Nitrogen-doped PVC (polyvinyl chloride) dechlorination carbon-loaded iron catalyst, preparation method thereof and application of nitrogen-doped PVC dechlorination carbon-loaded iron catalyst in polyolefin pyrolysis reaction

Abstract

The invention discloses a nitrogen-doped PVC (polyvinyl chloride) dechlorination carbon-loaded iron catalyst, a preparation method thereof and application of the nitrogen-doped PVC dechlorination carbon-loaded iron catalyst in a polyolefin pyrolysis reaction. The nitrogen-doped PVC dechlorination carbon-loaded iron catalyst is obtained by taking ferric oxide as an iron source, melamine as a nitrogen source and PVC as a carbon source through high-temperature calcination. The catalyst prepared by the invention can realize the pyrolysis of polyolefin in an inert atmosphere at 360 DEG C, and the yield of pyrolysis oil can reach 88% (greater than or equal to C6). The catalyst prepared by the invention can be expanded to pyrolysis of commercial waste polyolefin plastics, and pyrolysis oil with the yield of 80-88% is obtained. Compared with a traditional pyrolysis catalyst, the catalyst is high in catalytic activity, easy to prepare, low in cost, good in stability and easy to recover, 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] This invention belongs to the field of plastic degradation technology, specifically relating to a nitrogen-doped carbon-supported iron catalyst for dechlorination of PVC, its preparation method, and its application in the pyrolysis reaction of polyolefins. Background Technology

[0002] Polyethylene (PE), as one of the most widely used general-purpose plastics, is extensively used in packaging, agriculture, construction, automobiles, and many other fields due to its excellent chemical resistance, mechanical strength, and processing performance. It is the world's largest-produced plastic variety. However, of the 380 million tons of plastic produced globally each year, nearly 75% is discarded after single use. The PE molecular chain possesses extremely strong chemical stability and hydrophobicity, making it difficult for microorganisms to degrade it in the natural environment. The long-term accumulation of large quantities of waste PE products, forming "white pollution," has caused serious harm to soil, water bodies, ecosystems, and human health, becoming a major environmental problem urgently needing to be addressed globally. Pyrolysis technology, due to its simple operation and good adaptability to various raw materials, shows broad potential in industrial applications. When pyrolysis is carried out in a high-temperature, oxygen-free environment of 400–900℃, polyolefins can be converted into low-molecular-weight hydrocarbons. These products can be further refined into fuels or converted into other high-value-added materials. However, current pyrolysis technology still faces three key challenges: first, how to reduce the pyrolysis temperature to reduce energy consumption; second, how to reduce the amount of catalyst and improve catalytic efficiency; and third, how to regulate the carbon chain composition of the product to optimize the product value.

[0003] Catalysts play an indispensable and crucial role in pyrolysis, significantly reducing reaction temperature and improving efficiency. By rationally designing the morphology, pore structure, and pH of the catalyst support, the compositional distribution of the products can also be controlled. Acidic zeolites ZSM-5 and Beta, as well as solid acid supports loaded with noble metal clusters, are commonly used catalysts for polyolefin pyrolysis; however, their pyrolysis temperatures are as high as 400℃. While higher acidity can generate lighter hydrocarbons, it also accelerates coke formation, leading to catalyst deactivation. In recent years, researchers have made considerable efforts to address the problem of high pyrolysis energy consumption. Modified layered zeolites, ZSM-5 nanosheets (Angew. Chem. Int. Ed. 2024, 63, e202405252.), and SO4 / ZrO2-Al2O3 materials (Angew. Chem. Int. Ed. 2024, 64, e202417923.) can all achieve the pyrolysis of polyolefins in a relatively low temperature range (240–280 °C). However, these methods suffer from the problems of large catalyst dosage and long reaction time, which not only increases the recycling cost but also limits their feasibility for large-scale industrial applications.

