Catalyst for maleic acid water phase hydrogenation as well as preparation method and application thereof

Abstract

The invention provides a catalyst for maleic acid water phase hydrogenation as well as a preparation method and application thereof. The catalyst comprises a carrier, an active metal component and a nitrogen-doped carbon layer, wherein the nitrogen-doped carbon layer is formed by shrinking and carbonizing melamine and formaldehyde in pores of the carrier loaded with the active metal component, and the active metal component is coated with the nitrogen-doped carbon layer; the active metal component is selected from at least one of palladium, ruthenium, gold, rhodium and iridium, and the carrier is a spherical or granular porous material. The catalyst disclosed by the invention comprises the nitrogen-doped carbon layer formed by shrinking melamine and formaldehyde in pores of the carrier loaded with the active metal components and carbonizing, so that the active metal components of specific types can be promoted to be uniformly dispersed in the catalyst structure, and the catalytic performance is improved; meanwhile, by combining the effect of the carrier, the mechanical strength of the catalyst is effectively improved, so that the catalyst can still maintain high activity after being used in an acid environment for a long time, and the maleic acid water-phase hydrogenation reaction can be efficiently catalyzed in a long-life manner.

Description

Technical Field

[0001] This invention relates to the field of chemical synthesis technology, specifically to a catalyst for the aqueous hydrogenation of maleic acid, its preparation method, and its application. Background Technology

[0002] Succinic acid (also known as succinic acid) is an important "C4 platform compound" with wide applications in food, medicine, agriculture, surfactants, and solvents. It is also a key raw material for the synthesis of 1,4-butanediol, tetrahydrofuran, N-methylpyrrolidone, and biodegradable biopolymers. Of particular note is its significant market potential as a primary raw material for the biodegradable plastic polybutylene succinate.

[0003] Industrial methods for preparing succinic acid mainly include electrochemical synthesis, bio-fermentation, and catalytic hydrogenation. Electrochemical synthesis suffers from problems such as unfavorable process conditions, numerous byproducts, fragile ion-exchange membranes, and high energy consumption. While bio-fermentation offers mild and environmentally friendly conditions, product separation is difficult and costs are high. In contrast, catalytic hydrogenation offers comprehensive advantages, including a wide range of raw material sources, simple process, high conversion rate, and easy product separation, making it the lowest-cost and most efficient industrial production method currently available.

[0004] Catalytic hydrogenation processes can be divided into non-aqueous phase and aqueous phase methods. Non-aqueous phase methods typically use maleic anhydride as a raw material, hydrogenating it under organic solvent or solvent-free conditions followed by hydrolysis to obtain succinic acid. This method is prone to localized hot spots due to intense exothermic reactions, accelerating catalyst deactivation and triggering side reactions. While using organic solvents can alleviate this problem, it leads to high solvent consumption, requires subsequent solvent removal, and results in a complex process with a heavy environmental burden. Aqueous phase methods use water as a solvent, hydrolyzing maleic anhydride to maleic acid, followed by further hydrogenation to prepare succinic acid. This process is environmentally friendly, the product is easy to separate, and it has greater potential for industrial applications. However, the reaction system is highly acidic, easily causing the loss of active catalyst components and affecting its stability. Existing technologies improve catalyst acid resistance through carbon layer encapsulation and the use of inert supports, but these methods involve complex preparation processes, stringent conditions, and high costs, limiting large-scale application.

[0005] In recent years, some patents (such as CN114984994A and CN114505073A) have improved the acid resistance and activity of aqueous hydrogenation catalysts by introducing nitrogen-doped carbon, utilizing the interaction between nitrogen atoms and active components to promote dispersion and reduce agglomeration. However, these methods mostly achieve nitrogen doping by adding nitrogen-containing small-molecule organic compounds through physical adsorption. During long-term use, the nitrogen-doped carbon is prone to loss or structural collapse due to insufficient loading, making it difficult to meet the requirements for long-term stable operation. Therefore, developing efficient, corrosion-resistant, and long-life aqueous hydrogenation catalysts is a crucial breakthrough needed to achieve low-cost, low-pollution continuous production of succinic acid. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention discloses a catalyst for the aqueous hydrogenation of maleic acid, its preparation method, and its application. This catalyst has excellent corrosion resistance, long service life, and can efficiently catalyze the aqueous hydrogenation reaction of maleic acid.

