Pd catalyst as well as preparation method and application thereof

Abstract

The invention relates to the field of catalysts, and discloses a Pd catalyst and a preparation method and application thereof.The catalyst comprises a carrier and Pd nano-particles, based on the total mass of the catalyst, the mass percentage content of the Pd nano-particles is 0.1-0.2 wt%, and the mass percentage content of the carrier is 99.8-99.9 wt%; wherein the Pd nanoparticles are dispersed and loaded on the surface of the carrier. The catalyst comprises the carrier and the Pd nano-particles, the Pd nano-particles are dispersed and loaded on the surface of the carrier without agglomeration, the dosage of Pd can be greatly reduced under the condition that the activity requirement of the catalyst is met, meanwhile, the stability is good, and the preparation method is simple.

Description

Technical Field

[0001] The present invention relates to the field of catalysts, and particularly to a Pd catalyst, a preparation method thereof, and an application thereof. Background Art

[0002] Pyrolysis gasoline is an important by-product of ethylene. After pyrolysis gasoline is subjected to first-stage hydrogenation to remove styrene and diolefins, and then second-stage hydrogenation to remove monoolefins, sulfur, nitrogen and other impurities, it can be used for aromatics extraction. At present, the first-stage hydrogenation catalysts mainly use Pd-based catalysts and Ni-based catalysts. Among them, the Pd-based catalyst has the advantages of high space velocity, simple preparation, convenient startup, long service life, etc. Moreover, after the first investment, Pd can be recovered and the catalyst replacement cost is relatively low. However, at present, the vast majority of devices use Ni-based catalysts, especially newly built devices, because the initial investment cost of Pd-based catalysts is too high, which limits their application.

[0003] At present, the content of Pd in the Pd-based catalyst applied to the first-stage hydrogenation industrial device of pyrolysis gasoline is 0.25 - 0.30 wt%, and the cost is still relatively high. However, after reducing the Pd content in the catalyst, there is a problem of reduced catalyst activity and it cannot meet the requirements of long-term operation.

[0004] Therefore, there is a need for a method to reduce the Pd content in the catalyst without reducing the catalyst activity requirements and catalyst stability, so as to reduce the cost of the catalyst. Summary of the Invention

[0005] The object of the present invention is to overcome the problem in the prior art that while reducing the Pd content in the Pd catalyst, the catalyst activity is greatly reduced and it cannot meet the requirements of stable operation. The present invention provides a Pd catalyst, a preparation method thereof, and an application thereof. The catalyst comprises a carrier and Pd nanoparticles, and the Pd nanoparticles are dispersed and loaded on the surface of the carrier without forming agglomerates, which can greatly reduce the Pd dosage while meeting the catalyst activity requirements, and at the same time has good stability and a simple preparation method.

[0006] To achieve the above object, in the first aspect of the present invention, a Pd catalyst is provided, wherein the catalyst comprises a carrier and Pd nanoparticles. Based on the total mass of the catalyst, the mass percentage content of the Pd nanoparticles is 0.1 - 0.2 wt%, and the mass percentage content of the carrier is 99.8 - 99.9 wt%;

[0007] Among them, the Pd nanoparticles are dispersed and loaded on the surface of the carrier.

[0008] In the second aspect of the present invention, a preparation method of a Pd catalyst is provided, wherein the method comprises the following steps:

[0009] (1) Prepare a precursor solution containing a polyacrylic acid compound, an amine ligand, and a Pd source, and the precursor solution is alkaline;

[0010] (2) Immerse the carrier in the precursor solution and calcine to obtain the Pd catalyst.

[0011] The third aspect of the present invention provides a Pd catalyst prepared by the preparation method described in the second aspect.

[0012] The fourth aspect of the present invention provides an application of the Pd catalyst described in the first aspect or the third aspect in the hydrogenation reaction of pyrolysis gasoline.

