Method for preparing light aromatic hydrocarbon through catalytic pyrolysis of waste polyolefin plastic

Abstract

The invention belongs to the technical field of waste plastic treatment, and particularly relates to a method for preparing light aromatic hydrocarbon through catalytic pyrolysis of waste polyolefin plastic. According to the invention, a microporous molecular sieve ZSM-5 is coated in an ordered mesoporous layer SBA-15 by using a super-dilute liquid phase coating method, so that the coated catalyst ZSM-5-coated SBA-15 with the packaging thickness of 60-185 nm is prepared. The coated catalyst has an ordered mesoporous encapsulation layer, and can be pre-cracked and enrich plastic pyrolysis volatile components in the reaction, so that the mass transfer rate is increased, the generation of light aromatic hydrocarbons is promoted, the generation of macromolecular chain hydrocarbon substances in the catalytic pyrolysis process is inhibited, and the efficient preparation of light aromatic hydrocarbons by catalytic pyrolysis of waste polyolefin plastics is realized. Compared with the traditional ZSM-5, when the packaging thickness of the ZSM-5-coated SBA-15 is 90nm, the yield of light aromatic hydrocarbon is 31.02 wt.%, the amplification is up to 49.14%, and meanwhile, the heavy chain hydrocarbon byproduct is reduced by 42.17%.

Description

Technical Field

[0001] The present invention belongs to the technical field of waste plastic treatment, and particularly relates to a method for catalytic pyrolysis of waste polyolefin plastics to produce light aromatics. Background Art

[0002] Plastics are indispensable in the global economy and are widely used in industries such as electronics, transportation, construction, packaging, and healthcare. Among all types of plastics, polyolefins (including polyethylene and polypropylene) account for more than 50% of the global polymer production. More than 75% of plastics are discarded after single use, and due to their short-term use characteristics, most plastics are not recycled. It is estimated that about 381 million tons of plastic waste were generated in 2015, and it is expected to double by 2034. The generation and improper treatment of a large amount of plastic waste pose a serious threat to the environment, including water and soil quality as well as biological health. The traditional way to treat plastic waste is incineration to recover energy, but it will lead to greenhouse gas emissions and the generation of harmful pollutants. It is worth noting that plastics, as important carbon resources, using catalytic technology to directly convert them into high-value-added chemicals is of great significance for the rational utilization of resources and environmental protection.

[0003] Catalytic pyrolysis using microporous zeolites such as ZSM-5 as catalysts is an effective method to upgrade plastic waste to high-value-added chemicals (such as aromatics). Among these high-value-added compounds, light aromatics (BTEX) such as benzene, toluene, ethylbenzene, and xylene are highly favored because they play a key role in the production of pharmaceuticals, cosmetics, pesticides, and as gasoline octane number enhancers. It is worth noting that compared with fossil-based BTEX, BTEX from plastic waste has less impact on the environment, with greenhouse gas emissions reduced by about 12%. However, existing methods for catalytic pyrolysis of waste polyolefin plastics still have technical bottlenecks such as poor BTEX selectivity, too high a proportion of heavy chain hydrocarbons (C 10+ chain hydrocarbons), and serious catalyst coking deactivation. Therefore, there is an urgent need to develop a method for efficient production of light aromatics by catalytic pyrolysis of waste polyolefin plastics. Summary of the Invention

[0004] In order to overcome the above deficiencies of the prior art, the present invention synthesized a coated composite molecular sieve catalyst ZSM-5@SBA-15 in an acidic medium by an ultra-dilute liquid phase coating method and used it for the catalytic pyrolysis of polypropylene (PP) to produce light aromatics. This catalyst can pre-pyrolyze heavy volatiles and enrich chain hydrocarbon intermediates through the coating layer, thereby greatly increasing the yield of light aromatics in the catalytic pyrolysis process of waste polyolefin plastics.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] The present invention provides a method for catalytic pyrolysis of waste polyolefin plastics to produce light aromatics, specifically: first, the microporous molecular sieve ZSM-5 is coated in the ordered mesoporous layer SBA-15 by the ultra-dilute liquid phase coating method to prepare the coated catalyst ZSM-5@SBA-15, and then it is put into waste polyolefin plastics, and the waste polyolefin plastics are catalytically pyrolyzed at high temperature to obtain light aromatics.

