A preparation method and application of a homogeneous noble metal catalyst

Abstract

The application discloses a preparation method and application of a uniform noble metal catalyst. The method comprises the following steps: mixing a noble metal salt and citric acid or sodium citrate in water to form a soluble complex solution with a pH of 2.0-6.0 and a net negative charge; impregnating an alumina carrier with a particle size of 2-3 mm under normal pressure, so that the complex spontaneously migrates to the center of the carrier; after calcination, adding an aqueous formic acid solution at one time, and reducing for 2-4 hours at 25-50 DEG C; the formic acid simultaneously plays a dual function of removing surface citrate and reducing internal noble metal. The application realizes the bulk phase uniform distribution of the noble metal in the 2-3 mm alumina carrier, which is different from the eggshell type or egg white type catalyst, and the reduction process is safe and simple, and does not need to control reagents or complex equipment. The application can be used for medical synthesis, VOCs catalytic oxidation or CO2 reduction reaction.

Description

Technical Field

[0001] This invention belongs to the field of catalyst preparation technology, and specifically relates to a method for preparing a homogeneous noble metal catalyst and its application. Background Technology

[0002] Noble metal catalysts (such as palladium / alumina catalysts) are widely used in important industrial reaction processes such as pharmaceutical synthesis, VOCs catalytic oxidation, and CO2 reduction due to their excellent catalytic activity. Alumina, with its advantages of high specific surface area, good thermal stability, high mechanical strength, and moderate cost, is one of the most commonly used support materials for noble metal catalysts. Among them, spherical or irregular blocky alumina with a particle size of 2-3 mm has been widely used in industrial fixed-bed reactors due to its good industrial operability.

[0003] Currently, the industrial preparation of noble metal catalysts mainly employs the impregnation-reduction method. This method typically includes the following steps: mixing a noble metal precursor solution with an alumina support, followed by drying, calcination, and reduction to obtain the catalyst. Conventional drying processes also suffer from solute backflow: when the impregnated wet support is directly subjected to rapid drying at high temperatures, the solution within the pores flows from the inside to the surface, carrying the noble metal complex to the surface for enrichment, further exacerbating the uneven distribution of active components. Therefore, controlling the drying process is crucial for achieving uniform distribution. However, in catalysts prepared using the conventional impregnation method, the distribution of active components is affected by the mass transfer behavior of the solution during impregnation and drying, typically resulting in an "eggshell" distribution, where the active components are mainly concentrated near the outer surface of the support, with extremely low levels of active components inside. To improve this situation, researchers have developed "protein-type" or "thick eggshell-type" catalysts, distributing the active components in the subsurface region of the support. However, under these two distribution patterns, the noble metal still mainly concentrates on the surface or subsurface region of the support, lacking active components inside the support, which cannot participate in the catalytic reaction and is detrimental to improving catalytic efficiency.

[0004] Therefore, how to achieve a uniform bulk distribution of precious metals from the surface to the interior within a 2-3 mm alumina carrier is one of the technical problems that urgently need to be solved in this field.

[0005] Furthermore, the existing catalyst reduction process also has significant drawbacks: Hydrogen reduction requires high-temperature, high-pressure hydrogen cylinders and explosion-proof facilities. The equipment investment is large, the safety risks are high, and the operation requires professional training, making it unsuitable for small and medium-sized enterprises or general laboratories. Sodium borohydride and hydrazine hydrate reduction are both classified as easily explosive or hazardous controlled chemicals. Their purchase, storage, and use are strictly restricted. Moreover, the reduction process is violent and can easily lead to the aggregation or shedding of active components, making it difficult to control.

[0006] Therefore, how to achieve a uniform bulk distribution of precious metals within a 2-3 mm alumina carrier, while employing a safe, simple, and unregulated reduction process, remains an unsolved technical problem in this field. Summary of the Invention

[0007] This invention provides a method for preparing a uniform noble metal catalyst and its application, which can solve the problem of achieving a uniform bulk distribution of noble metals within a 2-3 mm alumina support in the prior art.