[0013] Through the above technical solutions, the following beneficial effects are achieved:

[0014] (1) For the Pd catalyst provided by the present invention, while the Pd content is reduced, high catalytic activity is ensured, meeting the requirements of stable operation, having good sulfur resistance, low cost, and is expected to achieve large-scale application;

[0015] (2) For the preparation method provided by the present invention, using a polyacrylic acid compound as a dispersant in combination with an amine ligand, through the anchoring effect of the polyacrylic acid compound, the dispersion of Pd particles is improved, and a large reduction in Pd dosage can be achieved while meeting the catalyst activity requirements, and at the same time, the stability is also good, and the preparation method is simple. Description of the Drawings

[0016] Figure 1 is the XRD pattern of the Pd catalyst prepared in Example 1;

[0017] Figure 2 is the TEM image of the Pd catalyst prepared in Example 1, Figure 2 (a) is the bright field image, Figure 2 (b) is the dark field image;

[0018] Figure 3 is the appearance image of the Pd catalyst prepared in Example 1;

[0019] Figure 4 is the TEM image of the Pd catalyst prepared in Comparative Example 1, Figure 4 (a) is the bright field image, Figure 4 (b) is the dark field image;

[0020] Figure 5 is the appearance image of the Pd catalyst prepared in Comparative Example 1. Detailed Embodiments

[0021] The endpoints and any values in the ranges disclosed herein are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed herein.

[0022] In the first aspect of the present invention, a Pd catalyst is provided, wherein the catalyst comprises a support and Pd nanoparticles. Based on the total mass of the catalyst, the mass percentage of the Pd nanoparticles is 0.1 - 0.2 wt%, and the mass percentage of the support is 99.8 - 99.9 wt%.

[0023] Among them, the Pd nanoparticles are dispersedly loaded on the surface of the support.

[0024] In the present invention, the catalyst comprises a support and Pd nanoparticles. The Pd nanoparticles are dispersedly loaded on the surface of the support without forming agglomerates. While meeting the catalyst activity requirements, a significant reduction in Pd usage is achieved, and the stability is relatively good.

[0025] According to the present invention, preferably, based on the total mass of the catalyst, the mass percentage of the Pd nanoparticles is 0.15 - 0.2 wt%, and the mass percentage of the support is 99.8 - 99.85 wt%.

[0026] According to the present invention, preferably, the Pd nanoparticles are anchored and dispersed on the surface of the support through a polyacrylic acid compound.

[0027] According to the present invention, preferably, the support comprises alumina, optional Na, and optional Si.

[0028] In the second aspect of the present invention, a method for preparing a Pd catalyst is provided, wherein the method comprises the following steps:

[0029] (1) Prepare a precursor solution containing a polyacrylic acid compound, an amine ligand, and a Pd source. The precursor solution is alkaline.

[0030] (2) Immerse the support in the precursor solution and calcine to obtain the Pd catalyst.

[0031] In the present invention, in the method for preparing a Pd catalyst, a polyacrylic acid compound is used as a dispersant in combination with an amine ligand. Through the anchoring effect of the polyacrylic acid compound, the dispersion of Pd particles is improved. While meeting the catalyst activity requirements, a significant reduction in Pd usage is achieved, and the stability is also relatively good. The preparation method is simple.

[0032] In the present invention, for the prepared Pd catalyst, the polymer is adsorbed on the surface of alumina as the anchoring site for Pd; meanwhile, the polymer also has a steric hindrance effect to prevent Pd from entering the bulk phase of the alumina support. Pd in the bulk phase of the catalyst support has diffusion limitations in the catalytic reaction and has low activity, which will lead to a decrease in the utilization rate of Pd. The steric hindrance effect of the polymer can prevent Pd from entering.

[0033] According to the present invention, preferably, the mass ratio of the support to the polyacrylic acid compound in the precursor solution is 1:0.005 - 0.03, such as 1:0.005, 1:0.1, 1:0.015, 0:0.2, 1:0.025, 1:0.03, or the range between any two of them, preferably 1:0.01 - 0.02.