[0007] To achieve the above technical objectives, this invention proposes a catalyst for the aqueous hydrogenation of maleic acid. The catalyst comprises a support, an active metal component, and a nitrogen-doped carbon layer. The nitrogen-doped carbon layer is formed by the carbonization of melamine and formaldehyde within the pores of a support loaded with the active metal component. The active metal component is coated by the nitrogen-doped carbon layer. The active metal component is selected from at least one of palladium, ruthenium, gold, rhodium, and iridium. The support is a spherical or granular porous material with a particle size of 3–8 mm and a BET specific surface area of ​​100–600 m². 2 / g, pore volume is 0.55~1.30mL / g, pore size is 10~20nm.

[0008] Melamine is a nitrogen-rich ligand that readily chelates with soluble metal ions. After treatment under an inert atmosphere, it easily yields highly dispersed, nitrogen-doped, carbon-coated metal-supported catalysts, thus exhibiting acid and alkali resistance and high-temperature stability. However, through extensive experiments, the research team of this invention discovered that conventional doping methods only distribute melamine in the support through physical adsorption. Such catalysts still experience loss and collapse during use due to weak bonding, thus affecting their lifespan.

[0009] In the above technical solution, the nitrogen-doped carbon layer with a high degree of crosslinking, obtained by in-situ condensation and carbonization of melamine and formaldehyde within the pores of a support loaded with active metal components, retains its crosslinking structure well even after inert atmosphere treatment. This prevents the loss and collapse of active components during use, thus extending the catalyst's lifespan. Furthermore, the introduction of melamine-formaldehyde resin into the catalyst preparation not only leverages the chelating properties of melamine with soluble metal ions to achieve high dispersion of active components and enhance catalyst activity, but also ensures stable distribution of active metal components within the support by constructing a highly crosslinked melamine-formaldehyde resin. The resulting nitrogen-doped carbon structure remains stably distributed within the support structure, allowing the catalyst to maintain high activity even after prolonged use in acidic environments.

[0010] Furthermore, the use of a support within a specific range of structural parameters in the above technical solution not only effectively improves the mechanical strength of the catalyst, preventing it from breaking and collapsing due to insufficient mechanical strength during use, thus affecting the catalyst's lifespan; but also, the porous structure of the support increases the specific surface area of ​​the catalyst, allowing the nitrogen-doped carbon structure and metal active components to be evenly distributed inside and outside the catalyst, effectively improving the catalyst's activity.

[0011] In a further example of the present invention, the molar ratio of melamine to the active metal component is (0.5~2):1. By precisely controlling the microstructure of the catalyst active center, sufficient metal-nitrogen coordination is formed between the active metal ions and melamine, thereby obtaining a composite structure in which the metal particles are highly dispersed and uniformly coated with a nitrogen-doped carbon layer.

[0012] In a further example of the present invention, the mass of the active metal component is 0.2wt% to 2wt% of the mass of the support, which can improve the high reactivity of the catalyst while maintaining excellent mass transfer efficiency and structural stability, thereby achieving efficient and long-lasting catalytic performance in the harsh acidic environment of aqueous hydrogenation to prepare succinic acid.

[0013] In a further example of the invention, the support is a spherical or granular porous material, which can increase the specific surface area of ​​the catalyst. In an optional embodiment of the invention, the support is selected from silica, titanium dioxide, cerium oxide, or molecular sieves.

[0014] On the other hand, the present invention proposes a method for preparing a catalyst for the aqueous hydrogenation of maleic acid, the method comprising the following steps: S1: The carrier is impregnated in a mixed salt solution containing melamine and active metal salt, and then dried to obtain the first precursor; S2: The first precursor undergoes in-situ condensation with formaldehyde under acidic conditions to obtain the second precursor; S3: The second precursor is carbonized and reduced to obtain the catalyst for aqueous hydrogenation of maleic acid.

