Nitrogen-doped porous carbon supported iron monatomic catalyst, preparation method thereof and application of nitrogen-doped porous carbon supported iron monatomic catalyst in polyolefin pyrolysis reaction

Abstract

The invention discloses a nitrogen-doped porous carbon supported iron monatomic catalyst, a preparation method thereof and application of the nitrogen-doped porous carbon supported iron monatomic catalyst in a polyolefin pyrolytic reaction, 2-methylimidazole, benzylamine, iron and zinc metal are used for preparing a precursor, and the precursor is subjected to high-temperature calcination to obtain the nitrogen-doped porous carbon supported iron monatomic catalyst. The prepared catalyst can realize the pyrolysis of polyolefin under the inert atmosphere of 300 DEG C, the oil yield of the pyrolysis product can reach 94%, the fracture of C-C bonds in the polyolefin can be effectively promoted, and the generation of gaseous hydrocarbon products is avoided. The prepared catalyst can be expanded to pyrolysis of commercial waste polyolefin plastics, and an olefin and alkane mixture obtained through pyrolysis can be separated through saponification reaction. Compared with a conventional pyrolysis method, the catalyst is stable in performance and high in catalytic activity, can bear high-temperature conditions of pyrolysis and can be repeatedly used, and after the activity of the catalyst is reduced, the activity of the catalyst can be recovered through simple high-temperature calcination.

Description

Technical Field

[0001] The present invention belongs to the technical field of single-atom catalysts, and particularly relates to an iron single-atom catalyst supported on nitrogen-doped porous carbon, a preparation method thereof, and an application thereof in the pyrolysis reaction of polyolefins. Background Art

[0002] Plastics, as an indispensable material in modern society, especially polyolefins, which account for 60% of all plastics (mainly composed of polyethylene PE and polypropylene PP), have become a major environmental problem in terms of their waste. Among the 380 million tons of plastics produced globally each year, nearly 75% are discarded after single use (Sci. Adv. 2017, 3, e1700782). Although traditional recycling methods such as mechanical recycling help to close the plastic loop to a certain extent, issues such as strict sorting requirements, polymer degradation, and low recovery rates limit their effectiveness. Polyolefins are composed of C–H and C–C bonds, and their chemical inertness and high production volume pose challenges for their depolymerization (ACS Sustainable Chem. Eng. 2021, 9, 15722-15738). Pyrolysis technology shows great potential on an industrial scale due to its simple operation and ability to handle various raw materials. Pyrolysis carried out in an oxygen-free environment at a high temperature of 400-900 °C can convert polyolefins into low-molecular-weight hydrocarbons, and these products can be further refined for use as fuels or converted into other valuable materials (Science, 2023, 381, 666-671) (Science, 2023, 381, 660-666). However, current pyrolysis technology still faces three key problems: one is how to reduce the pyrolysis temperature to reduce energy consumption, the second is how to reduce the catalyst dosage to improve the catalytic efficiency, and the third is how to control the carbon chain components of the products to optimize the product value.