[0007] The coated catalyst prepared by the present invention has an ordered mesoporous coating layer, which can locally enrich the volatile components of waste polyolefin plastics during the reaction, and at the same time improve the mass transfer rate. Through the synergistic effect of the two, it promotes the generation of light aromatics and inhibits the generation of macromolecular hydrocarbon substances during the catalytic pyrolysis of waste polyolefin plastics, realizing the efficient production of light aromatics by catalytic pyrolysis of waste polyolefin plastics. Compared with the traditional ZSM-5, when the encapsulation thickness of the coated catalyst ZSM-5@SBA-15 is 90 nm, the highest yield of light aromatics is 31.02 wt.%, with an increase of up to 49.14%, and at the same time, the by-products of heavy chain hydrocarbons are reduced by 42.17%.

[0008] Preferably, the mass ratio of the coated catalyst ZSM-5@SBA-15 to the waste polyolefin plastics is 1-2:2-3.

[0009] Preferably, the pyrolysis is carried out at 550-700 °C with nitrogen as the carrier gas, and the flow rate of the carrier gas is 80-130 mL / min.

[0010] Preferably, the waste polyolefin plastics are polypropylene.

[0011] Preferably, the preparation of the coated catalyst ZSM-5@SBA-15 includes the following steps:

[0012] S1. Dissolve tetrapropylammonium hydroxide, tetraethyl orthosilicate and NaAlO 2 in water, carry out hydrothermal crystallization, wash to neutrality, and then obtain Na-type ZSM-5 molecular sieve through drying and calcination;

[0013] S2. Perform ammonium exchange on the Na-type ZSM-5 molecular sieve with NH 4 Cl solution, dry and then calcine to obtain H-type ZSM-5 molecular sieve;

[0014] S3. Dissolve P123 and MgSO 4 in the HCl solution, then add the H-type ZSM-5 molecular sieve and TEOS, mix well, place the obtained suspension at 90-120 °C for reaction for 20-30 h, wash to neutrality after the reaction, and then obtain the coated catalyst ZSM-5@SBA-15 through drying and calcination.

[0015] More preferably, in the coated catalyst ZSM-5@SBA-15, the encapsulation thickness of ZSM-5 is 60-185 nm.

[0016] In the present invention, the microporous molecular sieve ZSM-5 is encapsulated in the ordered mesoporous layer SBA-15 by the ultra-dilute liquid phase encapsulation method to prepare the coated catalyst ZSM-5@SBA-15 with different encapsulation thicknesses, and it is applied to the catalytic pyrolysis of waste polyolefin plastics to light aromatics, which can provide cooperative acidic centers and composite pore structures, and is beneficial to obtaining light aromatic hydrocarbon products with high content. In addition, by adjusting the encapsulation thickness of the coated catalyst, the encapsulation thickness at the optimal light aromatic hydrocarbon yield is 90 nm.

[0017] More preferably, in S1, the temperature of the hydrothermal crystallization is 160-180 °C and the crystallization time is 2-4 days.

[0018] More preferably, in S1, S2, and S3, the calcination temperature is 500-600 °C and the time is 5-7 h.

[0019] More preferably, in S2, the concentration of the NH 4 C1 solution is 1.0-2.0 M, the temperature of the ammonium exchange is 70-90 °C, and the time is 3-5 h.

[0020] More preferably, in S3, the concentration of the HCl solution is 1-3 M, and the concentration of P123 in the HCl solution is 0.45-0.7 g / 200-400 mL.

[0021] Compared with the prior art, the beneficial effects of the present invention are:

[0022] The present invention discloses a method for catalytic pyrolysis of waste polyolefin plastics to light aromatics. First, the microporous molecular sieve ZSM-5 is encapsulated in the ordered mesoporous layer SBA-15 by the ultra-dilute liquid phase encapsulation method to prepare the coated catalyst ZSM-5@SBA-15 with different encapsulation thicknesses, and then it is applied to the catalytic pyrolysis of waste polyolefin plastics to light aromatics. The coated catalyst ZSM-5@SBA-15 provided by the present invention for the catalytic pyrolysis of waste polyolefin plastics to light aromatics has an ordered mesoporous coating layer, which can locally enrich the volatiles of waste polyolefin plastics during the reaction and improve the mass transfer rate at the same time. Through the synergistic effect of the two, the formation of light aromatics is promoted, and the formation of macromolecular hydrocarbon substances during the catalytic pyrolysis of waste polyolefin plastics is inhibited, thus significantly improving the light aromatic hydrocarbon yield. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1Transmission electron microscopy test images of the coated composite catalyst ZSM-5@SBA-15 with different coating thicknesses. (a) is ZSM-5@SBA-15(0.5), (b) is ZSM-5@SBA-15(1), (c) is ZSM-5@SBA-15(2), and (d) is ZSM-5@SBA-15(3).