[0004] Furthermore, the regulation of the chain length and distribution of pyrolysis products during polyolefin pyrolysis is also a key issue that needs to be addressed. Therefore, developing a greener and more efficient method to convert polyolefin pyrolysis into high-value liquid products is of urgent practical need and significant importance. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a nitrogen-doped carbon-supported iron catalyst for PVC dechlorination, its preparation method, and its application in polyolefin pyrolysis reactions. Compared with traditional pyrolysis catalysts, the catalyst obtained by this invention has high catalytic activity, is easy to prepare, has low cost, good stability, and is easy to recover.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The preparation method of nitrogen-doped PVC dechlorination carbon-supported iron catalyst includes the following steps: Step 1: Melamine is placed in a tube furnace and heated to 550°C at a rate of 5°C / min. It is then calcined under a nitrogen atmosphere for 4 hours to obtain C3N4. Step 2: Mix PVC and iron oxide, and dechlorinate them by thermal dechlorination at 220 °C for 30 min. The resulting solid is denoted as Fe@C. PVC ; Step 3: Add Fe@C PVC The nitrogen-doped PVC dechlorinated carbon supported iron catalyst was obtained by ball milling with the C3N4 obtained in step one at a mass ratio of 1:1 and heated to 700°C at a rate of 5°C / min under a nitrogen atmosphere and held for 2 h. After grinding, the nitrogen-doped PVC dechlorinated carbon supported iron catalyst was obtained.

[0007] Furthermore, in the method for preparing the nitrogen-doped PVC dechlorination carbon-supported iron catalyst, the mass ratio of PVC to iron oxide in step two is 20:1.

[0008] The present invention also provides a nitrogen-doped PVC dechlorination carbon-supported iron catalyst prepared by the method described above.

[0009] The present invention also provides the application of the nitrogen-doped PVC dechlorination carbon-supported iron catalyst in the pyrolysis reaction of polyolefins.

[0010] Furthermore, the application of nitrogen-doped PVC dechlorinated carbon supported iron catalyst in the pyrolysis reaction of polyolefins includes the following steps: placing the polyolefin and the nitrogen-doped PVC dechlorinated carbon supported iron catalyst in a reactor, mixing them evenly, exchanging gas with N2, and carrying out the pyrolysis reaction at 320-400℃ for 30-120 min. After the reaction is completed, the catalyst and pyrolysis products are separated.

[0011] Furthermore, the application of the nitrogen-doped PVC dechlorinated carbon supported iron catalyst in the pyrolysis reaction of polyolefins, wherein the mass ratio of the polyolefin to the nitrogen-doped PVC dechlorinated carbon supported iron catalyst is 40:0.5~2.

[0012] Furthermore, the application of the nitrogen-doped PVC dechlorinated carbon supported iron catalyst in the pyrolysis reaction of polyolefins, wherein the mass ratio of the polyolefin to the nitrogen-doped PVC dechlorinated carbon supported iron catalyst is 40:1.

[0013] Compared with the prior art, the beneficial effects of this invention are as follows:

[0014] This invention proposes an innovative solution for the value-added utilization of polyolefin pyrolysis. The developed iron catalyst (Fe@CN) possesses excellent thermal stability, low preparation cost, and scalable synthesis process. It exhibits highly efficient catalytic activity in the polyolefin pyrolysis reaction, with a clear mechanism of action: acidic Fe sites promote selective C / C bond cleavage, while basic N sites accelerate carbon chain dehydrogenation and hydrogen transfer processes. The synergistic effect of these two factors achieves efficient and directional pyrolysis of polyolefins. Furthermore, the catalyst demonstrates outstanding catalytic stability and can be directly extended to the pyrolysis treatment of industrial waste plastics.

[0015] Meanwhile, this invention employs a temperature gradient reactor, constructing a precise temperature gradient through a combined temperature control mode of bottom heating and sidewall condensation. The accompanying condensate circulation system effectively suppresses the formation of gaseous products, achieving precise control over the distribution of hydrocarbon products. Experimental data shows that under reaction conditions of 360℃, the yield of liquid hydrocarbon products can reach 88% after Fe@CN catalyzes the pyrolysis of polyolefins for 60 min; by adjusting the reaction time to 130 min, the product chain length can be precisely controlled within the gasoline fraction range (C6-C12).