[0003] Catalysts play a crucial role in the pyrolysis process. They can significantly reduce the pyrolysis reaction temperature, improve the pyrolysis efficiency, and control the product component distribution by reasonably designing the morphology, pore structure, and acidity of the catalyst support. Acidic zeolites ZSM-5, Beta, and solid acid support materials loaded with noble metal clusters are conventional choices for polyolefin pyrolysis catalysts. However, the pyrolysis temperature is as high as 400 °C. Higher acidity can produce lighter hydrocarbons but also promote coke formation, leading to catalyst deactivation (Ind. Eng. Chem. Res. 2013, 52, 10637-10645.) (ACS Catal. 2022,12, 14882-14901.). In recent years, researchers have made many efforts to solve the problem of high pyrolysis energy consumption. Modified layered zeolites, ZSM-5 nanosheets (Angew. Chem. Int. Ed. 2024, 63, e202405252.), Ru / HZSM-5 (Nat. Nanotechnol. 2023, 18, 772-779.), Pt / F-Al 2 O 3 (Chem, 2023, 9, 2318-2336.), Pt / γ-Al 2 O 3 (Science, 2020, 370, 437-441.), SO 4 / ZrO 2 -Al 2 O 3 materials (Angew. Chem. Int.Ed. 2024, 64, e202417923.) have achieved the pyrolysis of polyolefins at lower temperatures (240-280 °C). However, these methods require a large amount of catalyst and a long reaction time, thus increasing the recovery cost and limiting their feasibility in large-scale industrial applications. In addition, the Lim research group first demonstrated the feasibility of UiO-66 MOF containing unsaturated zirconium sites in polyolefin pyrolysis, but the pyrolysis temperature is relatively high (400 °C) (Angew. Chem. Int. Ed. 2024, 136,e202408718).

[0004] In addition, the control of the chain length and its distribution of pyrolysis products is also a key issue to be solved in the polyolefin pyrolysis process. In addition to catalyst design, Liu et al. found that by controlling the pyrolysis temperature gradient, it is also an effective method to regulate the chain length and its distribution of pyrolysis oil, achieving a shift in the product hydrocarbon distribution from wax to oil and obtaining a high α-olefin content, which provides a good reaction model for the catalytic pyrolysis of polyolefins (Nat. Sustain. 2024, 7, 1681-1690). Therefore, it is necessary to develop a more green and efficient method to pyrolyze polyolefins into high-value products. Summary of the Invention

[0005] The object of the present invention is to overcome the deficiencies of the prior art, and to provide an iron catalyst supported on nitrogen-doped porous carbon material that is recyclable, highly stable, has good catalytic activity and is simple to prepare, and its preparation method and application. In the reactor of the present invention, the iron catalyst supported on nitrogen-doped porous carbon material can efficiently pyrolyze polyolefins into liquid hydrocarbons under mild reaction conditions, and at the same time, the catalyst has high stability and can be recycled.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows: The present invention provides a preparation method of an iron single-atom catalyst supported on nitrogen-doped porous carbon, comprising the following steps: Step 1: Disperse 2-methylimidazole and benzylamine in deionized water to obtain a first mixture, wherein the molar ratio of 2-methylimidazole to benzylamine is 1:0.5 to 1:1.5; Step 2: Disperse Fe(NO 3 ) 3 ·9H 2 O and Zn(NO 3 ) 2 ·6H 2 O in deionized water to obtain a second mixture, wherein the molar ratio of Fe(NO 3 ) 3 ·9H 2 O to Zn(NO 3 ) 2 ·6H 2 O is 1:10 to 50; Step 3: Dropwise add the second mixture obtained in Step 2 to the first mixture obtained in Step 1, stir and react at 25-40 °C for 4-10 h, then wash and dry the obtained mixture to obtain a precursor ZIFs; Step 4: Calcinate the precursor ZIFs obtained in Step 3, and the calcination is specifically: heat up to 800-1000 °C at a heating rate of 5 °C / min and calcine for 1-3 h to prepare the iron single-atom catalyst supported on nitrogen-doped porous carbon.

[0007] The present invention also provides a nitrogen-doped porous carbon-supported iron single-atom catalyst prepared by the preparation method of the nitrogen-doped porous carbon-supported iron single-atom catalyst described above.

[0008] The present invention also provides the application of the nitrogen-doped porous carbon-supported iron single-atom catalyst in the polyolefin pyrolysis reaction.

[0009] Further, the application of the nitrogen-doped porous carbon-supported iron single-atom catalyst in the polyolefin pyrolysis reaction includes the following steps: placing the polyolefin and the nitrogen-doped porous carbon-supported iron single-atom catalyst in a reactor, mixing evenly, using N 2 to conduct gas exchange, and carrying out a pyrolysis reaction at 220-300 °C for 10-120 min. After the reaction ends, separating the catalyst and the pyrolysis product, and recycling and reusing the nitrogen-doped porous carbon-supported iron single-atom.