[0008] On the one hand, this invention provides a method for preparing a homogeneous noble metal catalyst, comprising the following steps: S1. Mix the noble metal salt, citric acid and / or sodium citrate in water at a molar ratio of noble metal to citrate of 1:1–1:5, adjust the pH to 2.0–6.0, and form a soluble noble metal-citric acid complex solution with a net negative charge. S2. The alumina support with a particle size of 2–3 mm is immersed in the soluble noble metal-citric acid complex solution at normal pressure and room temperature; S3. The impregnated alumina support is calcined at 150-300℃ to obtain an oxide-state noble metal precursor; S4. Add the formic acid aqueous solution to the oxidized noble metal precursor in one step, wherein the molar ratio of formic acid to noble metal is 20:1–100:1, and reduce for 2–4 hours at 25–50°C without inert gas protection and with only stirring.

[0009] Among them, the noble metal salt is palladium salt, including palladium nitrate and palladium chloride; the loading of noble metal in noble metal catalyst is 0.5–5 wt%; the concentration of formic acid aqueous solution is 5–20 wt%; and the calcination time in S3 is 0.5–4 hours.

[0010] Preferably, the alumina carrier is pretreated before use, which involves calcining at 400-600℃ for 1-4 hours.

[0011] Preferably, between S2 and S3, there is also a staged temperature-controlled drying process: the impregnated carrier is first left to stand at 0-5°C for 12-24 hours, then dried at 40-60°C for 2-4 hours, and finally dried at 100-120°C to constant weight.

[0012] Preferably, before S4, a pre-wetting treatment is also included: the oxidized noble metal precursor is pre-wetted with deionized water or a dilute formic acid solution with a concentration of 1-5 wt% to make the surface uniformly wetted, and then the formic acid aqueous solution is added after filtering out excess liquid.

[0013] On the other hand, the present invention provides the use of a homogeneous noble metal catalyst for pharmaceutical synthesis, catalytic oxidation of VOCs, or CO2 reduction reactions.

[0014] This invention achieves the active migration of noble metal ions into the positively charged alumina pores under electrostatic attraction by mixing noble metal salts with citric acid / sodium citrate in a specific molar ratio and adjusting the pH to 2.0–6.0. ​​This is different from the passive capillary diffusion in the prior art and lays the foundation for subsequent uniform distribution of the bulk phase.

[0015] By immersing a 2–3 mm alumina support in the complex solution at ambient pressure and room temperature, the complex spontaneously migrates to the geometric center region of the support particles under the synergistic effect of concentration gradient and electrostatic attraction, thereby obtaining a uniform bulk distribution of noble metal from the surface to the interior, overcoming the defect of missing active components inside eggshell-type and protein-type catalysts.

[0016] By employing low-temperature calcination at 150-300℃, the noble metal-citric acid complex was mildly decomposed, allowing the noble metal to be converted into its oxidized state in situ at its original location. This avoided the migration and agglomeration of the noble metal caused by high-temperature calcination and effectively locked in the uniformly distributed structure formed during the impregnation stage.

[0017] By adding an aqueous formic acid solution in a single step, controlling the molar ratio of formic acid to precious metal at 20:1–100:1, and reducing the metal at 25–50°C for 2–4 hours without inert gas protection and only with stirring, the dual function of formic acid is achieved: dissolving and removing residual citrate ions on the surface, and penetrating into the pores to reduce the oxidized precious metal to its metallic state. This reduction process requires no hydrogen cylinders, explosion-proof facilities, or controlled chemicals such as sodium borohydride and hydrazine hydrate, offering advantages of safety, simplicity, and non-regulation.

[0018] By pre-treating the alumina carrier by calcining it at 400-600℃ for 1-4 hours before use, the adsorbed moisture and impurities on the carrier surface are removed, which improves the consistency of the interaction between the complex and the carrier surface during the subsequent impregnation process.

[0019] By introducing staged temperature-controlled drying after impregnation and before calcination, the phenomenon of solute back migration in conventional one-step high-temperature drying was effectively suppressed, further ensuring the uniform distribution of precious metals within the carrier. In conventional one-step high-temperature drying, the solution on the carrier surface evaporates preferentially, and the solution within the pores flows from the inside to the outside under capillary action, carrying dissolved precious metal complexes to the surface for enrichment, forming an "egg-shell" distribution and disrupting the uniform distribution formed during the impregnation stage. In contrast, staged temperature-controlled drying involves extremely slow evaporation of the solution at low temperatures in the first stage, allowing the complexes to fully diffuse and reach equilibrium within the pores, with no back migration. In the second stage, slow evaporation causes the complexes to precipitate in situ within the pores, significantly reducing back migration. In the third stage, free water is essentially removed, with no liquid phase flow; only bound water is removed, and the position of the complexes remains unchanged. Through this staged control, in-situ precipitation of the complexes within the pores is achieved, effectively suppressing solute back migration and consolidating the uniform distribution of precious metals.