[0034] According to the present invention, preferably, the polyacrylic acid compound is selected from at least one of polyacrylic acid, sodium polyacrylate, and ammonium polyacrylate. In the present invention, the polyacrylic acid compound plays an anchoring role. Through the molecular chain of the polyacrylic acid compound, the support is connected to the Pd particles, preventing the Pd particles from further entering the interior of the support, ensuring that the Pd particles are loaded on the surface of the support, and ensuring high catalytic activity of the catalyst while reducing the Pd loading amount.

[0035] According to the present invention, preferably, the weight-average molecular weight of the polyacrylic acid is 3000 - 5000 g / mol. In the present invention, if the molecular weight of the polyacrylic acid is too small and the molecular chain is too short, its steric hindrance cannot prevent the Pd particles from entering the interior of the support, resulting in waste of Pd. If the molecular weight of the polyacrylic acid is too large and the molecular chain is too long, the loading amount will decrease and the catalytic activity of the catalyst will decline.

[0036] In the present invention, the weight-average molecular weight of the ammonium polyacrylate is not particularly limited, and the weight-average molecular weight is not used as an index to control the ammonium polyacrylate. In the present invention, the ammonium polyacrylate can play an anchoring role and at the same time can play a role in adjusting the pH value without introducing other elements. Preferably, the weight-average molecular weight of the ammonium polyacrylate is 5000 - 7000 g / mol.

[0037] In the present invention, the addition method of the polyacrylic acid compound is not particularly limited. The compound can be directly added or added after being dissolved in water.

[0038] According to the present invention, in the precursor solution, the addition amount of the amine ligand is not particularly limited. Preferably, the addition amount of the amine ligand is such that the pH value of the precursor solution is alkaline, and those skilled in the art can adaptively adjust the addition amount of the amine ligand according to the pH value of the precursor solution.

[0039] According to the present invention, preferably, the pH value of the precursor solution is 8 - 12, more preferably 9 - 11. Adjusting the pH value of the precursor solution to meet the above range limitation, if the pH value is too high, it will affect the distribution of Pd on the surface of the carrier and reduce the catalytic activity.

[0040] According to the present invention, preferably, the amine ligand is ethylenediamine and / or ammonia water, more preferably ethylenediamine. In the present invention, the above amine ligand is used to play a coordination role on the one hand and to adjust the pH on the other hand, without the need to additionally introduce a pH regulator.

[0041] In the present invention, the amine ligand is preferably ethylenediamine, which has a good coordination effect and will not produce precipitation when added to the precursor solution.

[0042] According to the present invention, preferably, in the precursor solution, the mass concentration of the Pd source is 0.03 - 0.1 wt%, more preferably 0.04 - 0.08 wt%.

[0043] According to the present invention, the type of the Pd source is not particularly limited and can be a conventional water-soluble Pd compound in the art. Preferably, the Pd source is selected from at least one of chloropalladic acid, palladium chloride, and palladium nitrate, more preferably chloropalladic acid.

[0044] In the present invention, the solvent of the precursor solution is water.

[0045] In the present invention, the preparation method of the precursor solution is not particularly limited. The polyacrylic acid compound, the amine ligand, and the Pd source can be added together or separately. According to a preferred embodiment of the present invention, an aqueous solution containing the Pd source is prepared, the polyacrylic acid compound is added, and then the amine ligand is added to make the pH value of the precursor solution alkaline.

[0046] According to the present invention, preferably, the mass ratio of the carrier to the precursor solution is 1:2 - 6, such as 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, 1:5, 1:5.5, 1:6, or the range between any two of them, more preferably 1:3 - 5.

[0047] In the present invention, the shape and source of the carrier are not particularly limited. It can be commercially purchased or prepared by using existing methods. Those skilled in the art can select a conventional carrier shape.

[0048] According to the present invention, preferably, the addition amounts of the carrier and the precursor solution are such that in the prepared catalyst, based on the total mass of the catalyst, the mass percentage of Pd is 0.1 - 0.2 wt%, more preferably 0.15 - 0.2 wt%; the mass percentage of the carrier is 99.8 - 99.9 wt%, more preferably 99.8 - 99.85 wt%.

[0049] In the present invention, the mass percentage content of Pd in the catalyst is obtained by testing with an inductively coupled plasma emission spectrometer (ICP).

