption of hydrogen by the cobalt active site, but also forms a strong interaction between the nitrogen atoms on its surface and the carbonyl group of methyl levulinate, first generating an imine intermediate with ammonia, and then catalyzing hydrogenation and ring closure by cobalt. Co@NC-600 achieves a conversion rate and yield of >99.9% at 80°C and 3MPa H2, as shown in Example 1. Under the same conditions, Co@NC-500 has a low cobalt particle dispersion and insufficient active site exposure due to insufficient pyrolysis temperature, and the yield is only 48.7% as shown in Example 2, which completely overcomes the problem of precious metal dependence in the background technology.

[0022] The nitrogen-doped carbon derived from ZIF-67 of the present invention has a high specific surface area and open pores, allowing methyl levulinate to effectively diffuse into cobalt active sites even at high concentrations of 20-100%. The lipophilic carbon skeleton captures substrate molecules through π-π interactions and polar adsorption, avoiding side reactions caused by localized excessive concentrations. As shown in Table 1, the yield remains >99.9% when the substrate concentration increases from 20% to 60%, as shown in Examples 4-7. Even at a 100% concentration, it still reaches 93.2%, as shown in Example 8, significantly simplifying the subsequent separation process and reducing energy consumption.

[0023] The present invention achieves high conversion rates at low temperatures (60-100°C) and low- to medium-pressure hydrogen (2-4 MPa). Nitrogen-doped carbon in Co@NC accelerates electron transfer to cobalt active sites, significantly reducing the activation energy of the imine hydrogenation step. When the temperature rises from 60°C to 80°C, the yield jumps from 19.4% to >99.9%, as shown in Examples 1 and 17, confirming that the amination reaction is kinetically limited at low temperatures. Furthermore, when the hydrogen pressure increases from 2 MPa to 3 MPa, the yield increases from 67.1% to >99.9%, as shown in Examples 1 and 13, demonstrating that the catalyst can still efficiently activate hydrogen under low-pressure conditions. This characteristic significantly reduces equipment explosion-proof rating requirements and operational risks.

[0024] The present invention achieves catalyst regeneration through high-temperature thermal activation in a nitrogen atmosphere. The high-temperature nitrogen flow effectively burns off carbon deposits on the catalyst surface while simultaneously reconstructing the cobalt-nitrogen coordination bonds and restoring active sites. As shown in Table 1, the yield remained at 86.9% after eight cycles, as shown in Example 28, and no significant sintering of the cobalt particles was observed during the cycles. The present regeneration process is simple to operate, low-cost, and can be directly integrated into industrial continuous production processes.

[0025] Of course, any product implementing the present invention does not necessarily need to achieve all of the advantages described above at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0027] Figure 1 Schematic diagram of X-ray diffraction patterns of the catalysts of Example 1 and Comparative Examples 1 and 2 of the present invention;

[0028] Figure 2 Schematic diagram of Raman spectra of the catalysts of Example 1 and Comparative Examples 1 and 2 of the present invention;

[0029] Figure 3Schematic diagram of gas chromatography-mass spectrometry of the present invention. DETAILED DESCRIPTION

[0030] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0031] Example 1

[0032] 0.15 g of Co@CN-X (X = 600°C) catalyst, 1.5 g of methyl levulinate, and 8 mL of methanol (at a mass ratio of methyl levulinate to catalyst of 10:1) were added to a sealed high-pressure reactor and mixed thoroughly. The mixture was then reacted at 80°C for 6 h under 3 MPa of hydrogen. After completion of hydrogenation and reductive amination, the target product, 5-methyl-2-pyrrolidone, was obtained. The reaction was terminated, cooled to room temperature, and the catalyst was recovered by filtration. The product, 5-methyl-2-pyrrolidone, was qualitatively and quantitatively analyzed using GC-MS and GC. The test results in this example are listed in Table 1, numbered 1.

