Hollow carbon sphere supported palladium-based catalyst, its preparation method and application

Abstract

The application discloses a hollow carbon sphere loaded palladium-based catalyst and a preparation method and application thereof. The application is characterized in that, by means of the Stober method, ammonia water, tetrapropyl orthosilicate, resorcinol and formaldehyde solution are fully reacted in a mixed solvent of deionized water and ethanol, and then, after centrifugation, washing, drying, calcination and etching, hollow carbon spheres are obtained; then, the vacuum impregnation method is adopted to uniformly load Pd metal; and then, after reduction by anhydrous methanol, the Pd active component is loaded in the cavity of the carbon sphere; the prepared hollow carbon sphere loaded palladium-based catalyst has uniform Pd metal particle size and small average size, can utilize the confinement effect of the cavity to enrich reaction substrates and hydrogen, has wide substrate universality, has good catalytic activity and selectivity for deoxygenation and hydrogenation reactions of various raw materials, and can realize efficient catalytic deoxygenation and hydrogenation reactions of vanillin.

Description

Technical Field

[0001] This invention belongs to the field of catalyst preparation technology, specifically relating to a hollow carbon sphere supported palladium-based catalyst, its preparation method, and its application. Background Technology

[0002] Lignin is a renewable biomass resource that can be converted into high-value-added chemicals. However, lignin pyrolysis products have a high oxygen content, which makes them unsuitable as biofuels due to their low energy density, unstable combustion, and excessive corrosion.

[0003] Hydrodeoxygenation (HDO) is an effective method for reducing the oxygen content in lignin. Vanillin can be converted to 2-methoxy-4-methylphenol (MMP) through hydrodeoxygenation, and MMP has wide applications in biofuels and pharmaceutical intermediates. Hydrogen is commonly used as a hydrogen source for catalytic hydrogenation reactions. However, the industrial use of hydrogen requires extremely complex and stringent measures. Formic acid is a safe and convenient liquid fuel, and it has attracted significant attention in the field of renewable energy storage due to its high hydrogen content, non-toxicity, easy availability, and high stability at room temperature.

[0004] The selective hydrogenation and deoxygenation of vanillin to generate MMP faces three main challenges: First, the reaction pathway is complex, as the aldehyde group, benzene ring, and methoxy group of vanillin may all participate in hydrogenation, easily generating byproducts such as guaiacol, leading to a decrease in the selectivity of the target product MMP; second, the reaction conditions are harsh, as traditional catalysts are prone to over-hydrogenation or CO bond breakage under high temperature and pressure, resulting in catalyst deactivation or difficulty in product separation; third, catalyst deactivation may occur, as carbon deposits or strongly adsorbed intermediates may be generated during the reaction, blocking active sites, and acidic media or high temperatures can easily lead to sintering or leaching of metal particles.

[0005] Palladium-based catalysts exhibit significant advantages in this reaction: high hydrogenation activity and moderate CO bond activation ability. Pd shows high hydrogenation activity for aldehydes and benzene rings, while also possessing moderate cleavage ability for CO bonds of methoxy groups, which is beneficial for selective deoxygenation under mild conditions. They also demonstrate strong resistance to poisoning; compared to other noble metals, Pd exhibits moderate adsorption strength for oxygen-containing intermediates, reducing deactivation caused by carbon deposition. Structural tunability: Their electronic properties and selectivity can be further optimized through particle size control, alloying, or support interactions.

[0006] In summary, the key to addressing the challenges of selective hydrogenation and deoxygenation of vanillin to prepare MMP lies in designing a highly efficient catalytic system that can precisely control the reaction pathway and suppress side reactions. Summary of the Invention

[0007] To address the problems in the prior art, the present invention aims to provide a hollow carbon sphere supported palladium-based catalyst, its preparation method, and its application.