[0050] According to the present invention, preferably, the carrier comprises alumina, optional Na and optional Si.

[0051] In the present invention, unless otherwise specified, the "optional" means containing or not containing, adding or not adding, adopting or not adopting. Specifically, the optional Na means that the carrier may contain Na or may not contain Na.

[0052] According to the present invention, preferably, the mass percentage content of Na in the carrier is 0-1 wt%. In the present invention, the mass percentage content of Na in the carrier is obtained by surface scanning test with a scanning electron microscope (SEM).

[0053] According to the present invention, preferably, the mass percentage content of Si in the carrier is 0-16 wt%, preferably 8-16 wt%. In the present invention, the additional introduction of Si can adjust the acidity of the carrier and increase the number of Pd anchoring sites.

[0054] In the present invention, there is no particular limitation on the introduction method of Si in the carrier. According to a preferred embodiment of the present invention, Si in the carrier is introduced by adding silica sol during the kneading process of the alumina carrier.

[0055] In the present invention, the pore diameter of the carrier is 5-25 nm, preferably 8-15 nm. Selecting a carrier with a larger pore diameter can improve the stability of the catalyst, but if the pore diameter is too large, the bulk weight of the catalyst will decrease and the strength will decline. The pore diameter of the carrier is obtained by BET nitrogen adsorption method.

[0056] According to the present invention, there is no particular limitation on the impregnation conditions, and those skilled in the art can adaptively select conventional impregnation conditions to ensure that Pd is impregnated onto the carrier. Preferably, the impregnation conditions include: at room temperature, the impregnation time is 20-80 min, preferably 30-60 min.

[0057] In the present invention, preferably, the impregnation process further includes stirring and / or ultrasonic treatment, and there is no particular limitation on the stirring conditions and ultrasonic conditions, and those skilled in the art can select conventional stirring conditions and ultrasonic conditions.

[0058] According to the present invention, preferably, step (2) further includes a process of drying the impregnated product.

[0059] According to the present invention, the drying method and drying conditions are not particularly limited, and those skilled in the art can select conventional drying methods and drying conditions. According to a preferred embodiment of the present invention, after the impregnated product is filtered, it is dried. Preferably, the drying conditions include: the drying temperature is 100-120°C; the drying time is 1-4 h.

[0060] According to the present invention, preferably, the calcination conditions include: the calcination temperature is 400-550°C, preferably 420-550°C; the calcination time is 2-6 h, preferably 3-5 h. Preferably, the calcination is carried out in an air atmosphere.

[0061] The third aspect of the present invention provides a Pd catalyst prepared by the preparation method described in the second aspect.

[0062] According to the present invention, preferably, the catalyst comprises a carrier and Pd nanoparticles. Based on the total mass of the catalyst, the mass percentage content of the Pd nanoparticles is 0.1-0.2 wt%, preferably 0.15-0.2 wt%, and the mass percentage content of the carrier is 99.8-99.9 wt%, preferably 99.8-99.85 wt%.

[0063] In the present invention, for the Pd catalyst prepared by the preparation method described in the second aspect, the Pd nanoparticles are uniformly loaded on the surface of the carrier. While reducing the Pd content in the catalyst, it ensures that the catalyst has high catalytic activity and significantly reduces the production cost.

[0064] The fourth aspect of the present invention provides an application of the Pd catalyst described in the first aspect or the third aspect in the hydrocracking of pyrolysis gasoline.

[0065] In the present invention, the above Pd catalyst is used in the hydrocracking of pyrolysis gasoline, has high catalytic activity and good sulfur resistance, and can meet the requirements of long-term operation of the catalyst.

[0066] According to the present invention, the conditions for the hydrocracking of pyrolysis gasoline are not particularly limited, and those skilled in the art can select conventional conditions for the hydrocracking of pyrolysis gasoline. Preferably, the conditions for the hydrocracking of pyrolysis gasoline include: the inlet temperature is 40-50°C, the pressure is 2-4 MPa, and the volume space velocity is 3-5 h -1 .