[0016] This catalytic system has wide applicability, exhibiting stable catalytic performance on low-density polyethylene (LDPE), mixtures of LDPE and high-density polyethylene (HDPE), and complex matrices such as actual waste sealed bags and waste plastic bottles. This invention lays the foundation for the industrial application of iron catalysts in the resource utilization of waste plastics, and opens up a new low-cost, highly selective pyrolysis value-added pathway for polyolefins. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the quartz reactor used for polyolefin pyrolysis in Examples 2, 3, 5 and 6-8 of the present invention; Figure 2 This is a schematic diagram of the metal reactor for the pyrolysis of polyolefins in Example 4 of the present invention; Figure 3The XRD pattern of the Fe@CN catalyst prepared in Example 1 of this invention: 2θ angles of 26.5° and 44.5° are characteristic of graphitic carbon, and about 30° is the crystal phase peak of iron; Figure 4 (a) is the N element XPS diagram of the Fe@CN catalyst prepared in Example 1 of this invention; Figure 4 (b) is the Fe element XPS diagram of the Fe@CN catalyst prepared in Example 1 of this invention; XPS detected signals of Fe and N, and the surface portion of Fe 2+ Iron is oxidized to Fe 3+ The iron atom cluster coordinates with N to form Fe-N bonds.

[0018] Figure 5 This is a GC spectrum analysis of the liquid products separated by polyolefin pyrolysis in Example 3 of the present invention. This is a chromatogram of hydrocarbon distribution of polyolefin pyrolysis products. It can be seen that after catalysis by the Fe@CN catalyst of the present invention, the products cover C6-C34 hydrocarbons and the peak intensity of high-value hydrocarbon components is prominent, which reflects its advantage of being able to efficiently control the carbon chain distribution of products and improve the yield of target liquid hydrocarbons. Figure 6 The image shows a TEM image of the Fe@CN catalyst in Example 1 of this invention. The sheet-like carbon layer coats carbon nanotubes, and the carbon nanotubes encapsulate iron nanoclusters.

[0019] Among them, 1. External surrounding condenser; 2. First thermometer; 3. Second thermometer; 4. First valve; 5. Second valve. Detailed Implementation

[0020] The present invention will now be described in further detail with reference to the embodiments.

[0021] Those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be construed as limiting the scope of the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed in accordance with the techniques or conditions described in the literature in the field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased.

[0022] Example 1 This embodiment provides a method for preparing a nitrogen-doped PVC dechlorination carbon-supported iron catalyst, which includes the following steps: Step 1: Place melamine (2 g) in a tube furnace and heat it to 550°C at a rate of 5°C / min. Calcine it under a nitrogen atmosphere for 4 h to obtain C3N4 (1.38 g). Step 2: Mix 100 mg of iron oxide with 2 g of PVC in... Figure 1After dechlorination at 220 °C for 30 min in the closed reactor shown, 1.42 g of dechlorinated carbon material (denoted as Fe@C) was obtained. PVC The mechanism is that iron oxide catalyzes the dechlorination of PVC to produce HCl; iron oxide (Fe2O3) and HCl undergo a neutralization reaction to produce ferric chloride.

[0023] Step 3: Fe@C PVC The mixture was ball-milled with the C3N4 obtained in step one at a mass ratio of 1:1, and then heated to 700 °C at a rate of 5 °C / min under a nitrogen atmosphere and held for 2 h. After grinding, a black solid Fe@CN was obtained, which is a nitrogen-doped PVC dechlorination carbon supported iron catalyst.

[0024] Example 2 This embodiment provides a pyrolysis reaction preparation process for low-density polyethylene (LDPE), specifically: The reaction is carried out in a custom-designed quartz reactor (e.g., Figure 1 (As shown). 2 g of low-density polyethylene and 50 mg of Fe@CN prepared in Example 1 were placed in a custom-made quartz reactor. After thoroughly mixing the reactants and catalyst by mechanical stirring, the top of the reactor was sealed with a circular quartz lid and a fluororubber ring. The sides were tightly sealed with sealing film and reinforced with four clamps. The gas outlet was sealed with a fluorinated plug and secured with a Teflon cap. N2 was introduced into the gas inlet, and degassing was maintained for 1 min. Then, the gas inlet was opened and maintained for 1 min, repeated three times. The condensate was then turned on, and the heating plate temperature was set to 400 °C to begin heating; the actual reaction temperature was 360 °C. After white smoke was emitted from the reactor (approximately 1.5 min), the inlet and outlet were sealed, and the reaction continued for 60 min. After the reaction was completed, the product was cooled to room temperature and dissolved in dichloromethane. The solid and liquid phases were separated by filtration. The solid phase was dried and weighed, and the liquid phase product was rotary evaporated and weighed. 1.76 g of C6~C34 liquid phase product and 0.14 g of solid were obtained. The PE conversion rate was 95%, the liquid phase product yield was 88%, and the C1-C5 yield was 7%.

