A photo-thermal synergistic catalyst for polyolefin cracking, a preparation method thereof and a polyolefin cracking method

Abstract

The application provides a photo-thermal synergistic catalyst for polyolefin cracking, a preparation method thereof and a polyolefin cracking method. The preparation method comprises the following steps: dropping a transition metal salt solution into carbon nanotubes, carrying out first drying and calcination treatment to obtain a calcination product; mixing the calcination product with a directing agent solution, adjusting the pH value to 9-10, and then adding a silicon source; after first centrifugal separation and refluxing to remove the directing agent, performing activation treatment on the obtained product to obtain the photo-thermal synergistic catalyst for polyolefin cracking. The application can load transition metal active sites on the carbon nanotube substrate and coat a porous silica protective layer, so that a catalyst with the advantages of photo-catalysis and thermal catalysis can be prepared, and the requirements of anti-carbon deposition and anti-sintering design can be met, thereby effectively preventing carbon deposition and sintering, prolonging the service life of the catalyst, and solving the problems of low catalytic efficiency and high energy consumption in polyolefin pyrolysis.

Description

Technical Field

[0001] This invention belongs to the field of photothermal catalytic decomposition technology for polyolefin plastics, specifically relating to a photothermal synergistic catalyst for polyolefin decomposition, its preparation method, and a polyolefin decomposition method. Background Technology

[0002] Currently, the efficient recycling of waste plastics is of great significance. Common methods for treating waste plastics include incineration, mechanical processing, biodegradation, chemical treatment, pyrolysis, and photocatalytic pyrolysis. However, each of these methods has its limitations and shortcomings. Incineration produces large amounts of harmful gases, causing environmental pollution. Mechanical processing cannot effectively decompose or recover its constituent materials, requiring further treatment through other methods. Biodegradation is costly and has stringent environmental requirements, resulting in low degradation efficiency. Chemical treatment requires large amounts of solvents, acids, alkalis, and other chemical reagents, generating significant amounts of wastewater and waste gas byproducts, causing secondary pollution and increased energy consumption. Pyrolytic pyrolysis is highly efficient, but requires high temperatures and consumes a lot of energy. Photolysis is environmentally friendly, but suffers from low decomposition efficiency.

[0003] In contrast, photothermal synergistic catalytic cracking of polyolefins is an effective method. It solves the problems of high energy consumption and high temperature requirements when using pyrolysis alone, as well as the problem of low efficiency when using photocatalysis alone. It can achieve the reaction under mild reaction conditions to obtain specific hydrocarbons with selectivity.

[0004] Currently, existing technologies utilize semiconductor materials, such as titanium dioxide, as photothermal catalyst components to absorb photon energy. These materials are widely used and researched due to their non-toxicity, stable physicochemical properties, and low cost. However, this material primarily responds to ultraviolet light with wavelengths below 387 nm, resulting in low light energy utilization. This necessitates the use of instruments to collect and supplement ultraviolet light during the reaction, increasing energy consumption. Furthermore, the use of TiO2 catalysts may activate additional oxidation reactions, causing partial oxidation of the plastic to CO2, thus affecting the composition of pyrolysis products and reducing efficiency. In addition, catalysts without anti-coking and anti-sintering designs may produce carbon deposits when reacting directly with the plastic, severely impacting the catalyst's lifespan.

[0005] Therefore, when designing catalysts for photothermal decomposition of polyolefins, how to provide a catalyst with high light energy utilization, good thermal conductivity, active catalytic sites, and the ability to meet the requirements of anti-coking and anti-sintering is an urgent problem to be solved. Summary of the Invention

[0006] To address the aforementioned technical problems, the present invention aims to provide a photothermal synergistic catalyst for polyolefin pyrolysis, its preparation method, and a polyolefin pyrolysis method.