[0008] The following inventive concept was adopted in the preparation process of hollow carbon spheres in this invention: 1) Tetrapropyl orthosilicate is first hydrolyzed in an alkaline solution to generate a SiO2 template; 2) In the process of preparing hollow carbon spheres by the Stöber method, resorcinol and formaldehyde mainly serve as precursor monomers for forming the carbon shell. They react on the surface of the SiO2 template through a sol-gel process to generate a phenolic resin (RF) layer, which is finally carbonized into a carbon skeleton under high-temperature heat treatment; 3) Finally, etching is performed under the action of NaOH solution to remove the SiO2 template, thereby leaving hollow cavities and mesoporous pore walls, which is beneficial for subsequently loading metal Pd into the cavity and using the confined enrichment effect of the cavity to improve catalytic performance.

[0009] Hollow carbon spheres, as a support, possess confinement and dispersibility effects. The hollow cavity of the support can restrict the growth of Pd metal particles inside, improve the dispersion and stability of Pd nanoparticles, and prevent agglomeration during the reaction. The porous shell and hollow structure of the support can promote the diffusion of reactants / products, shorten the residence time, and reduce over-reaction. The carbon material of hollow carbon spheres has high tolerance to acidic or neutral reaction environments, and the hollow structure helps to buffer volume changes during the reaction process, extending the catalyst lifetime.

[0010] The use of a palladium-based catalyst effectively combines high hydrogenation activity with moderate CO bond activation capability, providing a reaction basis for the highly selective generation of MMPs. Meanwhile, the hollow carbon sphere support, through its unique confinement effect, optimized mass transfer channels, and stable chemical environment, significantly improves the dispersion, stability, and resistance to deactivation of the palladium active centers. The synergistic effect of these two components constitutes a catalytic system exhibiting excellent activity, selectivity, and durability, providing a promising technical solution for the high-value conversion of vanillin.

[0011] The technical solution adopted in this invention is as follows: A method for preparing a palladium-based catalyst supported on hollow carbon spheres includes the following steps: S1: Add tetrapropyl orthosilicate dropwise into anhydrous ethanol-deionized water mixed solvent containing ammonia, and stir at room temperature for 10-30 min; S2: Then add resorcinol and formaldehyde solution, stir the reaction thoroughly, centrifuge to obtain the reaction product, wash and dry it, heat treat it at high temperature under an inert atmosphere, and then etch it with NaOH solution to obtain hollow carbon spheres. S3: Disperse the hollow carbon spheres obtained in step S2 in anhydrous ethanol, add Pd source for ultrasonic impregnation, and then evaporate under vacuum until the anhydrous ethanol is dried. After drying, the product is immersed in anhydrous methanol for stirring and reduction to obtain the hollow carbon sphere supported palladium-based catalyst. The mass of Pd atoms in the Pd source is 5-10% of the mass of the hollow carbon spheres.

[0012] Further, in step S1, the volume ratio of anhydrous ethanol to deionized water is 3-12:1, the final molar concentration of ammonia in the mixture in step S1 is 0.1-0.2M, and the final volume concentration of tetrapropyl orthosilicate in the mixture in step S1 is 2-6%, preferably 4-5%.

[0013] Further, in step S2, the mass ratio of resorcinol to formaldehyde is 1g:0.5-2mmol, preferably 1g:1.0-1.2mmol; the mass ratio of resorcinol in step S2 to the volume ratio of tetrapropyl orthosilicate in step S1 is 1g:5-12mL, preferably 1g:8-10mL. After adding resorcinol and formaldehyde solution in step S2, react at 25-40℃ for 20-30 hours.

[0014] Further, in step S1, the concentration of the NaOH solution is 1-3M, preferably 1.5-2M; the etching temperature is 70-90℃, preferably 75-80℃, and the etching time is 20-30h.

[0015] Further, the mass of Pd atoms in the Pd source is 6-7% of the mass of the hollow carbon spheres; in step S3, the concentration of Pd atoms in the Pd source dispersed in anhydrous ethanol is 0.1-0.5 mg / mL, preferably 0.3-0.4 mg / mL, and the ultrasonic impregnation time is 20-60 min; in step S3, the reduction temperature is room temperature, and the reduction time is 30-60 min.

[0016] Furthermore, in step S3, the rotary evaporation temperature is 55-65℃, and the vacuum degree of rotary evaporation is 20-40 kPa, preferably 25-30 kPa.