[0025] Example 3 This embodiment provides a pyrolysis reaction preparation process for high-density polyethylene (HDPE), specifically: The reaction is carried out in a custom-designed quartz reactor (e.g., Figure 1(As shown). 2 g of high-density polyethylene and 50 mg of Fe@CN prepared in Example 1 were placed in a custom-made quartz reactor. After thoroughly mixing the reactants and catalyst by mechanical stirring, the top of the reactor was sealed with a circular quartz lid and a fluororubber ring. The sides were tightly sealed with sealing film and reinforced with four clamps. The gas outlet was sealed with a fluorinated plug and fixed with a Teflon cap. N2 was introduced into the gas inlet, and degassing was maintained for 1 min. Then, the gas inlet was opened and maintained for 1 min, repeated three times. The condensate was then turned on, and the heating plate temperature was set to 400 °C to begin heating. The actual reaction temperature was 360 °C. After white smoke was emitted from the reactor (approximately 1.5 min), the inlet and outlet were sealed, and the reaction continued for 60 min. After the reaction was completed, the product was cooled to room temperature and dissolved in dichloromethane. The solid and liquid phases were separated by filtration. The solid phase was dried and weighed, and the liquid phase product was rotary evaporated and weighed. The conversion rate was calculated to be 73%, the selectivity of C1~C5 gas phase products was 1%, and the selectivity of C6~C34 liquid phase products was 72%.

[0026] Example 4 This embodiment provides a pyrolysis reaction preparation process for a mixture of high-density polyethylene and low-density polyethylene, specifically: The reaction is carried out in a custom-made metal reactor (e.g.) Figure 2 (As shown). 2.5 kg of high-density polyethylene and 2.5 kg of low-density polyethylene were placed in a custom-made metal reactor with Fe@CN (125 g) prepared in Example 1. After thoroughly mixing the reactants and catalyst by mechanical stirring, the top of the reactor was sealed with a circular metal lid and reinforced with screws. The gas outlet valve was closed, and N2 was introduced into the gas inlet for 1 min. Then the gas outlet was opened, and degassing continued for 10 min before the gas inlet and outlet were closed. The condensate was then turned on, and the heating mantle temperature was set to 500 °C to start heating. The actual reaction temperature was 360 °C, and the reaction was continued for 6-8 h. After the reaction, the mixture was cooled to room temperature and the liquid phase product was dissolved in dichloromethane. The solid and liquid phases were separated by filtration. The solid phase was dried and weighed, and the liquid phase product was rotary evaporated and weighed. The conversion rate was calculated to be 80%, the selectivity of the C1-C5 gas phase products was 3%, and the yield of the C6-C34 liquid phase products was 77%.

[0027] Comparative Example 1 No catalyst is added during the pyrolysis reaction of low-density polyethylene. Specifically: The reaction was carried out in a custom-designed quartz reactor. Figure 1Two g of high-density polyethylene and 50 mg of Fe@CN prepared in Example 1 were placed in a custom-made quartz reactor. The reactants and catalyst were thoroughly mixed by mechanical stirring. The top of the reactor was sealed with a circular quartz lid and a fluororubber ring, the sides were tightly sealed with sealing film, and reinforced with four clamps. The gas outlet was sealed with a fluorinated plug and secured with a Teflon cap. N2 was introduced into the gas inlet, and degassing was maintained for 1 min. Then the gas inlet was opened, and gas was introduced for 1 min, repeated three times. The condensate was then turned on, and the heating plate temperature was set to 400°C. o Heating begins at C, with the actual reaction temperature at 360°C. o C. After white smoke is emitted from the reactor (approximately 1.5 min), seal the inlet and outlet ports and continue the reaction for 60 min. After the reaction is complete, cool to room temperature and dissolve the liquid product in dichloromethane. Filter to separate the solid and liquid phases. Dry the solid phase and weigh it. Rotary evaporate the liquid product and weigh it. The conversion rate is calculated to be 42%, the selectivity of the C1-C5 gaseous products is 3%, and the selectivity of the C6-C34 liquid products is 39%.