[0007] To achieve the above objectives, the present invention provides a method for preparing a photothermal synergistic catalyst for polyolefin cracking, wherein the preparation method includes:

[0008] Step 1: A transition metal salt solution is dropped into carbon nanotubes, and after a first drying and calcination treatment, a calcined product is obtained; wherein the mass ratio of transition metal salt to carbon nanotubes is (0.05-0.2):(0.5-2.5);

[0009] Step 2: Mix the calcined product with the directing agent solution and adjust the pH to 9-10, then add the silicon source; after the first centrifugation and reflux to remove the directing agent, activate the obtained product to obtain a photothermal synergistic catalyst for polyolefin pyrolysis; wherein the mass ratio of the carbon nanotubes to the silicon source is 5:1 to 6:1, and the silicon source is calculated as silicon dioxide; the amount ratio of the directing agent to the silicon source is (0.15-0.2) g:(600-800) μL.

[0010] According to a specific embodiment of the present invention, preferably, the transition metal salt includes one or a combination of two or more of ruthenium chloride, nickel chloride, copper chloride, chloroplatinic acid, and palladium chloride; more preferably, it is ruthenium chloride. Among these, ruthenium metal has high catalytic activity and can effectively improve the degradation rate of polyolefins, exhibiting the best activity. Therefore, the addition of ruthenium metal has a dual function: on the one hand, it can further improve the overall blackness of the catalyst; on the other hand, it can act as an active site to catalyze the breaking and reforming of carbon-carbon and carbon-hydrogen bonds in plastics, demonstrating excellent activation ability.

[0011] According to a specific embodiment of the present invention, preferably, the solvent of the transition metal salt solution is an aqueous alcohol solution, wherein the volume ratio of water to ethanol in the aqueous alcohol solution is 1:1 to 3:1, and the amount of solvent added is 6-8 times the water absorption capacity of the carbon nanotubes.

[0012] In some specific implementations, preferably, the first drying conditions are vacuum drying at 80-90°C for 10-12 hours.

[0013] In some specific implementations, preferably, the process of dripping the transition metal salt solution into the carbon nanotube in step one further includes the following steps: dripping the transition metal salt solution into the carbon nanotube in 6-8 portions, with each dripping amount being equivalent to the water absorption capacity of the carbon nanotube, and stirring and drying after each dripping.

[0014] In some specific implementations, preferably, the third drying condition is vacuum drying at 80-90°C for 1-2 hours.

[0015] According to a specific embodiment of the present invention, preferably, the silicon source includes tetraethyl orthosilicate (TEOS) and / or silica aerogel. When silica aerogel is used, it is dissolved in ethanol and then added to the system as a solution.

[0016] According to a specific embodiment of the present invention, preferably, the silicon source is added as follows: 120-400 μL (more preferably 120-200 μL) is added dropwise every 30 minutes, followed by stirring at room temperature for 6-8 hours after the addition is complete. The present invention requires control of the silicon source addition rate. If the addition is too rapid, it will significantly affect the silica coating, resulting in silica being loaded onto the surface of the calcined product in the form of numerous silicon spheres. By using silica as an insulating material, the silica shell coats the carbon nanotube substrate and the ruthenium active sites, reducing the adhesion of carbon products to the catalyst and effectively preventing catalyst deactivation due to sintering and carbon buildup during the reaction.

[0017] According to a specific embodiment of the present invention, preferably, the directing agent is hexadecyltrimethylammonium bromide, the concentration of the directing agent solution is 1.5-3.1 mg / mL, and the solvent is a combination of water and ethanol with a volume ratio of 1:1 to 3.1:1.

[0018] In some specific embodiments, preferably, the carbon nanotubes are commercially available carbon nanotubes with a diameter of 10-15 nm. This invention uses carbon nanotubes as the substrate for the catalyst. Compared with other materials, carbon nanotubes have excellent light absorption and light energy utilization. Chiral carbon nanotubes absorb light in the 0-30 eV band, including the terahertz band, infrared band, visible light band, and a small portion of the ultraviolet band, exhibiting a wide absorption range and better absorption of visible light. This results in light absorption-induced temperature rise and high light energy utilization. Furthermore, carbon nanotubes have excellent thermal conductivity; their axial thermal conductivity exceeds that of most heat dissipation conductors, which is beneficial for uniform heating of the material, thereby ensuring reaction stability and reducing energy consumption. Therefore, using carbon nanotubes as the substrate enables natural light-assisted pyrolysis without the need for additional ultraviolet light, effectively increasing the local temperature of the catalyst and offering better cost-effectiveness and ease of use.