[0015] The present invention provides a method for preparing a catalyst for the aqueous hydrogenation of maleic acid by in-situ condensation and carbonization of melamine and formaldehyde to obtain a nitrogen-doped carbon layer for uniformly distributing active metal components and cross-linking with the support. The resulting catalyst has acid and alkali resistance and high temperature resistance. Combined with specific metal active components, it can stably and efficiently catalyze the reaction in the aqueous hydrogenation of maleic acid to prepare succinic acid.

[0016] Furthermore, the concentration of melamine in the mixed salt solution is 5~25 mmol / L.

[0017] It should be noted that the present invention does not limit the specific type of soluble salt of the active metal salt, and can be selected as nitrate, sulfate or chloride salt of the corresponding metal. Those skilled in the art can select according to their needs in the actual preparation process.

[0018] Further, the support is a spherical or granular porous material, preferably one of silica, titanium dioxide, cerium oxide, or molecular sieve. Further, the particle size of the support is 3–8 mm, and the BET specific surface area is 100–600 m². 2 / g, pore volume is 0.55~1.30 mL / g, pore size is 10~20 nm.

[0019] Further, step S2 includes contacting the first precursor with a mixed aqueous solution containing formaldehyde and phosphoric acid. Further still, the mass ratio of the first precursor to the mixed aqueous solution is 1:(1~2). Further still, in the mixed aqueous solution, the molar ratio of formaldehyde to melamine is (2~4):1, and the mass fraction of phosphoric acid is 0.5wt%~1.5wt%. Further still, the in-situ condensation reaction is carried out at a temperature of 60~95℃ for 4~10h.

[0020] Optionally, step S2 further includes solid-liquid separation of the material after in-situ condensation reaction and drying of the solid phase to obtain the second precursor. The specific operations of solid-liquid separation and drying are not limited in this invention; those skilled in the art can choose appropriate operation methods according to actual conditions.

[0021] The carbonization process is used to partially pyrolyze and remove non-carbon elements (such as H, O, etc.) from the resin, resulting in a nitrogen-doped carbon layer with carbon as the main framework and nitrogen atoms doped within it. Furthermore, the carbonization process is carried out under an inert atmosphere; the operating temperature is 400~600℃, and the operating time is 3~8h.

[0022] Furthermore, the reduction operation is carried out in a hydrogen-containing atmosphere, preferably in a hydrogen atmosphere or a mixed atmosphere containing hydrogen and an inert gas; when the reduction operation is carried out in a mixed atmosphere, hydrogen accounts for 5% to 20% of the volume of the mixed atmosphere. Furthermore, the temperature of the reduction operation is 150°C to 200°C, and the reduction time is 2 to 4 hours.

[0023] On the other hand, the present invention proposes the application of the above-mentioned catalyst for aqueous hydrogenation of maleic acid in the preparation of succinic acid.

[0024] Optionally, the application includes a reaction process for the hydrogenation of maleic acid to obtain succinic acid, specifically including the reaction of an aqueous solution of maleic acid with the catalyst in the presence of hydrogen to obtain succinic acid.

[0025] Further optionally, the concentration of the maleic acid aqueous solution is 5-30 wt%.

[0026] Further optionally, the temperature of the contact reaction is 80~130℃ and the reaction pressure is 0.5~2.5MPa.

[0027] Further optionally, the hydrogen gas velocity is 0.5–2.5 L / min, and the maleic acid aqueous solution feed mass hourly space velocity is 1–30 h⁻¹. -1 .

[0028] Alternatively, the contact reaction process may be carried out in a fixed-bed reactor.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows: The catalyst of the present invention includes a support, an active metal component, and a nitrogen-doped carbon layer formed by the condensation and carbonization of melamine and formaldehyde in the pores of the support loaded with the active metal component. This can promote the uniform dispersion of specific types of active metal components in the catalyst structure and improve catalytic performance. At the same time, the combined effect of the support effectively improves the mechanical strength of the catalyst, preventing the catalyst from breaking and collapsing due to insufficient mechanical strength during use. This allows the catalyst to maintain high activity even after long-term use in acidic environments, and to efficiently and with a long lifespan catalyze the aqueous hydrogenation reaction of maleic acid. Attached Figure Description

[0030] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 A product diagram of the catalyst prepared in Example 2 of the present invention is shown.