[0067] According to the present invention, preferably, the distillation range of the pyrolysis gasoline is 50-204°C, the diolefin value is 15-30 gI 2 / 100 g of oil, the bromine value is 20-40 gBr 2 / 100 g of oil, and the gum content is 0-100 ppm.

[0068] In the present invention, the diolefin value of pyrolysis gasoline is measured by the maleic anhydride method (UOP326-65); the bromine value is measured by a moisture meter, and the gum content is measured by the method of drying and weighing.

[0069] In the present invention, before the Pd catalyst is used in the pyrolysis gasoline hydrogenation reaction, it further includes a step of reducing the Pd catalyst, and the reduction conditions are conventional catalyst reduction conditions in the art and are not particularly limited.

[0070] According to a particularly preferred embodiment of the present invention, a method for preparing a Pd catalyst, the method comprising the following steps:

[0071] (1) Prepare a precursor solution containing a polyacrylic acid compound, an amine ligand, and a Pd source, and the precursor solution is alkaline;

[0072] (2) Immerse the carrier in the precursor solution and calcine to obtain a Pd catalyst;

[0073] The mass ratio of the carrier to the polyacrylic acid compound in the precursor solution is 1:0.01-0.02;

[0074] The polyacrylic acid compound is selected from at least one of polyacrylic acid, sodium polyacrylate, and ammonium polyacrylate;

[0075] The pH value of the precursor solution is 9-11;

[0076] The amine ligand is ethylenediamine.

[0077] The present invention will be described in detail below through examples and comparative examples. In the following examples and comparative examples, unless otherwise specified, the reagents used in the present invention are all commercially available;

[0078] The test methods for the mass percentage content of Pd, the diolefin value, the bromine value, and the gum content of pyrolysis gasoline in the catalyst are described in the foregoing specification and will not be elaborated here.

[0079] Example 1

[0080] (1) Prepare 80 g of an aqueous solution of chloropalladic acid with a palladium content of 0.05 wt%, adjust the pH to 10.5 by adding ethylenediamine, and then add 1 g of 40 wt% ammonium polyacrylate (Aladdin product number P304873) to obtain a precursor solution;

[0081] (2) Take 20 g of an alumina carrier (the mass percentage content of Na is 1 wt% and the mass percentage content of Si is 8 wt%), slowly add the precursor solution to the carrier for impregnation, stir at room temperature for 45 min, filter the impregnated product, dry at 120 °C for 2 h, and calcine at 500 °C for 4 h to obtain a catalyst.

[0082] Figure 1 is the XRD pattern of the Pd catalyst prepared in Example 1. It can be seen from Figure 1 that there are no large Pd particles in the catalyst, and the Pd particles are relatively small.

[0083] Figure 2 is the TEM image of the Pd catalyst prepared in Example 1. Figure 2 (a) is the bright-field image. Figure 2 (b) is the dark-field image. It can be seen from Figure 2 (a) that after adding the ammonium polyacrylate additive, the polymer additive is the anchoring site of Pd, and the Pd nanoparticles are distributed in a strip shape with small particle sizes. Figure 2 It can be seen from (b) that the Pd particles are dispersedly distributed on the surface of the carrier with good dispersion.

[0084] Figure 3 is the appearance image of the Pd catalyst prepared in Example 1. It can be seen from Figure 3 that the Pd on the surface of the carrier is evenly dispersed.

[0085] Example 2

[0086] (1) Prepare an 80 g chloro-palladic acid-aqueous solution with a palladium content of 0.08 wt%, add ethylenediamine to adjust the pH to 10.5, and add 1 g of 40 wt% ammonium polyacrylate (Aladdin catalog number P304873) to obtain a precursor solution;

[0087] (2) Take 20 g of alumina carrier (the mass percentage content of Na is 1 wt% and the mass percentage content of Si is 8 wt%), slowly add the precursor solution to the carrier for impregnation, stir at room temperature for 45 min, filter the impregnated product, dry at 120 °C for 2 h, and calcine at 550 °C for 4 h to obtain the catalyst.