[0019] According to a specific embodiment of the present invention, preferably, the process of reflux removal of the directing agent is as follows: the product obtained by the first centrifugal separation is redispersed in a mixture of ethanol and concentrated hydrochloric acid, and refluxed at 80-85°C for 24-30 hours.

[0020] According to a specific embodiment of the present invention, preferably, in the mixture of ethanol and concentrated hydrochloric acid, the volume ratio of ethanol to hydrochloric acid is (10-15):1.

[0021] In some specific embodiments, preferably, the conditions for the first centrifugal separation are: centrifugation at 6000-8000 r / min for 5-10 min, followed by washing with 200-250 mL of ethanol.

[0022] In some specific embodiments, preferably, before the activation treatment, the product obtained by reflux is further subjected to a second centrifugal separation and a second drying step; more preferably, the conditions for the second centrifugal separation are: first washing with 200-250 mL of ethanol, and then centrifuging at a speed of 8000-9000 r / min for 5-15 min.

[0023] In some specific embodiments, preferably, the second drying conditions are drying at 75-100°C for 18-24 hours.

[0024] According to a specific embodiment of the present invention, preferably, the calcination treatment is carried out at 500-550°C for 4-8 hours.

[0025] According to a specific embodiment of the present invention, preferably, the activation treatment is performed under a nitrogen atmosphere at a temperature of 550-600°C for 1-2 hours.

[0026] In some specific embodiments, preferably, before the calcined product is mixed with the directing agent solution, an ultrasonic dispersion process is further included, specifically: the calcined product is ultrasonically dispersed in ethanol for 30-60 min to obtain a dispersion with a calcined product concentration of 0.5-1 mg / mL; more preferably, after the calcined product is mixed with the directing agent solution, ultrasonic treatment is continued for 30-60 min.

[0027] In some specific implementations, preferably, the pH adjustment process is carried out by using concentrated ammonia water and stirring for 30-40 minutes.

[0028] The present invention also provides a photothermal synergistic catalyst for polyolefin cracking, which is prepared by the above-described preparation method.

[0029] According to a specific embodiment of the present invention, preferably, the substrate of the photothermal synergistic catalyst is a carbon nanotube, on which a transition metal is loaded, wherein the loading amount of the transition metal on the carbon nanotube is 1wt%-20wt%; and a porous silica protective layer is coated on the carbon nanotube loaded with the transition metal.

[0030] The present invention also provides a method for polyolefin pyrolysis, wherein the steps of the method include:

[0031] Under a nitrogen atmosphere at normal pressure, polyolefins are brought into contact with a photothermal synergistic catalyst for pyrolysis, wherein the photothermal synergistic catalyst is the aforementioned photothermal synergistic catalyst for polyolefin pyrolysis.

[0032] According to a specific embodiment of the present invention, preferably, the pyrolysis reaction conditions are: reaction at a temperature of 150-300℃ under a 50W xenon lamp for 1 hour. The present invention, under xenon lamp irradiation, can utilize photo-assisted thermocatalysis technology to complete the pyrolysis reaction of polyolefins within a relatively low temperature range and efficiently obtain pyrolysis products.

[0033] According to a specific embodiment of the present invention, preferably, the polyolefin includes one or more of high-density polyethylene, low-density polyethylene, polypropylene, and polystyrene.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0035] The present invention provides a method for preparing a photothermal synergistic catalyst for polyolefin pyrolysis. By loading transition metal active sites onto a carbon nanotube substrate and coating it with a porous silica protective layer, a catalyst possessing the advantages of both photocatalysis and thermocatalysis can be prepared. When used for polyolefin pyrolysis, this catalyst, employing a photo-assisted thermocatalysis technology route, overcomes the limitations of traditional pyrolysis methods, significantly reducing the external heat source temperature required for polyolefin pyrolysis, thereby substantially reducing energy input. This method features a simple preparation process, low production cost, ease of operation, and abundant raw materials.