[0033] Example 2

[0034] 0.15 g of Co@CN-X (X = 500°C) catalyst, 1.5 g of methyl levulinate, and 8 mL of methanol (at a mass ratio of methyl levulinate to catalyst of 10:1) were added to a sealed high-pressure reactor and mixed thoroughly. The mixture was then reacted at 80°C for 6 h under 3 MPa of hydrogen. After completion of hydrogenation and reductive amination, the target product, 5-methyl-2-pyrrolidone, was obtained. The reaction was terminated, cooled to room temperature, and the catalyst was recovered by filtration. The product, 5-methyl-2-pyrrolidone, was qualitatively and quantitatively analyzed using GC-MS and GC. The test results in this example are listed in Table 1, number 2.

[0035] Example 3

[0036] 0.15 g of Co@CN-X (X = 700°C) catalyst, 1.5 g of methyl levulinate, and 8 mL of methanol (at a mass ratio of methyl levulinate to catalyst of 10:1) were added to a sealed high-pressure reactor and mixed thoroughly. The mixture was then reacted at 80°C for 6 h under 3 MPa of hydrogen. After completion of hydrogenation and reductive amination, the target product, 5-methyl-2-pyrrolidone, was obtained. The reaction was terminated, cooled to room temperature, and the catalyst was recovered by filtration. The product, 5-methyl-2-pyrrolidone, was qualitatively and quantitatively analyzed using GC-MS and GC. The test results in this example are listed in Table 1, number 3.

[0037] Example 4

[0038] Co@CN-600 catalyst, methyl levulinate, and methanol solvent were added to a sealed high-pressure reactor with a substrate concentration of 30% and a mass ratio of methyl levulinate to catalyst of 10:1. The mixture was mixed evenly and then reacted at 80°C for 6 hours under 3 MPa of hydrogen. After completion of hydrogenation and reductive amination, the target product, 5-methyl-2-pyrrolidone, was obtained. The reaction was terminated, cooled to room temperature, and filtered to recover the catalyst. The product, 5-methyl-2-pyrrolidone, was qualitatively and quantitatively analyzed using GC-MS and GC. The test results in this example are listed in Table 1, number 4.

[0039] Example 5

[0040] Co@CN-600 catalyst, methyl levulinate, and methanol solvent were added to a sealed high-pressure reactor with a substrate concentration of 40% and a mass ratio of methyl levulinate to catalyst of 10:1. The mixture was mixed thoroughly and then reacted at 80°C for 6 hours under 3 MPa of hydrogen. After completion of hydrogenation and reductive amination, the target product, 5-methyl-2-pyrrolidone, was obtained. The reaction was terminated, cooled to room temperature, and filtered to recover the catalyst. The product, 5-methyl-2-pyrrolidone, was qualitatively and quantitatively analyzed using GC-MS and GC. The test results in this example are listed in Table 1, numbered 5.

[0041] Example 6

[0042] Co@CN-600 catalyst, methyl levulinate, and methanol solvent were added to a sealed high-pressure reactor with a substrate concentration of 50% and a mass ratio of methyl levulinate to catalyst of 10:1. The mixture was mixed thoroughly and then reacted at 80°C for 6 hours under 3 MPa of hydrogen. After completion of hydrogenation and reductive amination, the target product, 5-methyl-2-pyrrolidone, was obtained. The reaction was terminated, cooled to room temperature, and the catalyst was recovered by filtration. The product, 5-methyl-2-pyrrolidone, was qualitatively and quantitatively analyzed using GC-MS and GC. The test results in this example are listed in Table 1, number 6.

[0043] Example 7

[0044] Co@CN-600 catalyst, methyl levulinate, and methanol solvent were added to a sealed high-pressure reactor with a substrate concentration of 60% and a mass ratio of methyl levulinate to catalyst of 10:1. The mixture was mixed thoroughly and then reacted at 80°C under 3 MPa of hydrogen for 6 hours. After completion of hydrogenation and reductive amination, the target product, 5-methyl-2-pyrrolidone, was obtained. The reaction was terminated, cooled to room temperature, and filtered to recover the catalyst. The product, 5-methyl-2-pyrrolidone, was qualitatively and quantitatively analyzed using GC-MS and GC. The test results in this example are listed in Table 1, number 7.