[0024] Figure 2 Schematic diagram of the experimental process device for catalytic pyrolysis of waste polyolefin plastics to produce light aromatics;

[0025] Figure 2 In it: 1 - nitrogen gas cylinder, 2 - gas mixing flow device, 3 - quartz tube, 4 - timer, 5 - vertical tube furnace, 6 - hanging basket, 7 - collection bottle, 8 - washing bottle, 9 - washing bottle filled with pure water, 10, 11 - washing bottles filled with cotton, 12 - aluminum foil gas bag.

[0026] Figure 3 For the aromatics yield, light aromatics (BTEX) yield, heavy aromatics (C 9+ aromatics) yield, heavy chain hydrocarbons (C 10+ chain hydrocarbons) yield and the yields of benzene, toluene, ethylbenzene and xylene during the catalytic pyrolysis of waste polyolefin plastics when the coating thickness of the coated catalyst ZSM-5@SBA-15 is 60 nm.

[0027] Figure 4 For the aromatics yield, light aromatics (BTEX) yield, heavy aromatics (C 9+ aromatics) yield, heavy chain hydrocarbons (C 10+ chain hydrocarbons) yield and the yields of benzene, toluene, ethylbenzene and xylene during the catalytic pyrolysis of waste polyolefin plastics when the coating thickness of the coated catalyst ZSM-5@SBA-15 is 90 nm.

[0028] Figure 5 For the aromatics yield, light aromatics (BTEX) yield, heavy aromatics (C 9+ aromatics) yield, heavy chain hydrocarbons (C 10+ chain hydrocarbons) yield and the yields of benzene, toluene, ethylbenzene and xylene during the catalytic pyrolysis of waste polyolefin plastics when the coating thickness of the coated catalyst ZSM-5@SBA-15 is 130 nm.

[0029] Figure 6 For the aromatics yield, light aromatics (BTEX) yield, heavy aromatics (C 9+ aromatics) yield, heavy chain hydrocarbons (C 10+ chain hydrocarbons) yield and the yields of benzene, toluene, ethylbenzene and xylene during the catalytic pyrolysis of waste polyolefin plastics when the coating thickness of the coated catalyst ZSM-5@SBA-15 is 185 nm.

[0030] Figure 7 When the catalyst is the traditional microporous molecular sieve ZSM-5, the yields of aromatics, light aromatics (BTEX), heavy aromatics (C 9+ aromatics), heavy chain hydrocarbons (C 10+ chain hydrocarbons) and the yields of benzene, toluene, ethylbenzene and xylene in the catalytic pyrolysis of waste polyolefin plastics.

[0031] Figure 8 For the coated composite catalyst ZSM-5@SBA-15 and the traditional molecular sieve catalyst ZSM-5 prepared in Examples 1-4 and Comparative Example 1, the yields of light aromatics (BTEX) and heavy chain hydrocarbons (C 10+ chain hydrocarbons) in the catalytic pyrolysis of waste polyolefin plastics. Detailed implementation manners

[0032] The following further describes the detailed implementation manners of the present invention. It should be noted here that the description of these implementation manners is used to help understand the present invention, but does not constitute a limitation on the present invention. In addition, the technical features involved in the various implementation manners of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0033] The experimental methods in the following examples are all conventional methods unless otherwise specified, and the test materials used in the following examples are all available through conventional commercial channels unless otherwise specified.

[0034] The following examples and comparative examples respectively synthesize ZSM-5 and the coated catalyst ZSM-5@SBA-15 by the hydrothermal method and the ultra-dilute liquid phase coating method. The specific process is as follows:

[0035] (1) Weigh 26 g of H 2 O and 15.65 g of TPAOH (tetrapropylammonium hydroxide) and add them to a 100 mL polytetrafluoroethylene inner liner, and stir for 10 min until completely dissolved.

[0036] (2) Weigh 8.68 g of TEOS (tetraethyl orthosilicate) and add it to the solution in step (1), and continuously stir at a speed of 600 rpm for 6 h.

[0037] (3) Weigh 0.0344 g of NaAlO 2 and add it to the solution in step (2), and stir for 30 min, then transfer it to a stainless steel hydrothermal autoclave and crystallize at 170 °C for 3 days.