[0036] The photothermal synergistic catalyst for polyolefin pyrolysis provided by this invention can absorb photon energy and has good thermal conductivity. By using carbon nanotubes as a substrate and support, this photothermal synergistic catalyst achieves excellent photothermal conversion performance and thermal conductivity. While loading metal active catalytic sites, it also meets the requirements of anti-coking and anti-sintering design by coating with a silica protective layer, thereby effectively preventing coking and sintering, extending the service life of the catalyst, and solving the problems of low catalytic efficiency and high energy consumption in polyolefin pyrolysis. Attached Figure Description

[0037] Figure 1 SEM image of the photothermal synergistic catalyst for polyolefin pyrolysis prepared in Example 1.

[0038] Figure 2 SEM image of the photothermal synergistic catalyst for polyolefin pyrolysis prepared in Example 1.

[0039] Figure 3 This is a schematic diagram of the catalytic principle of the photothermal synergistic catalyst for polyolefin cracking provided by the present invention.

[0040] Figure 4 The distribution diagram of the pyrolysis products after the photothermal synergistic catalyst for polyolefin pyrolysis prepared in Example 1 is used to pyrolyze polyolefins. Detailed Implementation

[0041] In order to provide a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the technical solution of the present invention will now be described in detail below, but it should not be construed as limiting the scope of implementation of the present invention.

[0042] Example 1:

[0043] This embodiment provides a photothermal synergistic catalyst for polyolefin cracking, specifically including the following steps:

[0044] (1) Take 10 mg of commercially purchased carbon nanotubes with a diameter of 10-15 nm and determine the water absorption of the carbon nanotubes, which is recorded as x mL / g. Dissolve 0.01 mmol RuCl3 in a mixed solution of deionized water and ethanol (the volume ratio of water to ethanol is 2:1) to obtain a RuCl3 solution with a total volume of 6 times the water absorption of the carbon nanotubes. The mass ratio of dry RuCl3 to carbon nanotubes is 1:4.8.

[0045] (2) The active sites were loaded using the initial wet impregnation method. The RuCl3 solution was added to the carbon nanotubes in 6 drops, with the amount added each time being equivalent to the water absorption of the carbon nanotubes. After each addition, the mixture was stirred and dried under vacuum at 80°C for 1-2 hours. After the addition was completed, the mixture was dried under vacuum at 80°C overnight. Then, it was transferred to a muffle furnace and calcined at 500°C for 6 hours to obtain the calcined product, i.e., carbon nanotubes loaded with ruthenium, denoted as Ru-CNT, with a ruthenium loading of 10.1 wt%.

[0046] (3) Disperse 5.0 mg Ru-CNT in 10 mL of ethanol by ultrasonication for 30 min to obtain a Ru-CNT dispersion with a concentration of 0.5 mg / mL. Mix 165 mg cetyltrimethylammonium bromide (CTAB) with 50 mL of water and 16.3 mL of ethanol to obtain a directing agent solution. Add the directing agent solution to the Ru-CNT dispersion and sonicate for 30 min. Then add 550 μL of concentrated ammonia water and stir gently for 30 min to adjust the pH of the system to 10.

[0047] (4) Add tetraethyl orthosilicate to the solution obtained in step (3), and control the dripping rate to 180 μL every 30 min, for a total of 720 μL. After the dripping is completed, stir at room temperature for 6 h; then perform the first centrifugation separation under the following conditions: centrifuge at 6000 r / min for 5 min, and wash with 200 mL of ethanol.