[0010] Even further, the polyolefin is at least one of LDPE, HDPE, PP, and LLDPE.

[0011] Even further, the mass ratio of the polyolefin to the nitrogen-doped porous carbon-supported iron single-atom catalyst is 80:1 to 30.

[0012] Further, in the present invention, the reactor is a cylindrical container, which is provided with a lid at the top, and an air vent is respectively provided on both sides of the top end. The outer wall of the reactor is sleeved with a glass outer cover, and a condensate inlet and an outlet are provided on the glass outer cover.

[0013] Specifically, the cylindrical container is a quartz container, and the lid provided at the top is a circular quartz lid for sealing the reactor. When performing the sealing operation, it can be sealed in cooperation with a fluororubber ring, and the seal is tightened with a sealing film. The inner wall diameter of the reactor is 5-7 cm, the height of the reactor is 10-15 cm, the distance between the outer wall of the condenser, i.e., the glass outer cover, and the outer wall of the reactor is 1-2 cm, and an air vent is respectively provided at a position 1-2 cm away from the quartz lid on both sides of the top end of the reactor. The total volume of the reactor is 0.2-0.6 L, and the bottom surface area is 20-40 cm 2 .

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0015] The present invention firstly proves that the thermally stable, low-cost and scalable synthetic ZIF-derived Fe single-atom catalyst (Fe 1@NC) in the efficient catalytic performance of polyolefin pyrolysis, the temperature gradient regulation reaction model can reduce the generation of gaseous component products and obtain high-value liquid hydrocarbons with high selectivity. Acidic Fe sites promote the cleavage of C-C bonds, and basic N sites can promote the dehydrogenation or hydrogen transfer process of carbon chains, thus realizing the efficient pyrolysis of polyolefins. The catalyst shows excellent catalytic stability for the pyrolysis reaction of polyolefins, can be extended to the pyrolysis of commercial waste plastics, and the catalyst can be recycled after the reaction.

[0016] In the present invention, the customized temperature gradient reactor controls the temperature gradient for polyolefin pyrolysis by bottom heating and sidewall condensation of the reactor. The circulating cooling water plays an important role in avoiding the generation of gaseous products and can effectively regulate the product hydrocarbon distribution. Fe 1 @NC shows excellent catalytic performance in the reaction of producing high-value pyrolysis oil from polyolefin pyrolysis. At 300 °C for 50 min, the conversion rate can reach 97%, and the liquid hydrocarbon yield is as high as 94%. Adjusting the reaction time can control the liquid hydrocarbon distribution, and the product distribution can be controlled within the gasoline chain length range (C6-C12) after reacting for 130 min. The synthesized pyrolysis oil has a high concentration of olefin products (73%), and olefins and alkanes can be separated by a simple saponification reaction.

[0017] In the present invention, Fe 1 @NC shows good catalytic performance in the pyrolysis processes of LDPE, the mixture of LDPE and HDPE, PP, and waste sealed bags and bottles. Fe 1 @NC also shows excellent thermal stability and recyclability. Although the catalytic activity decreases after the fourth run, the catalytic performance can be restored after simple high-temperature calcination. Therefore, the present invention opens up a new way for the future value-added utilization of waste plastics using porous carbon-supported single atoms. Brief Description of the Drawings