[0038] (4) Wash the suspension obtained in step (3) several times with a large amount of water until neutral, then dry it at 80 °C for 12 h, and then place the obtained powder in a muffle furnace at 550 °C and calcine it for 6 h to obtain Na-type ZSM-5 molecular sieve.

[0039] (5) The ZSM-5 powder obtained in step (4) is subjected to ammonium exchange with NH 4 Cl (1.0 M) solution at 80 °C for 4 h (mixing ratio: 1 g ZSM-5 / 50 mL solution), repeated twice. After drying, it is calcined at 550 °C for 6 h to obtain H-type ZSM-5 molecular sieve.

[0040] (6) Weigh 0.58 g of P123 (polyethylene oxide - polypropylene oxide - polyethylene oxide triblock copolymer) and add it to 300 mL of 2 M HCl solution, and stir until dissolved.

[0041] (7) Weigh 1.74 g of MgSO 4 and add it to the solution in step (6), and stir until completely dissolved.

[0042] (8) Weigh 1.5 g of the molecular sieve ZSM-5 powder obtained in step (5) and add it to the solution obtained in step (7), and then ultrasonicate for 30 min.

[0043] (9) Weigh 1.5 g of TEOS and drop it into the solution obtained in step (8), stir at 600 rpm for 24 h, and then transfer the suspension to an autoclave and react at 100 °C for 24 h.

[0044] (10) Wash the suspension obtained in step (9) several times with a large amount of water until neutral, then dry at 80 °C for 12 h, and then calcine the obtained powder at 550 °C for 6 h to obtain the coated catalyst ZSM-5@SBA-15 with a coating thickness of 90 nm.

[0045] According to the above method, by changing the amount of TEOS weighed in step (9) to 0.75 g, 3 g, and 4.5 g respectively, the coated catalysts ZSM-5@SBA-15 with coating thicknesses of 60 nm, 130 nm, and 185 nm can be prepared.

[0046] From Figure 1 it can be observed that the coating thickness of the coated catalyst ZSM-5@SBA-15 increases with the increase in the addition amount of the silicon source (TEOS), indicating that the coating thickness can be regulated.

[0047] Examples and comparative examples:

[0048] The coated catalyst ZSM-5@SBA-15 is used for the catalytic pyrolysis of waste polyolefin plastics to produce light aromatics. The following takes polypropylene as an example for testing.

[0049] 1. Catalytic pyrolysis of waste polyolefin plastics to produce light aromatics

[0050] The schematic diagram of the experimental device for the catalytic pyrolysis of waste polyolefins is as Figure 2 shown, and the specific process is as follows:

[0051] (1) Weigh 0.5 g of the catalyst and 1 g of polypropylene separately. Additionally, weigh the quartz tube 3 and the collection bottle 7. Select nitrogen as the carrier gas (supplied by nitrogen cylinder 1), and control the gas flow rate at 100 mL / min using the gas mixing flow device 1;

[0052] (2) Put the catalyst weighed in step (1) into the quartz tube 3 and spread it evenly in the lower quartz layer. The polypropylene powder is placed in the quartz hanging basket 6 with an opening at the bottom and is suspended by a hook at the upper inner part (outside the heating zone) of the quartz tube 3;

[0053] (3) After heating the vertical tube furnace 5 to 600 °C, introduce nitrogen for 20 min to make the pyrolysis device reach a stable state;

[0054] (4) During the reaction, send the quartz hanging basket 6 into the heating zone for reaction for 60 min to fully react and collect the products. Both the pyrolysis reaction and the catalytic reaction occur in the heating zone in the middle section of the tube furnace cavity;

[0055] (5) During the pyrolysis process, set the pyrolysis temperature at 600 °C. The waste polyolefin plastic is placed in the quartz hanging basket. The condensable vapor is purged by the carrier gas and collected at low temperature (-20 °C) in a collection bottle 7 containing an absorbent liquid (20 mL of dichloromethane) and an empty collection bottle. The tail gas passes through an empty washing bottle 8 (to prevent backflow), a washing bottle 9 filled with pure water (to observe the gas velocity stability through bubbles), and two washing bottles 10 and 11 filled with cotton (to filter the tail gas), and is then collected using an 8 L aluminum foil gas bag 12;

[0056] (6) After the reaction, collect the waste catalyst in the quartz tube 3 and weigh the dried masses of the quartz tube 3 and the collection bottle 7.

[0057] According to the above process, set up 4 examples and 1 comparative example. The types of catalysts used and their encapsulation thicknesses are shown in Table 1.