[0020] By adding a pre-wetting treatment before formic acid reduction, the wettability of the dried precursor surface is improved, allowing the formic acid reducing solution to penetrate into the carrier pores more evenly, thus avoiding incomplete reduction or uneven distribution of active components caused by uneven local wetting.

[0021] Through the comprehensive application of the above-mentioned technical means, the homogeneous noble metal catalyst prepared by this invention exhibits superior catalytic performance compared to traditional eggshell catalysts in pharmaceutical synthesis, VOCs catalytic oxidation, and CO2 reduction reactions.

[0022] The beneficial effects of this invention are: 1. This invention overturns the traditional understanding of citric acid as a competitive adsorbent. Utilizing the spontaneous migration characteristics of noble metal-citric acid complexes with a net negative charge under electrostatic attraction and concentration gradient diffusion, it achieves a uniform bulk distribution of the noble metal within a 2-3 mm alumina support, unlike the eggshell or protein-like distribution of existing technologies. This uniform distribution structure allows noble metal atoms within the support to participate in the catalytic reaction, significantly improving the utilization rate of the noble metal.

[0023] 2. This invention employs a staged temperature-controlled drying process after impregnation: "low-temperature equilibration, medium-temperature pre-drying, and high-temperature final drying." First, the mixture is allowed to stand at 0-5℃ to ensure sufficient equilibrium distribution within the pores. Then, free water is slowly removed at 40-60℃, and finally, the mixture is thoroughly dried at 100-120℃. This process effectively suppresses the problem of solute migration back to the surface during conventional one-step high-temperature drying, further ensuring the uniform distribution of precious metals within the carrier.

[0024] 3. This invention adds a pre-wetting treatment before formic acid reduction, uniformly wetting the surface of the oxidized noble metal precursor with deionized water or dilute formic acid solution. This treatment improves the wettability of the dried precursor surface, allowing the subsequently added formic acid reducing solution to penetrate more evenly into the carrier pores, avoiding incomplete reduction or uneven distribution of active components due to uneven local wetting.

[0025] 4. The homogeneous catalyst prepared by this invention outperforms traditional eggshell catalysts in reactions controlled by internal diffusion. It exhibits lower ignition and complete conversion temperatures in VOCs catalytic oxidation reactions; significantly faster reaction rates in the hydrogenation of pharmaceutical intermediates; and higher CO2 conversion and better selectivity for target products in CO2 hydrogenation reactions. Attached Figure Description

[0026] Figure 1 This is a flowchart illustrating the preparation process of the homogeneous noble metal catalyst in Example 1 of the present invention.

[0027] Figure 2 The diagram shows a comparison of the distribution of active components in the cross-section of the catalyst support, where (a) is the eggshell-shaped distribution in Comparative Example 1 and (b) is the uniform distribution of the present invention.

[0028] Figure 3 This is a schematic diagram of the migration mechanism of noble metal-citric acid complexes, in which (a) in the absence of citric acid, noble metal ions are electrostatically repelled and cannot enter the pores, and (b) after forming a negatively charged complex, they are electrostatically attracted into the pores. Detailed Implementation

[0029] The specific embodiments of the present invention will be described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0030] Example Example 1 provides a method for preparing a homogeneous noble metal catalyst. The specific implementation method is as follows: Carrier pretreatment Take 100g of spherical γ-alumina carrier with a particle size of 2-3mm, its specific surface area is 200 20 m² / g, pore volume 0.5 0.1 cm³ / g, with an average pore size of 10 nm, was placed in a muffle furnace and calcined at 500 °C for 2 hours to remove moisture and impurities adsorbed on the carrier surface. After cooling to room temperature, it was ready for use.

[0031] Preparation of palladium-citric acid complex solution Weigh out 2.53 g of palladium nitrate (Pd(NO3)2·2H2O), with a palladium content of 39.5%; 1.5 g of citric acid (C6H8O7·H2O), analytical grade; and 0.5 g of sodium citrate (Na3C6H5O7·2H2O), analytical grade. This is equivalent to a molar ratio of palladium metal to citrate ions of 1:1.