[0017] Furthermore, in step S3, the reduction temperature is 15-25℃, preferably 20-22℃, and the reduction time is 40-45 min.

[0018] The present invention also discloses the application of the hollow carbon sphere supported palladium-based catalyst in catalytic hydrogenation reaction, wherein the catalyst is applied to the thermal catalytic hydrogenation deoxygenation reaction of vanillin to prepare 2-methoxy-4-methylphenol.

[0019] Unlike existing technologies, this invention has the following advantages: 1. This invention provides a novel method for preparing palladium-based catalysts supported on hollow carbon spheres. Using the Stöber method, ammonia, tetrapropyl orthosilicate, resorcinol, and formaldehyde solution are reacted thoroughly in a mixed solvent of deionized water and anhydrous ethanol. After centrifugation, washing, drying, calcination, and etching, hollow carbon spheres are obtained. Then, a vacuum impregnation method is used to achieve uniform dispersion of the active components, and the active components are anchored within the cavities of the hollow carbon spheres under the weak reducing effect of anhydrous methanol, thus obtaining the palladium-based catalyst supported on hollow carbon spheres. The prepared palladium-based catalyst exhibits good structural stability under high temperature and high pressure conditions. The preparation method is highly economical and operable, the raw materials are widely available and common, the process has good repeatability, and it is suitable for mass production.

[0020] 2. In existing technologies for impregnating loaded Pd, traditional atmospheric pressure impregnation often results in the air inside the porous hollow support being blocked by the liquid, forming an airlock. This makes it difficult for the Pd precursor solution to enter the internal cavity of the support, and most of the metallic Pd is deposited on the support surface. In contrast, the hollow carbon spheres of this invention are dispersed in anhydrous ethanol. After ultrasonic impregnation with the addition of the Pd source, impregnation is carried out under vacuum followed by rotary evaporation. This facilitates the loading of metallic Pd within the cavity of the hollow carbon spheres, utilizing the confined enrichment effect of the cavity to improve catalytic performance.

[0021] 3. Compared with the prior art, the palladium-based catalyst of the present invention achieves uniform dispersion of active components through vacuum impregnation and anchors Pd active components in the cavity of hollow carbon spheres under the weak reduction of anhydrous methanol. The palladium metal achieves high loading and small particle size under the action of anhydrous methanol, exposing more active sites and effectively catalyzing the selective hydrogenation and deoxygenation of vanillin to MMP. Therefore, the control parameters of reduction temperature and time under the action of anhydrous methanol are also very important.

[0022] 4. This invention also achieves the loading of active components within the cavity of hollow carbon spheres. The confinement effect of the cavity not only enriches the reaction substrate and hydrogen but also physically isolates harmful substances such as coke. The combination of the above methods effectively solves the problem that existing technologies require high temperature and high pressure to catalyze the hydrogenation and deoxygenation of lignin, and achieves highly selective catalytic conversion of vanillin to MMP under mild conditions.

[0023] 5. The hollow carbon sphere supported palladium-based catalyst provided by this invention has uniform metal particle size and small average size, which can utilize the confinement effect of the cavity to enrich the reaction substrate and hydrogen. It has broad substrate versatility and has good catalytic activity and selectivity for deoxygenation hydrogenation reactions of various lignin as raw materials. It can realize the highly efficient catalytic selective deoxygenation hydrogenation reaction of vanillin. Attached Figure Description

[0024] Figure 1This is a transmission electron microscope (TEM) image of the catalyst used for the selective hydrogenation and deoxygenation of vanillin to MMP as described in Example 1 of the present invention. Detailed Implementation

[0025] The present invention will be further described below with reference to the accompanying drawings and preferred embodiments. The embodiments given are only for illustrating the present invention and are not intended to limit the scope of the present invention.

[0026] Unless otherwise specified, all materials and reagents used in the following examples are commercially available; and all methods used in the following examples are conventional methods.