[0031] Figure 2 The image shows a SEM mapping scan of the catalyst prepared in Example 2 of the present invention. Detailed Implementation

[0032] To facilitate understanding of the present invention, a more comprehensive description will be provided below, along with preferred embodiments. However, it should be understood that these embodiments are merely for more detailed explanation and should not be construed as limiting the invention in any way, i.e., not intended to limit the scope of protection of the invention.

[0033] Unless otherwise defined, the technical terms used in the following embodiments have the same meanings as commonly understood by those skilled in the art to which this invention pertains. Unless otherwise specified, the experimental reagents used in the following embodiments are conventional biochemical reagents; and the experimental methods described are conventional methods.

[0034] Furthermore, it should be noted that although the various steps of the preparation method of the present invention are described in a specific order in the description of the present invention, these orders are not restrictive. Without departing from the basic principles of the present invention, those skilled in the art can perform the steps in different orders.

[0035] Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "multiple" or "at least one" means two or more.

[0036] All numerical designations, such as temperature, pressure, flow rate, and range, are approximate values. It should be understood that, while not always explicitly stated, all numerical designations are preceded by the term "approximately." It should also be understood that, while not always explicitly stated, the reagents described herein are merely examples, and their equivalents are known in the art.

[0037] When a quantity, concentration, or other value or parameter is expressed as a range, a preferred range, or a range defined by a series of upper and lower preferred values, this should be understood as specifically disclosing all ranges formed by any pair of any upper or preferred value with any lower or preferred value, regardless of whether the range is disclosed individually. For example, when the range “1–5” is disclosed, the described range should be interpreted as including ranges “1–4”, “1–3”, “1–2”, “1–2 and 4–5”, “1–3 and 5”, etc. When numerical ranges are described herein, unless otherwise stated, the range is intended to include its endpoints and all integers and fractions within that range.

[0038] The content of each component in the reaction product was analyzed by gas chromatography, and the conversion rate and selectivity were calculated according to the following formulas: Maleic acid conversion rate = (moles of maleic acid consumed in the reaction / moles of maleic acid fed) × 100%; Succinic acid selectivity = (moles of succinic acid produced in the reaction / moles of maleic acid consumed in the reaction) × 100%.

[0039] Example 1

[0040] A method for preparing a catalyst for the aqueous-phase hydrogenation of maleic acid, specifically: S1: Dissolve 5 mmol of melamine and 10 mmol of palladium nitrate in 1 L of 50°C hot water. After thorough dissolution, mix the aqueous solution with 532.1 g of a support (spherical silica support, average particle size 5 mm, specific surface area 270 m²).2 The first precursor was obtained by uniformly mixing (palladium ions at 0.2 wt% of the support mass) with a pore volume of 0.95 mL / g and an average pore size of 12.5 nm, followed by rotary evaporation and drying. S2: The first precursor was added to an equal mass of a mixed aqueous solution containing 20 mmol of formaldehyde and 0.5 wt% phosphoric acid, and reacted at 95°C for 4 h. After the reaction was completed, the mixture was filtered and dried to obtain the second precursor. S3: The second precursor was treated in a nitrogen atmosphere at 600°C for 3 hours; then reduced at 200°C with 5% hydrogen (volume fraction) and 95% nitrogen (volume fraction) for 2 hours to obtain catalyst 1.

[0041] Furthermore, the catalytic performance of the catalyst was tested in this embodiment: 30g of catalyst was placed in a fixed-bed reactor with an inner diameter of 2.5 cm and a height of 80 cm. Inert ceramic balls were packed at both ends of the catalyst. After a leak test with nitrogen, the reaction system was heated and reactants were introduced. The concentration of maleic acid aqueous solution was 5wt%, the reaction temperature was 80℃, the reaction pressure was 0.5MPa, the hydrogen gas velocity was 0.5L / min, and the feed mass hourly space velocity was 1h. -1 After the reaction stabilized, the resulting mixture was cooled, depressurized, and subjected to gas-liquid separation. Gas chromatography was then used for analysis. The catalyst evaluation results are shown in Table 1.