[0032] Add the above materials sequentially to 500 mL of deionized water and stir at 300 rpm for 30 minutes at room temperature until the solid is completely dissolved. Adjust the pH of the solution to 4.0 ± 0.2 with 0.1 mol / L dilute nitric acid. At this point, the Pd²⁺ content in the solution... + It forms a soluble complex with citrate ions carrying a net negative charge, primarily [Pd(C6H5O7)2]. 4- Add deionized water to the solution to bring the total volume to 500 mL, resulting in a final palladium ion concentration of 2.0 g / L.

[0033] Immersion Add 100g of pretreated alumina carrier to the above complex solution, ensuring the solution completely submerges the carrier. Allow the solution to stand and impregnate for 8 hours at room temperature and pressure, stirring once every hour at 100 rpm for 1 minute each time, to ensure sufficient liquid-solid contact and avoid mechanical damage to the carrier particles.

[0034] During the impregnation process, the negatively charged palladium-citric acid complex is continuously drawn inward by the positively charged alumina channels under electrostatic attraction, and diffuses towards the particle center under the drive of the concentration gradient.

[0035] Filtration and washing After impregnation, the carrier and residual liquid were separated by vacuum filtration. The carrier was washed three times with deionized water, using 200 mL each time, to remove excess complexes that were physically adsorbed on the carrier surface but did not enter the pores. The washing endpoint was determined by measuring the conductivity of the washing solution; washing was stopped when the conductivity of the washing solution was close to that of deionized water, i.e., less than 5 μS / cm.

[0036] dry The washed carrier was laid flat on a stainless steel tray and placed in a forced-air drying oven at 120℃ for 4 hours. It was turned over every hour during the drying process to prevent localized overheating. The dried carrier was light yellow.

[0037] roasting The dried support was placed in a muffle furnace and heated to 300°C at a heating rate of 5°C / min. It was then calcined at 300°C for 3 hours and allowed to cool naturally to room temperature. During calcination, the palladium-citrate complex decomposed, the citrate ion was oxidized and removed, and palladium was converted to palladium oxide (PdO), yielding the palladium oxide-alumina precursor. The calcined support was grayish-brown.

[0038] Preparation of formic acid reducing solution Take 11.8 mL of an 85 wt% formic acid aqueous solution, dilute it to 100 mL with deionized water to prepare a 10 wt% formic acid aqueous solution. The amount of formic acid in this solution is 0.22 mol.

[0039] reduction The obtained palladium oxide-alumina precursor was completely added to the formic acid solution, with a liquid-to-solid volume ratio of approximately 1:1. The mixture was then stirred at 150 rpm for 3 hours in a constant-temperature water bath at 40°C. A lidded conical flask was used to reduce formic acid evaporation, but it was not required to be sealed, and no inert gas was introduced for protection.

[0040] During the reduction process, formic acid plays the following role: dissolving and removing trace amounts of undecomposed citrate ions and their decomposition products remaining on the surface of the carrier, and penetrating into the carrier pores to reduce palladium oxide inside the pores to metallic palladium.

[0041] Reduction reaction equation: PdO + HCOOH → Pd + CO2↑ + H2O Washing and drying After reduction, the formic acid waste liquid was filtered off, and the catalyst was washed five times with deionized water (200 mL each time) until the pH of the washing solution reached 7.0. The washed catalyst was then dried in an 80°C forced-air drying oven for 3 hours to obtain a homogeneous palladium / alumina catalyst.

[0042] Uniformity verification method Take 3-5 catalyst particles, cut them in half lengthwise with a blade, and immerse the cut surfaces in a 5wt% nitric acid solution for 1 minute. Rinse with deionized water afterward. Because palladium reacts with nitric acid to form a brown palladium nitrite complex, the areas where palladium is present will appear brownish-yellow to brown. Observe the color distribution of the cut surfaces: If the cut surface has a dark brown edge and a colorless or light-colored center, it indicates an eggshell-shaped distribution. If the cross-section shows a uniform light brown color throughout, it indicates a uniform distribution. The catalyst prepared in this embodiment has a uniform light brown cross-section, indicating that palladium has been uniformly distributed inside the support.

[0043] Catalyst parameters According to inductively coupled plasma optical emission spectroscopy (ICP-OES), the actual palladium loading of the catalyst in this embodiment was 0.98 wt% (theoretical loading 1.0 wt%), and the yield was 98%.

