Method for hydrocracking waste plastics by using non-noble metal catalyst

Abstract

The invention belongs to the field of upgrading and recycling of waste plastics, and discloses a method for hydrocracking waste plastics by using a non-noble metal catalyst. The preparation method comprises the following steps: (1) respectively dispersing active metal salt and carrier metal salt into deionized water, and stirring at 15-45 DEG C until solids are completely dissolved to respectively obtain a transparent solution A and a transparent solution B; (2) uniformly mixing the transparent solution A and the transparent solution B to obtain a precursor solution C; (3) uniformly dipping the precursor solution C into a template agent to obtain an intermediate substance D; (4) completely drying the intermediate substance D, and roasting in air to obtain an intermediate E; and (5) carrying out reduction treatment on the intermediate E under certain conditions to obtain the catalyst. According to the catalyst, efficient activation and directional breakage of C-C bonds in waste plastic molecules are achieved based on the synergistic effect of the acid carrier and the non-noble metal, light alkanes such as methane and ethane hardly exist in gas products, and efficient upgrading and recycling of waste plastic hydrocracking are promoted.

Description

Technical Field

[0001] The present invention belongs to the field of upgraded recycling of waste plastics, and relates to a method for hydrocracking waste plastics by using a non-noble metal catalyst. Background Art

[0002] Plastic products have become an essential part of human life due to their low cost, good processability, and excellent corrosion resistance. The surging global demand for plastic products has driven the plastic production to increase exponentially. While the all-round plasticization promotes the convenience of modern life, it is also giving rise to an increasingly serious environmental crisis - the imbalance between the consumption and recycling of plastic products leads to the accumulation of waste, and the resulting persistent pollution not only destroys the stability of the natural ecosystem, but also ultimately backfires on the human living environment through the food chain enrichment effect. Plastic pollution is rising to the core challenge of the human sustainable development agenda, and the urgency of treating this environmental pollution is giving rise to a social governance consensus.

[0003] As typical petrochemical derivatives, the production process of plastic products not only consumes a large amount of fossil energy, but the rich carbon element resources contained in them also highlight the environmental protection and economic value of recycling waste plastics. By realizing the efficient recycling and resource treatment of waste plastics, the circular regeneration of carbon elements in the industrial system can be effectively promoted, which has important strategic significance for building a green and low-carbon circular economy model. At present, the waste plastic recycling methods include landfill, incineration, mechanical recycling, and chemical recycling. Landfilled waste plastics may leach harmful substances and cause soil and groundwater pollution; incinerating waste plastics will produce carbon dioxide emissions equivalent to three times the mass of the original plastics, and may also produce harmful substances to pollute the environment; plastics regenerated by mechanical methods often have degraded structural properties and are difficult to meet the production requirements of high-end products. Chemical recycling can convert plastic raw materials into high-value or high-quality chemicals through various catalysts, not only solving the environmental pollution problem, reducing carbon emissions, but also bringing additional value to chemical production, and realizing the upgraded recycling of plastic waste.

[0004] Due to the rich presence of highly stable covalent saturated C-C bonds in its molecular structure, waste plastics pose significant challenges in the chemical conversion process as the activation and cleavage of chemical bonds are difficult. Currently reported chemical recycling technologies include methods such as high-temperature pyrolysis and catalytic cracking. Among these technical routes, the heterogeneous catalytic hydrocracking process has attracted much attention from researchers due to its significant advantages: it can efficiently convert waste plastics into fuel components such as gasoline, diesel, and aviation kerosene under relatively mild reaction conditions, and also has characteristics such as easy separation of the catalyst and simple product purification. Based on these advantages, this technology has become a research hotspot in recent years. It is worth noting that current research on waste plastic hydrocracking catalysts mostly focuses on noble metal catalysts, and the high noble metal loading leads to high catalyst costs. Therefore, the development of non-noble metal catalysts for waste plastic hydrocracking is of great significance.

[0005] Chinese Patent CN108456328A discloses a method for treating waste plastics, which uses a non-noble metal modified zeolite catalyst to convert waste plastics into gas products such as ethylene and propylene and liquid products dissolved in cyclohexane and tetrahydrofuran, improving the added value of the resource utilization of waste plastics. Although this catalyst does not use noble metals and significantly reduces the reaction temperature under the premise of ensuring a high conversion rate in a hydrogen atmosphere, its preparation process is relatively complex, two solvents are added during the reaction, and the product separation process is cumbersome and the solvent cost is high. Currently reported non-noble metal catalytic systems in the field of waste plastic hydrocracking generally have two key technical bottlenecks: on the one hand, the non-noble metals have weak hydrogen dissociation ability, resulting in low waste plastic hydrocracking performance; on the other hand, additional zeolite additives often need to be introduced to increase the liquid fuel yield, increasing the process complexity (Green Chem., 2025, 27, 3398 - 3412). This current situation highlights the great scientific research value and application prospects of developing simple and efficient non-noble metal catalysts. Summary of the Invention

[0006] The object of the present invention is to provide a method for hydrocracking waste plastics using a non-noble metal catalyst. By adopting a solvent-free catalytic system, the efficient conversion of waste plastics can be achieved under mild reaction conditions; it exhibits excellent catalytic performance, including a prominent waste plastic conversion rate (>90%) and a high-value liquid fuel yield (>70%), with a relatively high selectivity for the gasoline fraction; the gas-phase product composition is ideal, with isobutane and pentane accounting for more than 75% and the content of methane and ethane being less than 0.5%, which can be used as liquefied petroleum gas after simple separation; the catalyst has good cyclic stability, its preparation process is simple, the cost is low, and rapid separation and recovery after the reaction can be achieved. This invention provides an economically efficient and environmentally friendly new way for the resource utilization of waste plastics.