[0048] (5) The product obtained in step (4) was dispersed in a mixture of 30 mL methanol and 2 mL concentrated hydrochloric acid (concentration 30%) and refluxed at 80 °C for 24 h to remove CTAB; then a second centrifugal separation was performed under the following conditions: first wash with 210 mL ethanol, then centrifuge at 8000 r / min for 5 min; the centrifuged product was dried at 85 °C for 24 h and activated at 600 °C for 1 h to obtain a photothermal synergistic catalyst for polyolefin cracking.

[0049] The SEM image of the photothermal synergistic catalyst for polyolefin pyrolysis prepared in this embodiment is as follows: Figure 1 and Figure 2 As shown, from Figure 1 and Figure 2 As can be seen, the skeleton structure of the wrinkled sheets is carbon nanotubes, the wrinkled sheets are a protective shell of silica, and the spheres are aggregates of excess silica.

[0050] The catalytic principle diagram of the photothermal synergistic catalyst prepared in this embodiment is as follows: Figure 3 As shown, from Figure 3 As can be seen, under the irradiation of the light source, the photothermal synergistic catalyst can absorb photon energy and convert it into heat. Then, when the polyolefin melts and diffuses, it can undergo cracking and desorption under the action of the photothermal synergistic catalyst.

[0051] Example 2:

[0052] This embodiment provides a photothermal synergistic catalyst for polyolefin cracking, specifically including the following steps:

[0053] (1) Take 100 mg of commercially purchased carbon nanotubes with a diameter of 10-15 nm and determine the water absorption of the carbon nanotubes, which is recorded as x mL / g. Dissolve 0.05 mmol RuCl3 in a mixed solution of deionized water and ethanol (the volume ratio of water to ethanol is 2:1) to obtain a RuCl3 solution with a total volume of 6 times the water absorption of the carbon nanotubes. The mass ratio of dry RuCl3 to carbon nanotubes is 1:9.6.

[0054] (2) The active sites were loaded using the initial wet impregnation method. The RuCl3 solution was added to the carbon nanotubes in 6 drops, with the amount added each time being equivalent to the water absorption of the carbon nanotubes. After each addition, the mixture was stirred and dried under vacuum at 80°C for 1-2 hours. After the addition was completed, the mixture was dried under vacuum at 80°C overnight. Then, it was transferred to a muffle furnace and calcined at 500-550°C for 6 hours to obtain the calcined product, i.e., the carbon nanotubes loaded with ruthenium, denoted as Ru-CNT, with a ruthenium loading of 5 wt%.

[0055] (3) Disperse 5.0 mg Ru-CNT in 10 mL of ethanol by ultrasonication for 30 min to obtain a Ru-CNT dispersion with a concentration of 0.5 mg / mL. Mix 165 mg cetyltrimethylammonium bromide (CTAB) with 50 mL of water and 16.3 mL of ethanol to obtain a directing agent solution. Add the directing agent solution to the Ru-CNT dispersion and sonicate for 30 min. Then add 550 μL of concentrated ammonia water and stir gently for 30 min to adjust the pH of the system to 9.

[0056] (4) Add tetraethyl orthosilicate to the solution obtained in step (3), and control the dripping rate to 180 μL every 30 min, for a total of 720 μL. After the dripping is completed, stir at room temperature for 6 h; then perform the first centrifugation separation under the following conditions: centrifuge at 6000 r / min for 5 min, and wash with 200 mL of ethanol.

[0057] (5) The product obtained in step (4) was dispersed in a mixture of 30 mL methanol and 2 mL concentrated hydrochloric acid (concentration 30%) and refluxed at 80 °C for 24 h to remove CTAB; then a second centrifugal separation was performed under the following conditions: first wash with 210 mL ethanol, then centrifuge at 8000 r / min for 5 min; the centrifuged product was dried at 85 °C for 24 h and activated at 600 °C for 1 h to obtain a photothermal synergistic catalyst for polyolefin cracking.