[0045] Example 8

[0046] Co@CN-600 catalyst, methyl levulinate, solvent methanol, substrate concentration of 100%, methyl levulinate to catalyst mass ratio of 10:1, added into a sealed high-pressure reactor and mixed uniformly, 3 MPa hydrogen, 80 °C, reaction time of 3-7 h, after the hydrogenation reduction amination was completed, the target product 5-methyl-2-pyrrolidinone was obtained, the reaction was ended, cooled to room temperature and the catalyst was recovered by filtration. The product 5-methyl-2-pyrrolidinone was qualitatively and quantitatively detected by GC-MS and GC, and the detection results in this example are listed in Table 1 as No. 9-12.

[0047] Examples 9-12

[0048] Co@CN-600 catalyst, methyl levulinate, solvent methanol, substrate concentration of 100%, methyl levulinate to catalyst mass ratio of 10:1, added into a sealed high-pressure reactor and mixed uniformly, 3 MPa hydrogen, 80 °C, reaction time of 3-7 h, after the hydrogenation reduction amination was completed, the target product 5-methyl-2-pyrrolidinone was obtained, the reaction was ended, cooled to room temperature and the catalyst was recovered by filtration. The product 5-methyl-2-pyrrolidinone was qualitatively and quantitatively detected by GC-MS and GC, and the detection results in this example are listed in Table 1 as No. 9-12.

[0049] Examples 13-16

[0050] Co@CN-600 catalyst, methyl levulinate, solvent methanol, substrate concentration of 100%, methyl levulinate to catalyst mass ratio of 10:1, added into a sealed high-pressure reactor and mixed uniformly, 2-4 MPa hydrogen, 80 °C, reaction for 6 h, after the hydrogenation reduction amination was completed, the target product 5-methyl-2-pyrrolidinone was obtained, the reaction was ended, cooled to room temperature and the catalyst was recovered by filtration. The product 5-methyl-2-pyrrolidinone was qualitatively and quantitatively detected by GC-MS and GC, and the detection results in this example are listed in Table 1 as No. 13-16.

[0051] Examples 17-20

[0052] Co@CN-600 catalyst, methyl levulinate, solvent methanol, substrate concentration of 100%, methyl levulinate to catalyst mass ratio of 10:1, added into a sealed high-pressure reactor and mixed uniformly, 3 MPa hydrogen, 60-100 °C, reaction for 6 h, after the hydrogenation reduction amination was completed, the target product 5-methyl-2-pyrrolidinone was obtained, the reaction was ended, cooled to room temperature and the catalyst was recovered by filtration. The product 5-methyl-2-pyrrolidinone was qualitatively and quantitatively detected by GC-MS and GC, and the detection results in this example are listed in Table 1 as No. 17-20.

[0053] Examples 21-28

[0054] The catalyst in Example 1 was reacted at 80°C for 6 hours. After the reaction, it was collected by centrifugation, washed three times with deionized water and anhydrous ethanol, dried at 50°C for 12 hours, and then thermally activated at 600°C in a tube furnace for 3 hours. The pyrolysis was carried out under N2 flow at a heating rate of 5°C / min. -1 , and was directly used in the next catalytic reaction. The catalytic conditions were as follows: 0.15g Co@CN-600, 1.5g ML, and 8mL ethanol were mixed and added to a sealed autoclave, heated to 80°C, reacted for 6 hours, cooled to room temperature, and sampled. Qualitative and quantitative analysis was performed using GC-MS and GC. The catalytic performance of the catalysts with different recycling times is listed in Table 1, numbered 21 to 28.