[0044] Example 2: The catalyst was prepared according to the steps of Example 1, except that the amount of palladium nitrate was adjusted.

[0045] Preparation of complex solutions Weigh 1.27 g of palladium nitrate (39.5% palladium content), 0.8 g of citric acid (analytical grade), and 0.3 g of sodium citrate (analytical grade). Add deionized water to 500 mL and adjust the pH to 4.0 to prepare a complex solution with a palladium ion concentration of 1.0 g / L. This is equivalent to a molar ratio of palladium metal to citrate ions of 1:1.

[0046] Carrier pretreatment Take 100g of spherical γ-alumina carrier with a particle size of 2-3mm, the same as in Example 1, and pretreat it according to the method in Example 1.1.

[0047] Next steps The steps of impregnation, filtration and washing, drying, reduction, washing and drying are exactly the same as in Example 1, except that the calcination time is changed to 2 hours.

[0048] result The catalyst is light gray. Verification by nitric acid staining of the cross-section showed that palladium was uniformly distributed on the support cross-section, with a slightly lighter color than in Example 1. ICP determination showed the actual palladium loading to be 0.49 wt%.

[0049] Example 3: The catalyst was prepared according to the steps of Example 1, except that the amount of palladium nitrate and the impregnation method were adjusted.

[0050] Preparation of complex solutions Weigh 12.65g of palladium nitrate (39.5% palladium content), 7.5g of citric acid (analytical grade), and 2.5g of sodium citrate (analytical grade). Add deionized water to 1000mL and adjust the pH to 4.0 to prepare a complex solution with a palladium ion concentration of 5.0g / L.

[0051] Carrier pretreatment Take 100g of spherical γ-alumina carrier with a particle size of 2-3mm, the same as in Example 1, and pretreat it according to the method in Example 1.1.

[0052] Immersion An equal-volume impregnation method was used. The saturated water absorption capacity of 100g alumina support was pre-determined to be 85mL. 85mL of the above complex solution was taken and evenly sprayed onto 100g alumina support, stirring constantly to ensure uniform absorption by the support. The mixture was then sealed and allowed to stand for 12 hours to allow for full penetration of the complex.

[0053] Next steps The steps of impregnation, filtration and washing, drying, roasting, washing and drying are the same as in Example 1, but the time in the reduction step is extended to 4 hours.

[0054] result ICP analysis revealed an actual palladium loading of 4.85 wt%. Nitric acid staining analysis of the cross-section showed that palladium was uniformly distributed across the support cross-section, with no obvious surface enrichment.

[0055] Example 4: The catalyst was prepared according to the steps of Example 1, except for the drying method.

[0056] Staged temperature-controlled drying The carrier, after impregnation, filtration, and washing, is dried in stages under controlled temperature according to the following steps: First stage: Place the carrier in a refrigerator at 0-5℃ and let it stand for 12 hours to allow the complex to be fully and evenly distributed in the carrier pores; Second stage: Transfer the carrier to a 50°C forced-air drying oven and dry for 4 hours to slowly remove free water; Third stage: Heat the carrier to 120℃ and dry it to constant weight.

[0057] Next steps The roasting, reduction, washing and drying steps are exactly the same as in Example 1.

[0058] result The nitric acid colorimetric assay showed that palladium exhibited a uniform light brown color on the cross-section of the support, consistent with the results in Example 1. Staged drying further helps to suppress the re-migration of solute to the surface during the drying process.

[0059] Example 5: The catalyst was prepared according to the steps of Example 1, except for the pre-wetting treatment before formic acid reduction.

[0060] Pre-wetting treatment The palladium oxide-alumina precursor was prepared according to the steps in Example 1. The precursor was placed in a beaker, and 50 mL of a 2 wt% dilute formic acid aqueous solution was added. The mixture was gently stirred for 1 minute to uniformly wet the surface of the precursor, and then the excess liquid was removed by filtration.

[0061] reduction The pre-wetted precursor was added to the formic acid reducing solution prepared according to Example 1, and the subsequent reduction, washing and drying steps were exactly the same as in Example 1.

[0062] result Nitric acid colorimetric analysis of the cross-section showed that palladium exhibited a uniform light brown color on the cross-section of the support, consistent with the results in Example 1. Pre-wetting treatment facilitates uniform penetration of the formic acid solution, especially for large-particle supports, and can further improve the uniformity of reduction.