[0042] Example 2

[0043] A method for preparing a catalyst for the aqueous-phase hydrogenation of maleic acid, specifically: S1: Dissolve 25 mmol of melamine and 25 mmol of ruthenium chloride in 1 L of 90℃ hot water. After thorough dissolution, mix the aqueous solution with 126.3 g of carrier (particulate silica carrier, average particle size 3 mm, specific surface area 365 m²). 2 The first precursor was obtained by mixing ruthenium ions (2 wt% of the support mass) with a pore volume of 1.22 mL / g and an average pore size of 13.5 nm, followed by rotary evaporation and drying. S2: The first precursor was added to an aqueous solution containing 3 mmol of formaldehyde and 1 wt% phosphoric acid at twice its mass and reacted at 60°C for 10 h. After the reaction was completed, the mixture was filtered and dried to obtain the second precursor. S3: The second precursor was treated in a nitrogen atmosphere at 400°C for 8 hours; then reduced at 150°C with 20% hydrogen (volume fraction) and 80% argon (volume fraction) for 4 hours to obtain catalyst 2. Figure 2The SEM mapping image of the catalyst in this embodiment is shown. It can be seen that C, N, and Ru are uniformly distributed in the visible field of view, indicating that the nitrogen-doped carbon structure has been successfully introduced into the support, confirming the successful synthesis of the catalyst.

[0044] Furthermore, the catalytic performance of the catalyst was tested in this embodiment: 30g of catalyst was placed in a fixed-bed reactor with an inner diameter of 2.5 cm and a height of 80 cm. Inert ceramic balls were packed at both ends of the catalyst. After a leak test with nitrogen, the reaction system was heated and reactants were introduced. The concentration of maleic acid aqueous solution was 30wt%, the reaction temperature was 130℃, the reaction pressure was 2.5MPa, the hydrogen gas velocity was 2.5L / min, and the feed mass hourly space velocity was 30h⁻¹. -1 After the reaction stabilized, the resulting mixture was cooled, depressurized, and subjected to gas-liquid separation. Gas chromatography was then used for analysis. The catalyst evaluation results are shown in Table 1.

[0045] Example 3

[0046] A method for preparing a catalyst for the aqueous-phase hydrogenation of maleic acid, specifically: S1: Dissolve 20 mmol of melamine and 10 mmol of iridium chloride in 1 L of 80℃ hot water. After thorough dissolution, mix the aqueous solution with 128.2 g of carrier (spherical molecular sieve carrier, average particle size 7 mm, specific surface area 150 m²). 2 The first precursor was obtained by mixing the iridium ions (1 wt% of the support mass) with a pore volume of 0.71 mL / g and an average pore size of 16.0 nm, and then rotary evaporating and drying. S2: The first precursor was added to an aqueous solution containing 40 mmol of formaldehyde and 1.5 wt% phosphoric acid at 1.5 times its mass and reacted at 70°C for 8 hours. After the reaction was completed, the solution was filtered and dried to obtain the second precursor. S3: The second precursor was treated in a nitrogen atmosphere at 450°C for 7 h; then reduced at 170°C with 15% hydrogen (volume fraction) and 85% argon (volume fraction) for 3 h to obtain catalyst 3.

[0047] Furthermore, the catalytic performance of the catalyst was tested in this embodiment: 30g of catalyst was placed in a fixed-bed reactor with an inner diameter of 2.5 cm and a height of 80 cm. Inert ceramic balls were packed at both ends of the catalyst. After a leak test with nitrogen, the reaction system was heated and reactants were introduced. The concentration of maleic acid aqueous solution was 20wt%, the reaction temperature was 120℃, the reaction pressure was 2.0MPa, the hydrogen gas velocity was 2.0L / min, and the feed mass hourly space velocity was 20h⁻¹. -1After the reaction stabilized, the resulting mixture was cooled, depressurized, and subjected to gas-liquid separation. Gas chromatography was then used for analysis. The catalyst evaluation results are shown in Table 1.