[0055] Table 1 Test results in each embodiment

[0056]

[0057]

[0058] Table 2 Structural characteristics of the catalyst of the present invention

[0059]

[0060] The X-ray diffraction (XRD) characterization results of the catalysts showed that the pyrolysis temperature significantly affected the crystal structure and specific surface area of ​​the catalysts. No obvious cobalt diffraction peak was observed in the catalyst (Co@CN-500) prepared at a lower temperature (500°C), indicating that the precursor was not completely pyrolyzed. As the pyrolysis temperature increased to 600°C and 700°C, a diffraction peak appeared and gradually strengthened at 44.1° in the XRD spectrum, corresponding to the (111) crystal plane of metallic cobalt (Co), confirming that a higher pyrolysis temperature is conducive to the full reduction and crystallization of the cobalt precursor. Raman spectroscopy analysis further revealed that all catalysts had characteristic peaks characterizing the degree of graphitization (G band) and structural defects (D band), as well as characteristic peaks attributed to Co-Ox species. Specific surface area tests showed that the Co@CN-600 and Co@CN-700 catalysts had significantly increased specific surface areas (358.78 m 2 / g and 243.63m 2 / g), much higher than Co@CN-500 (92.8m 2 / g). This high specific surface area and porous structure provide excellent mass transfer channels for reactants and abundant active sites, significantly enhancing catalytic performance. Gas chromatography-mass spectrometry (GC-MS) analysis of the catalytic reaction products confirmed that the main product peak was consistent with the target product, 5-methyl-2-pyrrolidone.

[0061] The preferred embodiments of the present invention disclosed above are intended only to help illustrate the present invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the present invention to the specific embodiments described. Obviously, many modifications and variations are possible based on the content of this specification. These embodiments are selected and described in detail in this specification to better explain the principles and practical applications of the present invention, thereby enabling those skilled in the art to better understand and utilize the present invention. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for preparing 5-methyl-2-pyrrolidone using cobalt-doped nitrogen-carbon catalysis, characterized in that: The method comprises the steps of adding a mixed gas of methyl acetyl propyl ester, a catalyst, ethanol, ammonia and hydrogen into a closed high-pressure reactor and mixing the mixed gas evenly, and obtaining the target product 5-methyl-2-pyrrolidone after the reaction is completed.

2. The method for preparing 5-methyl-2-pyrrolidone using cobalt-doped nitrogen-carbon catalysis according to claim 1, wherein: The specific steps include: S1: adding methyl levulinate and cobalt-doped nitrogen-carbon catalyst into a high-pressure reactor; S2: Introduce a mixture of ammonia and hydrogen, control the hydrogen pressure at 2-4 MPa, and control the ammonia pressure at 0.2-0.8 MPa; S3: reacting at 60-100° C. for 1-12 hours, and separating to obtain 5-methyl-2-pyrrolidone after the reaction is completed; The mass ratio of the catalyst to methyl levulinate is 10:1, and the substrate concentration is 20-100%.

3. The method for preparing 5-methyl-2-pyrrolidone using cobalt-doped nitrogen-carbon catalysis according to claim 2, wherein: The hydrogen pressure is 3 MPa, and the reaction time is 6 hours.

4. The method for preparing 5-methyl-2-pyrrolidone using cobalt-doped nitrogen-carbon catalysis according to claim 2, wherein: The preparation method of the cobalt-doped nitrogen-carbon catalyst in step S1 includes the following sub-steps: S1.1: Dissolve Co(NO₃)₂·6H₂O and 2-methylimidazole in 150 mL of methanol, then slowly add the metal salt solution to the 2-methylimidazole solution. Stir at room temperature for 6 h. Recover the resulting precipitate by centrifugation at 9000 ppm for 5 min, wash three times with methanol, and dry in a vacuum oven at 50°C for 24 h to obtain ZIF-67. S1.2: After drying, ZIF-67 was ground and then thermally activated in a tube furnace at a temperature range of 500-600 °C for 3 h to obtain Co@NC-X, where "X" represents the pyrolysis temperature. The pyrolysis was carried out under N2 flow and the heating rate was 5 °C / min. -1 .

5. The method for preparing 5-methyl-2-pyrrolidone using cobalt-doped nitrogen-carbon catalysis according to claim 4, wherein: The pyrolysis temperature in step S1.2 is 600°C.

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