[0063] Comparative Example Comparative Example 1: The catalyst was prepared according to the steps of Example 1, except for the preparation of the impregnation solution.

[0064] Preparation of impregnation solution Weigh 2.53g of palladium nitrate (Pd(NO3)2·2H2O) with a palladium content of 39.5%, dissolve it in 500mL of deionized water, and adjust the pH of the solution to 4.0 with 0.1mol / L dilute nitric acid. Do not add citric acid or sodium citrate.

[0065] Carrier pretreatment Take 100g of spherical γ-alumina carrier with a particle size of 2-3mm and pretreat it according to the method in Example 1.1.

[0066] Impregnation and subsequent steps The pretreated alumina carrier was added to the above impregnation solution, and the subsequent impregnation, filtration and washing, drying, calcination, formic acid reduction, washing and drying steps were exactly the same as in Example 1.

[0067] result During the impregnation process, due to Pd² + Positively charged, Pd² exhibits electrostatic repulsion with the similarly positively charged alumina surface. + It is difficult to penetrate deep into the pores and mainly adsorbs on the outer surface of the particles.

[0068] The precursor was light yellow after calcination, and the catalyst was gray after reduction. Nitric acid colorimetric analysis of the cut surface showed that the edges were dark brown and the central area was light yellow, exhibiting a typical eggshell-shaped distribution. This indicates that without citric acid, formic acid reduction alone cannot achieve a uniform distribution of palladium within the support.

[0069] Comparative Example 2: The catalyst was prepared according to the steps of Example 1, except for the reduction step.

[0070] Precursor preparation The palladium oxide-alumina precursor was prepared according to the steps in Example 1.

[0071] reduction The precursor was placed in a tubular reduction furnace, and high-purity hydrogen gas was introduced at a flow rate of 100 mL / min. The temperature was increased to 300 °C at a rate of 5 °C / min, and the reduction was carried out at this temperature for 3 hours. Then, the catalyst was cooled to room temperature in a hydrogen atmosphere. The reduced catalyst was then removed under nitrogen protection.

[0072] result The obtained catalyst cross-section was verified by nitric acid color development, showing that the cross-section was a uniform light brown color, and the palladium distribution was the same as in Example 1.

[0073] However, hydrogen reduction has the following problems: it requires equipment such as hydrogen cylinders, pressure reducing valves, gas pipelines, tubular reduction furnaces, and explosion-proof facilities; it operates at high temperatures and consumes a lot of energy; and hydrogen is a flammable and explosive gas, requiring professional training and meeting the requirements of an explosion-proof workshop. In contrast, the formic acid reduction in Example 1 of this invention only requires conventional glassware and a constant-temperature water bath, making the operation simple and safe.

[0074] Performance testing To verify the beneficial effects of the homogeneous noble metal catalyst prepared in this invention, the following performance tests were conducted.

[0075] 1. VOCs Catalytic Oxidation Test: The homogeneous catalyst prepared in Example 1 and the eggshell-shaped catalyst prepared in Comparative Example 1 were ground and sieved separately, and 40-60 mesh particles were collected for later use. A fixed-bed reactor with a 10 mm inner diameter quartz tube was used, with a catalyst loading of 0.5 g. The reaction gas composition was: toluene 1000 ppm, oxygen 21%, nitrogen balance, and a total flow rate of 300 mL / min. The reaction pressure was atmospheric pressure, and the reaction temperature was programmed to rise from room temperature to 400 °C at a rate of 5 °C / min. The toluene concentration in the reaction tail gas was analyzed online using a gas chromatograph (FID detector). Subsequently, under the above reaction conditions, the reaction temperature was kept constant at 240 °C, and the reaction was carried out continuously for 100 hours. Samples were then taken to analyze the toluene conversion rate and to examine the stability of the catalyst.

[0076] The results are shown in Table 1:

[0077] The T50 and T90 of the homogeneous catalyst are 35°C lower than those of the eggshell catalyst, and the conversion rate is 47 percentage points higher at 240°C. This is because in the catalytic oxidation of toluene, the reactants need to diffuse into the catalyst channels to contact the active sites. The active components of the eggshell catalyst are only distributed near the surface, and the internal pores are not utilized; the active components of the homogeneous catalyst are distributed throughout the entire particle, with more active sites per unit volume and a shorter diffusion path.