[0058] Table 1 Conditions of each example and comparative example

[0059]

[0060] 2. Detection of the aromatic hydrocarbon and hydrocarbon contents in the products and calculation of the yields

[0061] Detect the products collected in each of the above examples and comparative examples to determine the aromatic hydrocarbon content therein. The specific process is as follows:

[0062] (1) Dissolve and dilute the collected products with dichloromethane and make the volume constant at 20 mL;

[0063] (2) Measure 50 μL of the solution in step (1), make the volume constant at 0.5 mL with ethyl acetate, and then fill it into a brown injection vial;

[0064] (3) The above-mentioned sample to be tested was detected by GC-MS (gas chromatography-mass spectrometry) to determine the content of aromatics and each component therein.

[0065] (4) GC-FID (gas chromatography-flame ionization detector) was used to determine the mass of the gas collected in the aluminum foil gas bag.

[0066] (5) TGA (thermogravimetric analyzer) was used to determine the mass of the carbon deposition product on the spent catalyst.

[0067] (6) The mass of the paraffin product was determined according to the mass difference between the quartz tube and the collection bottle before and after the reaction.

[0068] The pretreated sample was detected by GC-MS (7890B-5977B, HP-5ms chromatographic column) for component identification. The scanning mode was 30-500 amu. The column oven temperature program was to hold at 35 °C for 8 min, then rise to 100 °C at a rate of 5 °C / min and hold for 3 min, then rise to 220 °C at a rate of 5 °C / min, and finally rise to 290 °C at a rate of 10 °C / min and hold for 5 min. The compounds corresponding to each peak were identified with reference to the NIST spectral library and relevant literature (http: / / doi.org / 10.1021 / acscatal.3c05098.; http: / / doi.org / 10.1016 / j.jhazmat.2023.131547.). The percentage of the peak area in the total area was recorded as the relative content (area%) of this component.

[0069] Gas detection was carried out using a gas chromatograph (GC9800), including an FID detector and a TCD detector. The sample could be injected when the temperature (60 °C) was ready, and continuous injection was carried out for about 30 s. At the same time, the exhaust situation of the carrier gas should be confirmed during injection. The overall detection time was about 20 min.

[0070] According to the law of conservation of mass, the calculation process of the yield of aromatics and its components is as follows:

[0071]

[0072] The yield of each component (wt.%) = relative content × liquid product yield × PP mass Eq.(5)

[0073] The detection results of the content of aromatics in the products of each example and comparative example are shown in Table 2, and the detection results of each component and its content of aromatics are as Figures 3 - 8 shown, where Figure 3 are the results of Example 1, Figure 4 are the results of Example 2, Figure 5 are the results of Example 3,Figure 6 Results of Example 4 Figure 7 Results of Comparative Example 1 Figure 8 Yields of light aromatics (BTEX) and heavy hydrocarbons (C 10+ hydrocarbons) in Examples 1 to 4 and Comparative Example 1

[0074] Table 2 Yields of each component of aromatics, heavy hydrocarbons and light aromatics in the products of each example and comparative example

[0075] Yield, wt.% Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Total amount of aromatics 33.53 34.71 26.83 21.21 23.76 Heavy aromatics 5.77 3.69 4.37 1.90 2.96 BTEX 27.76 31.02 22.47 19.31 20.80 Benzene 0.77 3.05 0.88 1.88 1.52 Toluene 6.23 13.27 7.09 7.66 7.65 Ethylbenzene 1.82 1.33 1.07 0.98 1.05 Xylene 18.94 13.37 13.43 8.79 10.59 Heavy chain hydrocarbons 14.17 10.92 15.58 12.74 18.88

[0076] From the comparison of the total amount of aromatics in Table 2, it can be seen that the total amount of aromatics in Examples 1 to 2 is significantly higher than that in Comparative Example 1. In addition, when comparing Examples 3 to 4 with Examples 1 to 2, and comparing Examples 3 to 4 with Comparative Example 1, it is found that when the encapsulation thickness of the coated catalyst gradually increases, the aromatic yield of catalytic waste polyolefin plastics first increases and then decreases, reaching the highest at an encapsulation thickness of 90 nm (Example 2), and when the encapsulation thickness is 185 nm (Example 4), it is lower than that of traditional ZSM-5.