[0018] Figure 1 XRD patterns of the catalysts used after Example 1 and Example 19 of the present invention and the iron catalyst supported on nitrogen-doped porous carbon material used in Example 20; Figure 2 TEM image of the iron catalyst supported on nitrogen-doped porous carbon material prepared in Example 1 of the present invention; Figure 3a XANES pattern of the iron single-atom catalyst supported on nitrogen-doped porous carbon material prepared in Example 1 of the present invention; Figure 3b EXAFS pattern of the iron single-atom catalyst supported on nitrogen-doped porous carbon material prepared in Example 1 of the present invention; Figure 4 Reactor model for polyolefin pyrolysis in Examples 3-20 of the present invention; Figure 5 GC spectrum analysis of the liquid product from pyrolysis separation of polyolefin in Example 3 of the present invention; Figure 6 Selectivity distribution of the liquid-phase products in Examples 3, 7 and 8 of the present invention; Figure 7 Selectivity distribution of the liquid-phase products in Examples 3, 9 and 10 of the present invention; Figure 8 Selectivity distribution of the liquid-phase products of the ionic detergent and the separation product in Example 16 of the present invention; Figure 9 TEM image of the catalyst after the reaction in Example 19 of the present invention; Figure 10 TEM image of the catalyst used in Example 20 of the present invention; Detailed implementation manners

[0019] The present invention will be further described in detail below in conjunction with examples.

[0020] Those skilled in the art will understand that the following examples are only for illustrating the present invention and should not be construed as limiting the scope of the present invention. For those not specified in the examples regarding specific technologies or conditions, the technologies or conditions described in the literature in this field or according to the product specifications shall be followed. For reagents or instruments without indicating the manufacturer, they are all conventional products that can be obtained by purchase.

[0021] Example 1 This example provides a preparation method of an iron single-atom catalyst supported on a nitrogen-doped porous carbon material, and the method includes the following steps: Step 1: Dissolve 2-methylimidazole (24 mmol) and benzylamine (24 mmol) in deionized water (80 mL) to obtain solution A; Step 2: Dissolve iron(III) nitrate nonahydrate (0.12 mmol) and zinc nitrate hexahydrate (6 mmol) in deionized water (80 mL) to obtain solution B; Step 3: Dropwise add solution B into solution A, stir and react at 25 °C for 4 h, then wash and dry the mixture with deionized water and methanol to obtain FeZn@BZIF; Step 4: Heat the precursor FeZn@BZIF in a tubular furnace under a nitrogen atmosphere at a heating rate of 5 °C / min to 900 °C and calcine for 2 h to prepare Fe 1 @NC.

[0022] It can be seen from Figure 1 that Fe 1 @NC at 25 o and 44 oThe two broad peaks at [[]] belong to the (002) and (101) crystal planes of the graphite carbon material respectively, and no Fe signal peak is detected, indicating that iron may exist in the form of single atoms in this catalyst.

[0023] As can be seen from Figure 2 Fe 1 @NC presents an irregular flaky structure, and some hollow carbon nanotubes are formed, and no Fe nanoparticles are observed.

[0024] As can be seen from Figure 3, Fe exists in the form of single atoms in Fe 1 @NC, there are 4 nitrogens outside the first shell, and the catalytic active center is the Fe-N 4 structure.

[0025] Example 2 The steps for preparing the catalyst are the same as in Example 1, except that in the preparation method described in Example 1, ferric nitrate nonahydrate is not added, and the precursor name is Zn@ZIF, and the NC catalyst is obtained by calcination.

[0026] Example 3 This example provides the preparation process of the pyrolysis reaction of a mixture of low-density polyethylene and high-density polyethylene. Specifically: Put 2 g of low-density polyethylene, 2 g of high-density polyethylene and the Fe 1 @NC (50 mg) prepared in Example 1 into a customized quartz reactor. After the reactants and the catalyst are fully mixed by mechanical stirring, seal the top of the reactor with a round quartz cover and a fluororubber ring, wind the side with a sealing film, and reinforce the seal with 4 clamps. Seal the gas outlet with a fluorinated plug and fix it with a Teflon cap. Pass N 2 into the gas inlet, keep the gas inlet for 1 min, then open the gas outlet and continue degassing for 10 min. Further turn on the condensed water, and set the temperature of the heating plate to 300 °C to start heating. After white smoke appears in the reactor (about 1.5 min), seal the inlet and outlet, and continue the reaction for 50 min. After the reaction is over, cool to room temperature and dissolve the liquid-phase product with dichloromethane. Filter to separate the solid and liquid phases. Weigh the dried solid phase, and weigh the liquid-phase product after rotary evaporation. Calculate the conversion rate to be 97%, the selectivity of C1~C5 gas-phase products to be 3%, and the selectivity of C5~C25 liquid-phase products to be 94%.