[0027] Example 1: This example provides a method for preparing a palladium-based catalyst supported on hollow carbon spheres, comprising the following steps: S1. Preparation of hollow carbon sphere carrier: 15 mL of ammonia water with a concentration of approximately 3.65 mol / L was added to a mixed solvent prepared from 350 mL of anhydrous ethanol and 30 mL of deionized water, and stirred for 10 min to mix thoroughly. 17.3 mL of tetrapropyl orthosilicate was added and stirred continuously at room temperature for 15 min to allow for complete reaction. Then, 2.0 g of resorcinol and 2.8 mL of formaldehyde solution with a concentration of approximately 0.8 mol / L were added and stirred continuously. The mixture was allowed to react completely at 30 °C for 24 h to obtain the reaction product. After centrifugation, the reaction product was centrifuged twice with deionized water and washed twice with anhydrous ethanol. It was then vacuum dried at 60 °C for 10 h and heat-treated at 700 °C for 5 h under an argon atmosphere. The resulting product was then added to a 2 mol / L sodium hydroxide solution and shaken to obtain a uniform reaction solution. The solution was allowed to stand and age at 80 °C for 24 h to allow for complete reaction. The resulting product was then washed until neutral and dried to obtain the hollow carbon sphere carrier. Step S2, Palladium-based catalyst supported: 150 mg of hollow carbon spheres obtained in step S1 were dispersed in 30 ml of anhydrous ethanol. After sonication for 5 min, 22.1 mg of palladium acetate was added to the resulting dispersion. After sonication at room temperature for 30 min, the mixture was transferred to a round-bottom flask and the anhydrous ethanol was evaporated at 60 °C and a vacuum of 30 kPa. After drying, the product was immersed in 30 ml of anhydrous methanol and reduced at 20 °C for 40 min. After centrifugation and drying, the hollow carbon sphere supported palladium-based catalyst was obtained.

[0028] TEM image of the palladium-based catalyst supported on hollow carbon spheres prepared in Example 1 is shown below. Figure 1 As shown. From Figure 1As can be clearly observed, the prepared material exhibits a unique hollow porous carbon sphere structure. The combination of its internal cavity and abundant channels on the shell not only significantly reduces the apparent density of the material but also provides efficient channels for mass transport. The metal nanoparticles uniformly loaded on the surface and within the channels of the carbon spheres have an extremely concentrated particle size distribution, with an average particle size of only about 1.37 nm, exhibiting highly uniform size characteristics. These sub-nanometer-scale metal particles possess extremely high surface energy, exposing a large number of low-coordination atomic sites. Furthermore, due to quantum size effects, they may exhibit unique electronic structures. The high specific surface area of ​​the hollow porous carbon spheres is conducive to the full exposure of active sites, while the mesoporous / microporous hierarchical channel structure promotes the rapid diffusion of reactant molecules and product release. In addition, the abundant catalytic active centers provided by the ultra-small metal particles are beneficial to improving the catalytic reaction rate and selectivity.

[0029] Example 2: The preparation method of the catalyst in Example 2 is the same as that in Example 1, except that in step S1, the volumes of anhydrous ethanol and deionized water in the mixed solvent are replaced with 300 mL and 50 mL respectively. All other conditions remain unchanged, and a hollow carbon sphere supported palladium-based catalyst is finally obtained.

[0030] The hollow carbon sphere supported palladium-based catalyst prepared in Example 2 has a similar structure to that in Example 1.

[0031] Example 3: The preparation method of the catalyst in Example 3 is the same as that in Example 1, except that in step S1, the volumes of anhydrous ethanol and deionized water in the mixed solvent are replaced with 300 mL and 100 mL, respectively. All other conditions remain unchanged, and a hollow carbon sphere supported palladium-based catalyst is finally obtained. The hollow carbon sphere supported palladium-based catalyst prepared in Example 3 has a similar structure to that in Example 1.

[0032] Comparative Example 1: The catalyst for Comparative Example 1 was the hollow carbon spheres prepared in step S1 of Example 1.