[0028] Examples 5-6 Examples 5 and 6 provide the same pyrolysis reaction preparation process for low-density polyethylene as in Example 2, except that only the reaction time is changed, as shown in Table 1.

[0029] Table 1. Effect of reaction time on catalytic activity in Examples 5-6

[0030] Example 7 Example 7 provides a pyrolysis reaction preparation process for low-density polyethylene that is the same as in Example 2, except that only the amount of catalyst used is changed, as shown in Table 2.

[0031] Table 2 Effect of catalyst dosage on catalytic activity in Example 7

[0032] Example 8 Example 8 provides a pyrolysis reaction preparation process for low-density polyethylene that is the same as in Example 2, except that only the reaction temperature is changed, as shown in Table 3.

[0033] Table 3 Effect of reaction temperature on catalytic activity in Example 8

[0034] Examples 9-11 The pyrolysis reaction preparation process of low-density polyethylene provided in Examples 9-11 is the same as that in Example 2. The difference between Examples 9-11 and Example 2 is the number of catalyst recovery cycles, as shown in Table 3.

[0035]

[0036] Based on Examples 1-11 and the comparative examples, it can be concluded that the catalyst Fe@CN prepared in Example 1 of this invention can not only effectively catalyze the degradation of PE to C5-C34, improving its liquid-phase selectivity and reducing the degradation temperature, but also restore its catalytic performance after high-temperature calcination and can be reused. The optimal conditions were determined to be a PE:Fe@CN mass ratio of 40:1, a reaction temperature of 360 ℃, and a reaction time of 60 min.

[0037] The above embodiments have provided a detailed description of the implementation of the present invention. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention. The above descriptions are merely preferred embodiments of the present invention and do not limit the scope of the present invention. All equivalent structural changes made based on the content of this specification are included within the scope of the present invention.

Claims

1. A method for preparing a nitrogen-doped PVC dechlorination carbon-supported iron catalyst, characterized in that, Includes the following steps: Step 1: Place melamine in a tube furnace and heat it to 550°C at a rate of 5°C / min. Calcine it under a nitrogen atmosphere for 4 hours to obtain C3N4. Step 2: Mix PVC and iron oxide, and dechlorinate them by heat at 220℃ for 30 minutes. The resulting solid is denoted as Fe@C. PVC ; Step 3: Add Fe@C PVC The mixture was ball-milled with the C3N4 obtained in step one at a mass ratio of 1:1, and heated to 700℃ at a rate of 5℃ / min under a nitrogen atmosphere, and held at that temperature for 2 h. The nitrogen-doped PVC dechlorination carbon-supported iron catalyst was obtained after grinding.

2. The method for preparing the nitrogen-doped PVC dechlorination carbon-supported iron catalyst according to claim 1, characterized in that, The mass ratio of PVC to iron oxide in step two is 20:

1.

3. The nitrogen-doped PVC dechlorination carbon-supported iron catalyst prepared by the method described in claim 1.

4. The application of the nitrogen-doped PVC dechlorination carbon-supported iron catalyst according to claim 3 in the pyrolysis reaction of polyolefins.

5. The application of the nitrogen-doped PVC dechlorination carbon-supported iron catalyst according to claim 4 in the polyolefin pyrolysis reaction, characterized in that, The process includes the following steps: placing the polyolefin and the nitrogen-doped PVC dechlorination carbon-supported iron catalyst in a reactor, mixing them thoroughly, exchanging the gas with N2, and maintaining the temperature at 320–400 °C. o Under temperature C, the pyrolysis reaction is carried out for 30–120 min. After the reaction is completed, the catalyst and pyrolysis products are separated.

6. The application of the nitrogen-doped PVC dechlorination carbon-supported iron catalyst according to claim 5 in the pyrolysis reaction of polyolefins, characterized in that, The mass ratio of the polyolefin and the nitrogen-doped PVC dechlorination carbon-supported iron catalyst is 40:0.5~2.

7. The application of the nitrogen-doped PVC dechlorination carbon-supported iron catalyst according to claim 6 in the polyolefin pyrolysis reaction, characterized in that, The mass ratio of the polyolefin and the nitrogen-doped PVC dechlorination carbon-supported iron catalyst is 40:1.

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