[0027] As can be seen from Figure 4 the reaction is carried out in a customized quartz reactor.

[0028] The selectivity of the liquid-phase components analyzed by GC is as Figure 5 shown.

[0029] Example 4 This embodiment provides a pyrolysis reaction preparation process of a mixture of low-density polyethylene and high-density polyethylene, which is the same as that of Example 3, except that the reaction time is 10 min. Specifically: 2 g of low-density polyethylene, 2 g of high-density polyethylene and the Fe 1 @NC (50 mg) was placed in a custom-made quartz reactor. After the reactants and catalyst were fully mixed by mechanical stirring, the top of the reactor was sealed with a round quartz cover and a fluororubber ring. The sides were tightly wrapped with a sealing film and reinforced with four clamps. The gas outlet was sealed with a fluorinated plug and fixed with a Teflon cap. The gas inlet was passed through N 2 , keep the air inlet for 1 min, then open the gas outlet and continue degassing for 10 min. Further open the condensed water, set the heating plate temperature to 300 ° C and start heating. After white smoke emerges in the reactor (about 1.5 min), seal the inlet and outlet, and continue the reaction for 10 min. After the reaction is completed, cool to room temperature and dissolve the liquid phase product with dichloromethane, filter and separate the solid and liquid phases, dry and weigh the solid phase, and weigh the liquid phase product after rotary evaporation. The calculated conversion rate is 73%, the selectivity of C1~C5 gas phase products is 2%, and the selectivity of C5~C25 liquid phase products is 71%.

[0030] Comparative Example 1 The NC catalyst prepared in Example 2 is used in the pyrolysis reaction preparation process of a mixture of low-density polyethylene and high-density polyethylene, specifically: 2 g of low-density polyethylene, 2 g of high-density polyethylene and the NC catalyst (50 mg) prepared in Example 2 were placed in a custom-made quartz reactor. After the reactants and catalyst were fully mixed by mechanical stirring, the top of the reactor was sealed with a round quartz cover and a fluororubber ring, the sides were tightly wrapped with a sealing film, and the seal was reinforced with 4 clamps. The gas outlet was sealed with a fluorinated plug and fixed with a Teflon cap, and the gas inlet was passed through N 2 , keep the air inlet for 1 min, then open the gas outlet and continue degassing for 10 min. Further open the condensed water, set the heating plate temperature to 300 ° C and start heating. After white smoke emerges in the reactor (about 1.5 min), seal the inlet and outlet, and continue the reaction for 10 min. After the reaction is completed, cool to room temperature and dissolve the liquid phase product with dichloromethane, filter and separate the solid and liquid phases, dry and weigh the solid phase, and weigh the liquid phase product after rotary evaporation. The calculated conversion rate is 42%, the selectivity of C1~C5 gas phase products is 3%, and the selectivity of C6~C35 liquid phase products is 39%.