[0077] As Figures 3 - 8 shown, by comparing the yields of BTEX and heavy aromatics in each example and comparative example, it is found that when the encapsulation thickness is 90 nm, the promotion effect on light aromatics is the most obvious when catalytic waste polyolefin plastics produce aromatics, and the yield of heavy aromatics is lower compared with the coated catalysts of other encapsulation thicknesses. In addition, among the components of BTEX, the most obvious increase in yield is toluene and xylene with a relatively large proportion. By comparing the yields of heavy hydrocarbons in each example and comparative example, it can be seen that the coated catalyst has an obvious inhibitory effect on the side reactions on the surface of ZSM-5, and the yield of heavy hydrocarbons in Example 2 is the lowest.

[0078] In summary, in the process of catalytic waste polyolefin plastics to produce aromatics, compared with traditional ZSM-5, the coated catalyst prepared by the method of the present invention can significantly improve the aromatic yield, especially the yield of light aromatics BTEX. And when the encapsulation thickness of the coated catalyst is 90 nm, the improvement effect on the aromatic yield, especially the BTEX yield, is the best.

[0079] The above has described the embodiments of the present invention in detail, but the present invention is not limited to the described embodiments. For those skilled in the art, without departing from the principle and spirit of the present invention, various changes, modifications, substitutions and variations of these embodiments still fall within the protection scope of the present invention.

Claims

1. A method for preparing light aromatic hydrocarbons by catalytic pyrolysis of waste polyolefin plastics, characterized in that: First, the microporous molecular sieve ZSM-5 is coated in the ordered mesoporous layer SBA-15 using the ultra-dilute liquid phase coating method to prepare the coated catalyst ZSM-5@SBA-15, which is then added to waste polyolefin plastics and catalyzed to thermally decompose the waste polyolefin plastics at high temperature to produce light aromatics.

2. The method for preparing light aromatic hydrocarbons by catalytic pyrolysis of waste polyolefin plastics according to claim 1, characterized in that: The mass ratio of the coated catalyst ZSM-5@SBA-15 to the waste polyolefin plastic is 1-2:2-3.

3. The method for preparing light aromatic hydrocarbons by catalytic pyrolysis of waste polyolefin plastics according to claim 1, characterized in that: The pyrolysis is carried out at 550-700° C. using nitrogen as carrier gas, and the amount of carrier gas introduced is 80-130 mL / min.

4. The method for preparing light aromatic hydrocarbons by catalytic pyrolysis of waste polyolefin plastics according to claim 1, characterized in that: The waste polyolefin plastic is polypropylene.

5. The method for preparing light aromatic hydrocarbons by catalytic pyrolysis of waste polyolefin plastics according to claim 1, characterized in that: The preparation of the coated catalyst ZSM-5@SBA-15 comprises the following steps: S1. Tetrapropylammonium hydroxide, tetraethyl orthosilicate and NaAlO2 are dissolved in water, washed to neutrality after hydrothermal crystallization, and then dried and calcined to obtain Na-type ZSM-5 molecular sieve; S2, using NH4Cl solution to carry out ammonium exchange on the Na-type ZSM-5 molecular sieve, drying and then calcining to obtain the H-type ZSM-5 molecular sieve; S3. Dissolve P123 and MgSO4 in HCl solution, add H-type ZSM-5 molecular sieve and TEOS, mix well, place the resulting suspension at 90-120°C for 20-30h, wash to neutrality after reaction, and then dry and calcine to obtain the coated catalyst ZSM-5@SBA-15.

6. The method for preparing light aromatic hydrocarbons by catalytic pyrolysis of waste polyolefin plastics according to claim 5, characterized in that: In the coated catalyst ZSM-5@SBA-15, the packaging thickness of ZSM-5 is 60-185 nm.

7. The method for preparing light aromatic hydrocarbons by catalytic pyrolysis of waste polyolefin plastics according to claim 5, characterized in that: The hydrothermal crystallization temperature in S1 is 160-180° C., and the crystallization time is 2-4 days.

8. The method for preparing light aromatic hydrocarbons by catalytic pyrolysis of waste polyolefin plastics according to claim 5, characterized in that: The calcination temperature of S1, S2 and S3 is 500-600°C and the calcination time is 5-7h.

9. The method for preparing light aromatic hydrocarbons by catalytic pyrolysis of waste polyolefin plastics according to claim 5, characterized in that: The concentration of the NH4C1 solution in S2 is 1.0-2.0M, the temperature of ammonium exchange is 70-90°C, and the time is 3-5h.

10. The method for preparing light aromatic hydrocarbons by catalytic pyrolysis of waste polyolefin plastics according to claim 5, characterized in that: The concentration of the HCl solution in S3 is 1-3M, and the concentration of P123 in the HCl solution is 0.45-0.7g / 200-400mL.

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