[0078] After continuous reaction at 240℃ for 100 hours, the toluene conversion rate of the homogeneous catalyst decreased from 92% to 88%, a decrease of 4%; the conversion rate of the eggshell catalyst decreased from 45% to 38% under the same conditions, a decrease of 7%. The homogeneous catalyst exhibited better stability.

[0079] 2. Hydrogenation test of pharmaceutical intermediates: The homogeneous catalyst prepared in Example 1 and the eggshell-shaped catalyst prepared in Comparative Example 1 were used. 0.1 mmol of p-nitrophenol, 10 mL of deionized water, and 0.5 mg of catalyst were added to a 50 mL three-necked flask. Hydrogen gas was introduced, and the reaction was stirred at 25 °C. Samples were taken every 5 minutes, and the concentration of p-nitrophenol was detected using a UV-Vis spectrophotometer (400 nm).

[0080] The results are shown in Table 2:

[0081] The reaction rate constant k of the homogeneous catalyst is 0.28 min. -¹, Eggshell-type catalyst k=0.15min - ¹. The reaction rate of the homogeneous catalyst is approximately 1.87 times that of the eggshell catalyst. Homogeneous catalyst: 25 minutes; eggshell catalyst: 50 minutes.

[0082] 3. CO2 Catalytic Hydrogenation Test: The homogeneous catalyst prepared in Example 1 and the eggshell-shaped catalyst prepared in Comparative Example 1 were used. A fixed-bed reactor was employed, with a catalyst loading of 1.0 g. The reaction gas composition was: CO2 24%, H2 72%, N2 4%, total flow rate 50 mL / min, reaction temperature 300 °C, and reaction pressure 2.0 MPa. The reaction tail gas was analyzed using a gas chromatograph (TCD detector).

[0083] The test results are shown in Table 3:

[0084] The homogeneous catalyst showed a 6.2 percentage point higher CO2 conversion rate and a 4 percentage point higher methane selectivity compared to the eggshell catalyst. This indicates that the homogeneous catalyst also has advantages in this reaction.

[0085] The above test results show that the homogeneous catalyst prepared in this invention exhibits superior catalytic performance compared to traditional eggshell-type catalysts in VOCs catalytic oxidation, pharmaceutical intermediate hydrogenation, and CO2 hydrogenation reactions. This is because the uniformly distributed structure makes the active components accessible throughout the entire support particle, shortening the reactant diffusion path and improving the utilization rate of active sites.

[0086] Based on the above embodiments, comparative examples, and performance tests, the following conclusions can be drawn: I. Citric acid / sodium citrate is key to uniform distribution. Comparative Example 1 shows that without the addition of citric acid and sodium citrate, palladium ions cannot penetrate into the carrier. Figure 3 As shown, only an eggshell-shaped distribution can be obtained. However, in Examples 1-5, by adding citric acid and sodium citrate, soluble palladium-citric acid complexes with net negative charges can be formed by complexing with palladium ions. Utilizing the electrostatic attraction between positive and negative charges, the active component is driven to migrate into the pores of the support. Combined with the concentration gradient diffusion effect, a uniform bulk distribution of palladium is ultimately achieved within the 2-3 mm large-particle alumina support. This fully demonstrates that citrate complexation modification and regulation of the charge characteristics of the active component are the core technologies for overcoming the problem of enrichment of noble metals on the surface of large-particle supports and preparing uniform palladium catalysts.

[0087] II. Formic acid is feasible and has significant advantages as the sole reducing agent. Examples 1-5 demonstrate that formic acid can effectively reduce palladium oxide to metallic palladium under mild conditions of 25-50°C, normal pressure, and without inert gas protection, with the reduction process completed in 2-4 hours. Comparative Example 2 shows that, compared to hydrogen reduction, formic acid reduction does not require complex equipment such as hydrogen cylinders, tubular furnaces, and explosion-proof facilities, making the operation safer and simpler.