[0031] Comparative Example 2 No catalyst is added during the pyrolysis reaction preparation process of the low-density polyethylene and high-density polyethylene mixture, specifically: Place 2 g of low-density polyethylene and 2 g of high-density polyethylene in a customized quartz reactor. After thoroughly mixing the reactants by mechanical stirring, seal the top of the reactor with a round quartz lid and a fluororubber ring. Wrap the sides tightly with sealing film and reinforce the seal with 4 clamps. Seal the gas outlet with a fluorinated plug and fix it with a Teflon cap. Pass N 2 into the gas inlet, keep the gas inlet open for 1 min, then open the gas outlet and continue degassing for 10 min. Further turn on the condensed water and set the temperature of the heating plate to 300 °C to start heating. After white smoke appears in the reactor (about 1.5 min), seal the inlet and outlet, and continue the reaction for 10 min. After the reaction is completed, cool to room temperature and dissolve the liquid-phase product with dichloromethane. Filter to separate the solid and liquid phases. Weigh the dried solid phase and weigh the liquid-phase product after rotary evaporation. Calculate the conversion rate to be 15%, the selectivity of C1-C5 gas-phase products to be 2%, and the selectivity of C5-C25 liquid-phase products to be 13%.

[0032] Examples 5-8 Examples 5-8 provide the preparation process of the pyrolysis reaction of the mixture of low-density polyethylene and high-density polyethylene, which is the same as that of Example 3. The difference from Example 3 is that only the reaction time is changed, as specifically shown in Table 1.

[0033] Table 1 Influence of reaction time on catalytic activity in Examples 5-8

[0034] The selectivity distribution of the liquid-phase products obtained at different reaction times is as Figure 6 shown.

[0035] Examples 9-10 Examples 9-10 provide the preparation process of the pyrolysis reaction of the mixture of low-density polyethylene and high-density polyethylene, which is the same as that of Example 3. The difference from Example 3 is that only the catalyst dosage is changed, as specifically shown in Table 2.

[0036] Table 2 Influence of catalyst dosage on catalytic activity in Examples 9-10

[0037] The selectivity distribution of the liquid-phase products obtained at different catalyst dosages is as Figure 7 shown.

[0038] Examples 11-15 Examples 11-15 provide the preparation process of the pyrolysis reaction of the mixture of low-density polyethylene and high-density polyethylene, which is the same as that of Example 3. The difference from Example 3 is that only the type of polyolefin substrate is changed, as specifically shown in Table 3.

[0039] Table 3 Influence of different polyolefin substrates on catalytic activity in Examples 11-15

[0040] Example 16

[0041] Place 100 mg of the product oil from Example 9 into a 25 mL pressure-resistant tube, place the pressure-resistant tube in an ice bath, dropwise add 50 mg of concentrated sulfuric acid while stirring, stir and react for 10 min, then transfer the pressure-resistant tube to a room temperature environment and stir and react for 10 min. Further, add 1.2 mL of 1 M KOH aqueous solution to neutralize the reaction solution to obtain an ionic detergent. The detergent is diluted to 8 mL with deionized water, and the unreacted alkane components are extracted with hexane. The separated alkane components are dried with Na 2 SO 4 dried, rotary evaporated and weighed. The selectivity of the olefin product is calculated to be 73%, and the selectivity of the alkane product is 27%. Further, the product distribution of the alkane components is obtained by GC analysis.

[0042] The ionic detergent and the separated alkane product distribution are as Figure 8 shown.

[0043] Examples 17 - 20: Catalyst Reusability Test The preparation processes of the pyrolysis reactions of the mixtures of low-density polyethylene and high-density polyethylene in Examples 17 - 20 are the same as those in Example 3. The difference from Example 3 is that the used Fe 1 @NC catalyst is filtered and washed, and directly used for the next reaction cycle after drying. The catalyst used in Example 17 is the catalyst after being used in Example 3, the catalyst used in Example 18 is the catalyst after being used in Example 17, the catalyst used in Example 19 is the catalyst after being used in Example 18. After the catalyst in Example 19 is used, the recovered catalyst is calcined and continuously used in the catalytic cycle of Example 20. See Table 4 for details.

[0044] Table 4 Catalytic Cycle Tests of Fe 1 @NC in Examples 17 - 20

[0045] It can be Figure 1 seen that in the XRD pattern of the recovered catalyst after the reaction in Example 19, the diffraction peak of graphite carbon becomes smaller, and obvious residual diffraction peaks of long-chain hydrocarbons appear. After further calcination, the diffraction peaks of long-chain hydrocarbons in the catalyst used in Example 20 almost disappear.