[0088] Example 3

[0089] Prepare the catalyst according to the method of Example 1, except that in step (1), ammonia water is added instead of ethylenediamine to adjust the pH value of the precursor solution to 10.5 to obtain the catalyst.

[0090] Example 4

[0091] (1) Prepare an 80 g chloro-palladic acid-aqueous solution with a palladium content of 0.05 wt%, add ethylenediamine to adjust the pH to 10.5, and then add 1 g of 30 wt% polyacrylic acid (weight average molecular weight is 3000 g / mol) to obtain a precursor solution;

[0092] (2) Take 20 g of alumina support (mass percentage of Na is 1 wt%, mass percentage of Si is 8 wt%), slowly add the precursor solution to the support for impregnation, stir at room temperature for 45 min, filter the impregnated product, dry at 120 °C for 2 h, and calcine at 500 °C for 4 h to obtain the catalyst.

[0093] Example 5

[0094] Prepare the catalyst according to the method of Example 1, except that the addition amount of ethylenediamine in step (1) is adjusted to make the pH value of the precursor solution 12 to obtain the catalyst.

[0095] Example 6

[0096] Prepare the catalyst according to the method of Example 4, except that polyacrylic acid is changed to polyacrylic acid with an equal mass and a weight-average molecular weight of 2000 g / mol to obtain the catalyst.

[0097] Comparative Example 1

[0098] Prepare the catalyst according to the method of Example 1, except that ammonium polyacrylate is not added in step (1) to obtain the catalyst.

[0099] Figure 4 It is the TEM image of the Pd catalyst prepared in Comparative Example 1. Figure 4 (a) is the bright-field image. Figure 4 (b) is the dark-field image. It can be seen from Figure 4 (a) that the Pd particles on the surface of the Pd catalyst without additives do not have a strip-like distribution. Figure 4 It can be observed from (b) that some Pd particles have an agglomeration phenomenon.

[0100] Figure 5 It is the appearance image of the Pd catalyst prepared in Comparative Example 1. It can be seen from Figure 5 that the color of the catalyst surface is close to that of the bulk phase, indicating that the Pd particles enter the alumina bulk phase.

[0101] Comparative Example 2

[0102] Prepare the catalyst according to the method of Example 1, except that ammonium polyacrylate is changed to ammonium citrate to obtain the catalyst.

[0103] Comparative Example 3

[0104] Prepare the catalyst according to the method of Example 1, except that no amine ligand is added in step (1), and sodium hydroxide is used to adjust the pH, and a precipitate is formed in the impregnation solution. The catalyst obtained in Comparative Example 3 does not meet the requirements of the hydrogenation reaction, and the hydrogenation reaction is not carried out.

[0105] Test Example

[0106] The prepared catalyst was used for the hydrogenation reaction of pyrolysis gasoline. The starting pyrolysis gasoline feedstock had a diene value of 20 - 25 gI 2 / 100 g of oil, a bromine number of 30 - 35 gBr 2 / 100 g of oil, a distillation range of 50 - 204 °C, and a gum content of 28 ppm.

[0107] Before starting the feed in a fixed - bed reactor, the catalyst was reduced by purging with hydrogen at 60 °C.

[0108] The reaction conditions were an inlet temperature of 40 °C, a pressure of 2.7 MPa, a volume hourly space velocity of the pyrolysis gasoline of 3 h -1 , a recycle ratio of 3.0, and a volume hourly space velocity of hydrogen of 100 h -1 . Samples were taken after running stably for 24 h, and the results are shown in Table 1.

[0109] The residual Pd amount in the impregnating solution was measured by an inductively coupled plasma emission spectrometer (ICP).

[0110] Table 1

[0111]

[0112]

[0113] From the results in Table 1, it can be seen that when the catalyst prepared by the preparation method of the embodiment of the present invention was used for the hydrogenation reaction of pyrolysis petroleum, the diene index and bromine number index of the product were lower, and the catalytic activity was greatly improved compared with conventional alkaline impregnation.