[0048] Example 4

[0049] A method for preparing a catalyst for the aqueous-phase hydrogenation of maleic acid, specifically: S1: Dissolve 20 mmol of melamine and 15 mmol of ruthenium chloride in 1 L of 70°C hot water. After thorough dissolution, mix the aqueous solution with 303.2 g of a carrier (spherical titanium dioxide carrier, average particle size 5 mm, specific surface area 240 m²). 2 The first precursor was obtained by mixing ruthenium ions (0.5 wt% of the support mass) with a pore volume of 1.13 mL / g and an average pore size of 11.5 nm, followed by rotary evaporation and drying. S2: The first precursor was added to an equal mass of an aqueous solution containing 60 mmol of formaldehyde and 1 wt% phosphoric acid, and reacted at 80°C for 6 h. After the reaction was completed, the mixture was filtered and dried to obtain the second precursor. S3: The second precursor was treated in a nitrogen atmosphere at 550°C for 4 hours; then reduced at 160°C with 10% hydrogen (volume fraction) and 90% argon (volume fraction) for 3 hours to obtain catalyst 4.

[0050] Furthermore, the catalytic performance of the catalyst was tested in this embodiment: 30g of catalyst was placed in a fixed-bed reactor with an inner diameter of 2.5 cm and a height of 80 cm. Inert ceramic balls were packed at both ends of the catalyst. After a leak test with nitrogen, the reaction system was heated and reactants were introduced. The concentration of maleic acid aqueous solution was 10wt%, the reaction temperature was 100℃, the reaction pressure was 1.5MPa, the hydrogen gas velocity was 1.0L / min, and the feed mass hourly space velocity was 10h⁻¹. -1 After the reaction stabilized, the resulting mixture was cooled, depressurized, and subjected to gas-liquid separation. Gas chromatography was then used for analysis. The catalyst evaluation results are shown in Table 1.

[0051] Comparative Example 1 Compared to Example 2, melamine was not added during catalyst preparation, while all other conditions remained the same as in Example 2. Catalyst evaluation results are shown in Table 1.

[0052] Comparative Example 2 Compared to Example 2, no formaldehyde was added during catalyst preparation, while all other conditions remained the same as in Example 2. The catalyst evaluation results are shown in Table 1.

[0053] Comparative Example 3 Compared to Example 2, melamine-formaldehyde resin was used instead of the catalyst support, and the preparation process is as follows: 136.2 g of melamine-formaldehyde resin was mixed evenly with 1 L of a solution containing 25 mmol of ruthenium chloride. After drying the catalyst precursor, it was calcined and reduced under the same conditions as in Example 2. The catalyst obtained by this method could not be directly molded. Instead, a catalyst with a diameter of 7 mm and a height of 7 mm was obtained by extrusion with the addition of a binder. The catalyst was evaluated according to the method in Example 2.

[0054] Comparative Example 4 Compared to Example 2, the molar ratio of formaldehyde to melamine added during catalyst preparation in this comparative example was 1:1, and all other conditions were the same as in Example 2. The catalyst evaluation results are shown in Table 1.

[0055] Comparative Example 5 Compared to Example 2, the molar ratio of formaldehyde to melamine added during catalyst preparation in this comparative example was 5:1, and all other conditions were the same as in Example 2. The catalyst evaluation results are shown in Table 1.

[0056] Table 1

[0057] As can be seen from Examples 1-4 in Table 1, the catalyst of the present invention can be used in the aqueous hydrogenation of maleic acid with high conversion and high selectivity. Furthermore, the results of the operation test of the catalysts of Example 2 and Comparative Examples 1 and 2 after 1000 hours confirm that the catalyst performance of the present invention does not suffer significant loss after continuous use for 1000 hours, and has excellent acid resistance, good stability, long service life, and is suitable for continuous industrial production applications.

[0058] As can be seen from Example 2 and Comparative Example 3, the catalyst obtained by directly using melamine-formaldehyde resin as a carrier and mixing, calcining, and reducing with the active component cannot be molded for performance evaluation. Although it can be obtained as a catalyst for evaluation by adding a binder and extruding, the catalyst of Comparative Example 3 has poor catalytic activity due to the lack of porous structure and low specific surface area.