[0058] Comparative Example 1:

[0059] This comparative example provides a photothermal synergistic catalyst for polyolefin cracking without a silica protective layer, specifically including the following steps:

[0060] (1) Take 10 mg of commercially purchased carbon nanotubes with a diameter of 10-15 nm and determine the water absorption of the carbon nanotubes, which is recorded as x mL / g. Dissolve 0.01 mmol RuCl3 in a mixed solution of deionized water and ethanol (the volume ratio of water to ethanol is 1:3) to obtain a RuCl3 solution with a total volume of 6-8 times the water absorption of the carbon nanotubes. The mass ratio of dry RuCl3 to carbon nanotubes is 1:4.8.

[0061] (2) The active sites were loaded using the initial wet impregnation method. The RuCl3 solution was added to the carbon nanotubes in 6-8 drops, with the amount added each time being equivalent to the water absorption of the carbon nanotubes. After each addition, the mixture was stirred and dried under vacuum at 80°C for 1-2 hours. After the addition was completed, the mixture was dried under vacuum at 80°C overnight. Then, it was transferred to a muffle furnace and calcined at 500-550°C for 6 hours to obtain the calcined product, i.e., carbon nanotubes loaded with ruthenium, denoted as Ru-CNT, with a ruthenium loading of 10.1 wt%.

[0062] (3) Disperse 5.0 mg Ru-CNT in 10 mL of ethanol by ultrasonication for 30 min to obtain a Ru-CNT dispersion with a concentration of 0.5 mg / mL;

[0063] (4) The dispersion obtained in step (3) was centrifuged at 8000 r / min for 5 min and washed with 200 mL of ethanol. The centrifuged product was dried at 85 °C for 24 h and activated at 600 °C for 1 h to obtain Ru-CNT catalyst without silica protective layer.

[0064] Comparative Example 2:

[0065] This comparative example provides a catalyst for polyolefin cracking, specifically: commercially available carbon nanotubes with a diameter of 10-15 nm are directly used as catalysts for photothermal synergistic catalytic reactions.

[0066] Test example:

[0067] The catalyst used in the photothermal synergistic catalysis process should have good thermal conductivity and light absorption properties, and be able to catalyze the cracking reaction. This test example investigates the photothermal synergistic catalytic performance of the catalysts prepared in Example 1 and Comparative Examples 1-2 for polyolefin cracking, as detailed below:

[0068] The catalysts prepared in Example 1 and Comparative Examples 1-2 were used to pyrolyze high-density polyethylene (HDPE) under photothermal synergistic heating conditions at 200°C and a 50W xenon lamp for 1 hour. The reaction was carried out in an inert atmosphere of nitrogen at atmospheric pressure. During the reaction, gaseous products were collected using a gas bag, and oil phase products were collected by water condensation. The gaseous products were quantified by passing them through a pre-calibrated gas chromatograph with nitrogen as an internal standard. The oil phase products were first dissolved in a chloroform solution, and cyclohexane was added as an internal standard before quantitative analysis by gas chromatography-mass spectrometry.

[0069] In this study, after catalytic reaction using the catalyst prepared in Example 1, the distribution of the pyrolysis products of high-density polyethylene was as follows: Figure 4 As shown. From Figure 4 As can be seen from the figure, for cracking products with different carbon numbers, the photothermal synergistic catalyst for polyolefin cracking prepared in Example 1 has good C5-C7 selectivity (as shown in the dotted line graph), and the overall mass conversion of HDPE (as shown in the bar graph) is greater than 10%, of which C5 hydrocarbons are the main products, with a yield of about 5%.

[0070] In contrast, none of the catalysts in Comparative Examples 1-2 showed any catalytic effect on HDPE, with a mass conversion rate of 0%.

[0071] Furthermore, the temperatures of the catalysts provided in Example 1 and Comparative Example 1 were tested under simulated illumination conditions of 200°C and a 50W xenon lamp. The results showed that the temperature of the photothermal synergistic catalyst for polyolefin cracking prepared in Example 1 could rise to 400°C, while the temperature of the Ru-CNT catalyst in Comparative Example 1 was only 230°C.