[0033] Comparative Example 2 provides a method for preparing a supported palladium-based catalyst, comprising the following steps: 150 mg of hollow carbon spheres prepared in Example 1 are dispersed in 30 ml of anhydrous ethanol, and after ultrasonic treatment for 5 min, 22.1 mg of palladium acetate is added to the resulting dispersion, and after ultrasonic treatment at room temperature for 30 min, the mixed solution is transferred to a round-bottom flask, stirred at 35 °C for 1 h, and centrifuged to obtain the reaction product. After drying, the product is reduced with 30 ml of anhydrous methanol at 20 °C for 40 min, centrifuged, and dried to obtain the hollow carbon sphere supported palladium-based catalyst.

[0034] Comparative Example 3 provides a method for preparing a palladium-based catalyst supported on hollow carbon spheres, comprising the following steps: 150 mg of the hollow carbon spheres prepared in Example 1 are dispersed in 30 ml of anhydrous ethanol. After ultrasonic treatment for 5 min, 22.1 mg of palladium acetate is added to the resulting dispersion, and the mixture is ultrasonically treated at room temperature for 30 min. The mixed solution is then transferred to a round-bottom flask and the anhydrous ethanol is rotary evaporated at 60 °C and a vacuum of 30 kPa. After drying, the product is subjected to thermal reduction treatment in a H2-Ar mixed atmosphere with a volume fraction of 20% H2 at 400 °C for 4 h. After cooling to room temperature, the palladium-based catalyst supported on hollow carbon spheres is obtained.

[0035] Comparative Example 4: The solid carbon spheres of Comparative Example 4 were prepared using the method described in step S1 of Example 1, except that tetrapropyl orthosilicate was not added.

[0036] Comparative Example 5 provides a method for preparing a palladium-based catalyst supported on solid carbon spheres, comprising the following steps: 150 mg of hollow carbon spheres prepared in Comparative Example 4 are dispersed in 30 ml of anhydrous ethanol, and ultrasonically treated for 5 min. 22.1 mg of palladium acetate is added to the resulting dispersion, and ultrasonically treated for another 30 min. The mixed solution is then transferred to a round-bottom flask, and the anhydrous ethanol is rotary evaporated at 60 °C and a vacuum of 30 kPa. After drying, the product is immersed in 30 ml of anhydrous methanol and reduced at 20 °C for 40 min to obtain the palladium-based catalyst supported on solid carbon spheres.

[0037] Application Example 1: Catalyst Performance Testing The catalytic performance of the catalyst in the selective hydrogenation and deoxygenation reaction of vanillin was evaluated using a micro high-pressure reactor. The solvent used in the reaction system was 5 mL isopropanol. 76 mg (0.5 mmol) of vanillin and 10 mg of catalyst were added to the micro reactor. Before the reaction, the air in the reactor was purged with hydrogen, and then a certain amount of hydrogen was introduced into the reactor. Finally, the pressure inside the reactor was increased to the target value of 0.3 MPa using hydrogen, and the reaction was carried out at 45 °C and 1000 r / min for 1 h. The catalytic effects of the catalysts prepared in Examples 1-3 and Comparative Examples 1-5 in the selective hydrogenation and deoxygenation reaction of vanillin were compared. The experimental results are shown in Table 1. Table 1

[0038] .

[0039] In addition, catalysts were prepared according to the method of Example 1, except that the temperature and reduction time of the reduction with anhydrous methanol in step S2 were changed, while the other conditions remained the same. A series of catalysts prepared under different reduction conditions were obtained. Then, the catalytic effect of the selective hydrogenation and deoxygenation reaction of vanillin was compared according to the catalytic evaluation experiment of Application Example 1. The experimental effect data are shown in Table 2. Table 2

[0040] .

[0041] The catalyst in Example 1 was tested according to the catalytic evaluation experiment in Example 1, except that the type of aromatic aldehyde substrate was changed and all other conditions remained the same. The catalytic effects of different aromatic aldehyde substrates in the selective hydrogenation and deoxygenation reaction were compared, and the experimental results are shown in Table 3. Table 3

[0042] .

[0043] As can be seen from Table 3, the catalyst has a good catalytic effect on the vast majority of aromatic aldehydes.

[0044] The specific surface area, pore size, and pore volume data of the catalysts in Examples 1-3 are shown in Table 4.