[0059] As can be seen from the comparison between Example 2 and Comparative Examples 4 and 5, optimizing the feed ratio of formaldehyde and melamine is beneficial to improving the stability of the catalyst structure.

[0060] It should be noted that the above description is a further detailed explanation of the present invention in conjunction with specific embodiments, and it should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, several simple improvements can be made without departing from the concept of the present invention, and all such improvements should be considered to fall within the scope of protection of the present invention.

Claims

1. A catalyst for the aqueous-phase hydrogenation of maleic acid, characterized in that, The catalyst comprises a support, an active metal component, and a nitrogen-doped carbon layer; wherein the nitrogen-doped carbon layer is formed by the condensation and carbonization of melamine and formaldehyde within the pores of a support loaded with the active metal component, and the active metal component is coated by the nitrogen-doped carbon layer; the active metal component is selected from at least one of palladium, ruthenium, gold, rhodium, and iridium; the support is a spherical or granular porous material with a particle size of 3–8 mm and a BET specific surface area of ​​100–600 m². 2 / g, pore volume is 0.55~1.30mL / g, pore size is 10~20nm.

2. The catalyst for aqueous hydrogenation of maleic acid according to claim 1, characterized in that, The molar ratio of melamine to the active metal component is (0.5~2):1; And / or, the mass of the active metal component is 0.2wt% to 2wt% of the mass of the carrier.

3. The catalyst for aqueous hydrogenation of maleic acid according to claim 1, characterized in that, The carrier is at least one of silicon dioxide, titanium dioxide, cerium oxide, or molecular sieve.

4. A method for preparing a catalyst for the aqueous hydrogenation of maleic acid according to any one of claims 1-3, characterized in that, Includes the following steps: S1: The carrier is impregnated in a mixed salt solution containing melamine and active metal salt, and then dried to obtain the first precursor; S2: The first precursor undergoes in-situ condensation with formaldehyde under acidic conditions to obtain the second precursor; S3: The second precursor is carbonized and reduced to obtain the catalyst for aqueous hydrogenation of maleic acid.

5. The method for preparing the catalyst for aqueous hydrogenation of maleic acid according to claim 4, characterized in that, The concentration of melamine in the mixed salt solution is 5~25 mmol / L.

6. The method for preparing the catalyst for aqueous hydrogenation of maleic acid according to claim 4, characterized in that, The carrier is a spherical or granular porous material, preferably one of silicon dioxide, titanium dioxide, cerium oxide, or molecular sieve.

7. The method for preparing the catalyst for aqueous hydrogenation of maleic acid according to claim 4, characterized in that, S2 includes contacting the first precursor with a mixed aqueous solution containing formaldehyde and phosphoric acid; Preferably, the mass ratio of the first precursor to the mixed aqueous solution is 1:(1~2). Preferably, in the mixed aqueous solution, the molar ratio of formaldehyde to melamine is (2~4):1, and the mass fraction of phosphoric acid is 0.5wt%~1.5wt%. Preferably, the in-situ condensation reaction is carried out at a temperature of 60-95°C for 4-10 hours.

8. The method for preparing the catalyst for aqueous hydrogenation of maleic acid according to claim 4, characterized in that, The carbonization operation is carried out under an inert atmosphere; the operating temperature is 400~600℃, and the operating time is 3~8h.

9. The method for preparing the catalyst for aqueous hydrogenation of maleic acid according to claim 4, characterized in that, The reduction operation is carried out in a hydrogen-containing atmosphere, preferably in a hydrogen atmosphere or a mixed atmosphere containing hydrogen and an inert gas; Preferably, when the reduction operation is carried out in a mixed atmosphere, hydrogen accounts for 5% to 20% of the volume of the mixed atmosphere; And / or, the temperature of the reduction operation is 150℃~200℃, and the reduction time is 2~4h.

10. The application of the catalyst for aqueous hydrogenation of maleic acid according to any one of claims 1-3 in the preparation of succinic acid.

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