[0072] This demonstrates that in the photothermal decomposition of polyolefins, a suitable catalyst can effectively increase the temperature of the active sites, assist in heating, reduce the required external heating temperature, and significantly accelerate the reaction rate. Furthermore, under light irradiation, the plastic and catalyst transition to localized liquid-phase contact, resulting in more uniform dispersion. Therefore, compared to single pyrolysis or photolysis, the photothermal synergistic catalyst provided by this invention helps to further decompose the plastic into products with smaller molecular weights.

Claims

1. A method for preparing a photothermal synergistic catalyst for polyolefin cracking, wherein, The preparation method includes: Step 1: A transition metal salt solution is dropped into carbon nanotubes, and after a first drying and calcination treatment, the calcined product is obtained; The mass ratio of transition metal salt to carbon nanotubes is (0.05-0.2):(0.5-2.5). Step 2: Mix the calcined product with the directing agent solution, adjust the pH to 9-10, and then add a silicon source; after the first centrifugal separation and reflux to remove the directing agent, the obtained product is activated to obtain a photothermal synergistic catalyst for polyolefin cracking; The mass ratio of the carbon nanotubes to the silicon source is 5:1 to 6:1, and the silicon source is calculated as silicon dioxide. The ratio of the directing agent to the silicon source is (0.15-0.2) g : (600-800) μL; The transition metal salt is ruthenium chloride; The activation treatment is performed under a nitrogen atmosphere at a temperature of 550-600°C for 1-2 hours.

2. The preparation method according to claim 1, wherein, The solvent for the transition metal salt solution is an aqueous alcohol solution, wherein the volume ratio of water to ethanol in the aqueous alcohol solution is 1:1 to 3:1, and the amount of solvent added is 6-8 times the water absorption capacity of the carbon nanotubes.

3. The preparation method according to claim 1, wherein, The silicon source includes tetraethyl orthosilicate.

4. The preparation method according to claim 1, wherein, The silicon source is added as follows: 120-400 microliters are added dropwise every 30 minutes, and the mixture is stirred at room temperature for 6-8 hours after the addition is complete.

5. The preparation method according to claim 1, wherein, The directing agent is hexadecyltrimethylammonium bromide, the concentration of the directing agent solution is 1.5-3.1 mg / mL, and the solvent is a combination of water and ethanol with a volume ratio of 1:1 to 3.1:

1.

6. The preparation method according to claim 1, wherein, The process of removing the directing agent by reflux is as follows: the product obtained by the first centrifugation is dispersed again in a mixture of ethanol and concentrated hydrochloric acid, and refluxed at 80-85℃ for 24-30h.

7. The preparation method according to claim 6, wherein, In the mixture of ethanol and concentrated hydrochloric acid, the volume ratio of ethanol to hydrochloric acid is (10-15):

1.

8. The preparation method according to claim 1, wherein, The calcination treatment conditions are calcination at 500-550℃ for 4-8 hours.

9. A photothermal synergistic catalyst for polyolefin cracking, which is prepared by the preparation method according to any one of claims 1-8.

10. The photothermal synergistic catalyst according to claim 9, wherein, The substrate of this photothermal synergistic catalyst is carbon nanotubes, on which transition metals are loaded at a loading amount of 1wt%-20wt%. A porous silica protective layer is coated onto carbon nanotubes loaded with transition metals.

11. A method for pyrolyzing polyolefins, wherein, The steps of this method include: Under a nitrogen atmosphere at normal pressure, polyolefins are brought into contact with a photothermal synergistic catalyst for pyrolysis, wherein the photothermal synergistic catalyst is the photothermal synergistic catalyst for polyolefin pyrolysis as described in claim 9 or 10.

12. The polyolefin pyrolysis method according to claim 11, wherein, The pyrolysis reaction conditions are: reacting at a temperature of 150-300℃ and under a 50-200W xenon lamp for 1-3 hours.

13. The polyolefin pyrolysis method according to claim 11, wherein, The polyolefin includes one or more of high-density polyethylene, low-density polyethylene, polypropylene, and polystyrene.

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