[0046] It can be Figure 9 seen that in the TEM pattern of the recovered catalyst after the reaction in Example 19, the morphology and pore structure of the catalyst are retained, but carbon deposits and hydrocarbon substances are attached to the surface and inside the nanotubes.

[0047] From Figure 10 It can be seen that in the TEM image of the catalyst of Example 19 after calcination (i.e., the catalyst used in Example 20), the surface morphology of the catalyst has returned to the state of the fresh catalyst.

[0048] The above embodiments have described the implementation manners of the present invention in detail. However, the present invention is not limited to the above implementation manners. Within the scope of knowledge possessed by those of ordinary skill in the art, various changes can be made without departing from the gist of the present invention. The above are only the preferred and feasible embodiments of the present invention, and thus do not limit the scope of the rights of the present invention. Any equivalent structural changes made by using the content of the specification of the present invention are included within the scope of the rights of the present invention.

Claims

1. A method for preparing a nitrogen-doped porous carbon-supported iron single-atom catalyst, characterized in that: The steps include: Step 1: dispersing 2-methylimidazole and benzylamine in deionized water to obtain a first mixture, wherein the molar ratio of the 2-methylimidazole to the benzylamine is 1:0.5 to 1:1.5; Step 2: Dispersing Fe(NO3)3·9H2O and Zn(NO3)2·6H2O in deionized water to obtain a second mixture, wherein the molar ratio of Fe(NO3)3·9H2O to Zn(NO3)2·6H2O is 1:10-50; Step 3: Add the second mixture obtained in step 2 dropwise to the first mixture obtained in step 1, stir and react at 25-40° C. for 4-10 hours, wash and dry the obtained mixture to obtain precursor ZIFs; Step 4: calcining the precursor ZIFs obtained in step 3, wherein the calcination is specifically as follows: heating the temperature to 800-1000°C at a heating rate of 5°C / min, and calcining for 1-3h to prepare the nitrogen-doped porous carbon-supported iron single atom catalyst.

2. The nitrogen-doped porous carbon-supported iron single-atom catalyst prepared by the preparation method of the nitrogen-doped porous carbon-supported iron single-atom catalyst according to claim 1.

3. Use of the nitrogen-doped porous carbon-supported iron single-atom catalyst according to claim 2 in the pyrolysis reaction of polyolefins.

4. The use of the nitrogen-doped porous carbon-supported iron single-atom catalyst in polyolefin pyrolysis reaction according to claim 3, characterized in that: The method comprises the following steps: placing polyolefin and the nitrogen-doped porous carbon-supported iron single atom catalyst in a reactor, mixing them evenly, performing gas exchange with N2, performing pyrolysis reaction at 220-300°C for 10-120 min, separating the catalyst and the pyrolysis product after the reaction, and recycling the nitrogen-doped porous carbon-supported iron single atom.

5. The use of the nitrogen-doped porous carbon-supported iron single-atom catalyst in polyolefin pyrolysis reaction according to claim 4, characterized in that: The polyolefin is at least one of LDPE, HDPE, PP and LLDPE.

6. The use of the nitrogen-doped porous carbon-supported iron single-atom catalyst in polyolefin pyrolysis reaction according to claim 4, characterized in that: The mass ratio of the polyolefin to the nitrogen-doped porous carbon-supported iron single-atom catalyst is 80:1-30.

7. The use of the nitrogen-doped porous carbon-supported iron single-atom catalyst in polyolefin pyrolysis reaction according to claim 3, characterized in that: The reactor is a cylindrical container with a cover on the top and a vent on both sides of the top. The outer wall of the reactor is covered with a glass cover, and the glass cover is provided with a condensed water inlet and outlet.

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