[0088] III. Wide range of process parameters and strong adaptability Examples 1-3 demonstrate that uniform distribution can be achieved within a wide loading range of 0.5-5 wt%. Examples 1 and 3 respectively verify the feasibility of conventional impregnation and equal-volume impregnation methods. The reduction temperature of 40°C and formic acid concentration of 10 wt% in Example 1 are typical conditions. Those skilled in the art will understand that a homogeneous catalyst can be obtained by adjusting the reduction temperature within the range of 25-50°C and the formic acid concentration within the range of 5-20 wt%. Based on this, Examples 4 and 5 further optimized the post-treatment process, confirming that the staged temperature-controlled drying process can effectively suppress the migration of the active component solute to the support surface during drying, and that the precursor pre-wetting treatment can promote uniform penetration of the reducing solution into the pores of the large-particle support, further improving the overall homogeneity and preparation stability of the catalyst. Simultaneously, those skilled in the art can flexibly adjust the process parameters within a reasonable range of 25-50°C reduction temperature and 5-20 wt% formic acid concentration to stably prepare qualified homogeneous palladium / alumina catalysts, demonstrating high process tolerance and strong stability for industrial mass production.

[0089] IV. Excellent catalytic performance Performance tests show that in the catalytic oxidation of VOCs, the T90 of the homogeneous catalyst is 35°C lower than that of the eggshell catalyst, and the conversion rate (92%) at 240°C is 47 percentage points higher than that of the eggshell catalyst (45%), with better stability. In the hydrogenation reaction of p-nitrophenol, the reaction rate of the homogeneous catalyst is about 1.87 times that of the eggshell catalyst, and the reaction completion time is halved. In the hydrogenation reaction of CO2, the CO2 conversion rate of the homogeneous catalyst is 6.2 percentage points higher than that of the eggshell catalyst, and the methane selectivity is increased by 4 percentage points.

[0090] In summary, the method for preparing uniform noble metal catalysts provided by this invention effectively solves the technical problems of difficulty in achieving uniform bulk distribution of 2-3 mm alumina supports and safety hazards in the reduction process in the prior art, and has significant progress and industrial application value.

[0091] The above-disclosed embodiments are merely a few specific examples of the present invention. However, the embodiments of the present invention are not limited thereto, and any variations that can be conceived by those skilled in the art should fall within the protection scope of the present invention.

Claims

1. A method for preparing a homogeneous noble metal catalyst, characterized in that, Includes the following steps: S1. Mix the noble metal salt, citric acid and / or sodium citrate in water at a molar ratio of noble metal to citrate of 1:1–1:5, adjust the pH to 2.0–6.0, and form a soluble noble metal-citric acid complex solution with a net negative charge. S2. The alumina support with a particle size of 2–3 mm is immersed in the soluble noble metal-citric acid complex solution at normal pressure and room temperature; S3. The impregnated alumina support is calcined at 150-300℃ to obtain an oxide-state noble metal precursor; S4. Add the formic acid aqueous solution to the oxidized noble metal precursor in one step, wherein the molar ratio of formic acid to noble metal is 20:1–100:1, and reduce for 2–4 hours at 25–50°C without inert gas protection and with only stirring.

2. The method according to claim 1, characterized in that, The alumina carrier is pretreated before use, which involves calcining at 400-600℃ for 1-4 hours.

3. The method according to claim 1, characterized in that, The precious metal salt is a palladium salt, including palladium nitrate and palladium chloride.

4. The method according to claim 1, characterized in that, The loading of the noble metal in the noble metal catalyst is 0.5–5 wt%.

5. The method according to claim 1, characterized in that, The impregnated alumina carrier obtained in step S2 is further subjected to staged temperature-controlled drying, specifically including the following steps: the impregnated alumina carrier is first left to stand at 0-5℃ for 12-24 hours, then dried at 40-60℃ for 2-4 hours, and finally dried at 100-120℃ to constant weight.

6. The method according to claim 1, characterized in that, S4 includes a pre-wetting treatment: the oxidized noble metal precursor is pre-wetted with deionized water or a dilute formic acid solution with a concentration of 1-5 wt% to make the surface uniformly wetted, and then the formic acid aqueous solution is added after filtering out excess liquid.

7. The method according to claim 1, characterized in that, The concentration of the formic acid aqueous solution is 5–20 wt%.

8. The method according to claim 1, characterized in that, The roasting time described in S3 is 0.5-4 hours.

9. A homogeneous noble metal catalyst, characterized in that, It is prepared by any one of the methods described in claims 1–8.

10. The application of a homogeneous noble metal catalyst, characterized in that, The homogeneous noble metal catalyst prepared by any one of claims 1–8 can be used in pharmaceutical synthesis, catalytic oxidation of VOCs, or CO2 reduction reactions.

Images