[0114] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A Pd catalyst, characterized in that, the catalyst comprises a support and Pd nanoparticles, and based on the total mass of the catalyst, the mass percentage content of the Pd nanoparticles is 0.1 - 0.2 wt%, and the mass percentage content of the support is 99.8 - 99.9 wt%; wherein, the Pd nanoparticles are dispersedly loaded on the surface of the support.

2. The Pd catalyst according to claim 1, wherein, based on the total mass of the catalyst, the mass percentage content of the Pd nanoparticles is 0.15 - 0.2 wt%, and the mass percentage content of the support is 99.8 - 99.85 wt%; preferably, the Pd nanoparticles are anchored and dispersed on the surface of the support through a polyacrylic acid compound; preferably, the support comprises alumina, optional Na and optional Si.

3. A preparation method of a Pd catalyst, characterized in that, the method comprises the following steps: (1) Prepare a precursor solution containing a polyacrylic acid compound, an amine ligand and a Pd source, and the precursor solution is alkaline; (2) Immerse the support in the precursor solution and calcine to obtain the Pd catalyst.

4. The preparation method according to claim 3, wherein, the mass ratio of the support to the polyacrylic acid compound in the precursor solution is 1:0.005 - 0.03, preferably 1:0.01 - 0.02; preferably, the polyacrylic acid compound is selected from at least one of polyacrylic acid, sodium polyacrylate and ammonium polyacrylate; preferably, the weight-average molecular weight of the polyacrylic acid is 3000 - 5000 g / mol.

5. The preparation method according to claim 3 or 4, wherein, in the precursor solution, the addition amount of the amine ligand is such that the pH value of the precursor solution is alkaline; preferably, the pH value of the precursor solution is 8 - 12, preferably 9 - 11; preferably, the amine ligand is ethylenediamine and / or ammonia water, preferably ethylenediamine.

6. The preparation method according to any one of claims 3 - 5, wherein, in the precursor solution, the mass concentration of the Pd source is 0.03 - 0.1 wt%, preferably 0.04 - 0.08 wt%; preferably, the Pd source is selected from at least one of chloropalladic acid, palladium chloride and palladium nitrate, preferably chloropalladic acid.

7. The preparation method according to any one of claims 3 - 6, wherein, the mass ratio of the support to the precursor solution is 1:2 - 6, preferably 1:3 - 5; preferably, the support comprises alumina, optional Na and optional Si; preferably, the mass percentage content of Na in the support is 0 - 1 wt%; preferably, the mass percentage content of Si in the support is 0 - 16 wt%, preferably 8 - 16 wt%.

8. The preparation method according to any one of claims 3 - 7, wherein, The addition amounts of the carrier and the precursor solution are such that in the prepared catalyst, based on the total mass of the catalyst, the mass percentage content of the Pd nanoparticles is 0.1 - 0.2 wt%, preferably 0.15 - 0.2 wt%; the mass percentage content of the carrier is 99.8 - 99.9 wt%, preferably 99.8 - 99.85 wt%.

9. The preparation method according to any one of claims 3 - 8, wherein, the conditions of the impregnation include: at room temperature, the impregnation time is 20 - 80 min, preferably 30 - 60 min; Preferably, step (2) further includes a process of drying the impregnated product; Preferably, the drying conditions include: the drying temperature is 100 - 120 °C; the drying time is 1 - 4 h; Preferably, the calcination conditions include: the calcination temperature is 400 - 550 °C, preferably 420 - 550 °C; the calcination time is 2 - 6 h, preferably 3 - 5 h.

10. The Pd catalyst prepared by the preparation method according to any one of claims 3 - 8.

11. The application of the Pd catalyst according to any one of claims 1 - 2, 10 in the hydrocracking of pyrolysis gasoline; Preferably, the conditions of the hydrocracking of pyrolysis gasoline include: The inlet temperature is 40 - 50 °C, the pressure is 2 - 4 MPa, and the volumetric space velocity is 3 - 5 h -1 ; Preferably, the cracked gasoline has a distillation range of 50 - 204 °C, a diolefin value of 15 - 30 gI 2 / 100 g of oil, a bromine number of 20 - 40 gBr 2 / 100 g of oil, and a gum content of 0 - 100 ppm.

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