[0045] Table 4 .

[0046] The results in Tables 1 and 4 show that different ethanol-water volume ratios have a significant impact on the properties of the catalyst. This is because: 1) Water is a reactant in the hydrolysis of TEOS. A high water concentration leads to faster hydrolysis and the formation of a looser gel network with more defects; a low water concentration leads to a denser structure. 2) In the TEOS hydrolysis system, a suitable water concentration is beneficial to increasing the specific surface area of ​​the final carbon shell, because it can form a large number of micropores and suitable mesopores. A low water concentration will form a dense carbon shell with fewer micropores, while a high water concentration will cause some micropores to merge or the shell to collapse.

[0047] The above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. For those skilled in the art, other variations or modifications can be made based on the above description. It is impossible to exhaustively list all the implementation methods here. All extensions of the technical solutions of the present invention are obvious.

Claims

1. A method for preparing a palladium-based catalyst supported on hollow carbon spheres, characterized in that, Includes the following steps: S1: Add tetrapropyl orthosilicate dropwise into anhydrous ethanol-deionized water mixed solvent containing ammonia, and stir at room temperature for 10-30 min; S2: Then add resorcinol and formaldehyde solution, stir the reaction thoroughly, centrifuge to obtain the reaction product, wash and dry it, heat treat it at high temperature under an inert atmosphere, and then etch it with NaOH solution to obtain hollow carbon spheres. S3: Disperse the hollow carbon spheres obtained in step S2 in anhydrous ethanol, add Pd source for ultrasonic impregnation, and then evaporate under vacuum until the anhydrous ethanol is dried. After drying, the product is immersed in anhydrous methanol for stirring and reduction to obtain the hollow carbon sphere supported palladium-based catalyst. The mass of Pd atoms in the Pd source is 5-10% of the mass of the hollow carbon spheres.

2. The preparation method according to claim 1, characterized in that, In step S1, the volume ratio of anhydrous ethanol to deionized water is 3-12:1, the final molar concentration of ammonia in the mixture in step S1 is 0.1-0.2M, and the final volume concentration of tetrapropyl orthosilicate in the mixture in step S1 is 2-6%, preferably 4-5%.

3. The preparation method according to claim 1, characterized in that, In step S2, the mass ratio of resorcinol to formaldehyde is 1g:0.5-2mmol, preferably 1g:1.0-1.2mmol; the mass ratio of resorcinol in step S2 to the volume ratio of tetrapropyl orthosilicate in step S1 is 1g:5-12mL, preferably 1g:8-10mL. After adding resorcinol and formaldehyde solution in step S2, react at 25-40℃ for 20-30 hours.

4. The preparation method according to claim 1, characterized in that, In step S1, the concentration of the NaOH solution is 1-3M, preferably 1.5-2M; the etching temperature is 70-90℃, preferably 75-80℃; and the etching time is 20-30h.

5. The preparation method according to claim 1, characterized in that, The mass of Pd atoms in the Pd source is 6-7% of the mass of the hollow carbon spheres; in step S3, the concentration of Pd atoms in the Pd source dispersed in anhydrous ethanol is 0.1-0.5 mg / mL, preferably 0.3-0.4 mg / mL, and the ultrasonic impregnation time is 20-60 min; in step S3, the reduction temperature is room temperature and the reduction time is 30-60 min.

6. The preparation method according to claim 1, characterized in that, In step S3, the rotary evaporation temperature is 55-65℃, and the vacuum degree of rotary evaporation is 20-40 kPa, preferably 25-30 kPa.

7. The preparation method according to claim 1, characterized in that, In step S3, the reduction temperature is 15-25℃, preferably 20-22℃, and the reduction time is 40-45min.

8. A hollow carbon sphere supported palladium-based catalyst prepared by the method described in any one of claims 1-7.

9. The application of the hollow carbon sphere supported palladium-based catalyst as described in claim 8 in catalytic hydrogenation reaction.

10. The application as described in claim 9, characterized in that, The catalyst is used in the thermocatalytic hydrogenation and deoxygenation reaction of vanillin to prepare 2-methoxy-4-methylphenol.

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