[0007] The technical solution of the present invention:

[0008] A method for hydrocracking waste plastics using a non-noble metal catalyst realizes efficient hydrocracking of waste plastics by preparing a supported non-noble metal catalyst. The non-noble metal catalyst includes a supported metal component and an oxide support. The supported metal component includes one or more mixtures of Fe, Co, Ni, Mo, Cu, Zn, Mn, Al, and Y. It interacts with the support to form active centers, enabling the gradual breakage of polymer chains and selectively breaking the C-C bonds on the main chain to avoid the breakage of end-group C-C bonds to form methane and ethane. Under relatively mild reaction conditions, efficient conversion of waste plastics can be achieved, showing a good liquid fuel yield.

[0009] A method for hydrocracking waste plastics using a non-noble metal catalyst comprises the following steps

[0010] (1) Disperse the active metal salt and the support metal salt into deionized water respectively, and stir at 15 - 45 °C until the solids are completely dissolved to obtain transparent solution A and transparent solution B respectively;

[0011] (2) Mix transparent solution A and transparent solution B evenly to obtain precursor solution C;

[0012] (3) Impregnate precursor solution C evenly onto the template agent to obtain intermediate D;

[0013] (4) After the intermediate D is completely dried, calcine it in air to obtain intermediate E;

[0014] (5) Reduce intermediate E under certain conditions to obtain the catalyst.

[0015] Mix the waste plastics and the catalyst evenly and load them into a high-pressure reactor. Without adding additional solvent, after the reaction ends, wait for the reactor to cool to room temperature and collect the products. The gas products are collected in an air bag, and the liquid products and the remaining solids are collected in a centrifuge tube and then centrifuged for separation.

[0016] Preferably, in step (1), the active metal salt includes one or more mixtures of the corresponding hydrates of iron nitrate, cobalt nitrate, nickel nitrate, ammonium molybdate, zinc nitrate, copper nitrate, manganese nitrate, aluminum nitrate, and yttrium nitrate; the support metal salt is one of the corresponding hydrates of zirconium nitrate, aluminum nitrate, and ammonium metatungstate; the mass ratio of the active metal salt to the support metal salt is 1:(5 - 50).

[0017] Preferably, in step (2), the concentration of precursor solution C is 0.1 - 30 mol / L.

[0018] Preferably, in step (3), the template agent is one of C3N4, graphite, SBA-15, carbon black, and silica; the mass ratio of the template agent to precursor solution C is 1:(5 - 20).

[0019] Preferably, in step (4), the drying temperature is 60 - 100°C, the calcination temperature is 400 - 800°C, and the calcination time is 4 - 7 hours.

[0020] Preferably, in step (5), the reduction temperature is 300 - 500°C, and the reduction treatment time is 1 - 4 hours.

[0021] The method for using the non - precious metal catalyst obtained by the above - mentioned preparation method in the hydrocracking of waste plastics is characterized in that the mass ratio of the non - precious metal catalyst to the waste plastics is 0.05 - 0.5; the hydrogen pressure charged at room temperature is 0 - 4 MPa; the reaction temperature is 200 - 300°C; and the reaction time is 4 - 16 hours.

[0022] Preferably, the waste plastics are one or more mixtures of polyethylene, polypropylene, and polystyrene.

[0023] Advantages of the present invention: The non - precious metal catalyst for the hydrocracking of waste plastics to produce liquid fuel and its preparation method of the present invention. The catalyst prepared by the above hard template method has a high conversion rate of waste plastics. The products obtained by this system are easy to separate and have high application value. The gasoline selectivity in the liquid products is relatively high. In addition, almost no light alkanes such as methane and ethane with low added value are produced in the gas products. A catalyst with excellent performance is prepared using inexpensive and resource - rich non - precious metals. The preparation process is simple, the conditions are mild, suitable for large - scale production; and it is non - toxic, the catalyst performance is stable, and it is easy to separate after the reaction. The whole process is environmentally friendly. The invention solves the technical bottlenecks such as the high cost of existing precious metal catalysts and the difficulty of product separation, and provides an economically feasible solution for the resource utilization of waste plastics. Description of the Drawings

[0024] Figure 1 XRD diffraction patterns of Ni / WO3 and WO3.

[0025] Figure 2 Effect of different loadings on the hydrocracking of polyethylene catalyzed by Ni / WO3.

[0026] Figure 3 Effect of different active metals on the hydrocracking of polyethylene catalyzed by Ni / WO3.

[0027] Figure 4 Effect of the catalyst Ni / WO3 on the hydrocracking of different waste plastics.

[0028] Figure 5 Product distribution of Ni / WO3 in the gas at 250°C, 3 MPa hydrogen, and reaction for 6 hours. Detailed Embodiments

[0029] The following further illustrates the specific embodiments of the present invention in conjunction with the accompanying drawings and technical solutions.

[0030] Example 1: Preparation of Ni / WO3 catalyst

[0031] Weigh 1.0626 g of ammonium metatungstate hydrate and 0.127 g of nickel nitrate hexahydrate, add 6 ml of deionized water and heat with stirring until all nitrates are dissolved. Weigh 3 g of template agent, impregnate the above mixed solution evenly on the template agent, and then dry at 60 °C for 12 hours to ensure complete removal of moisture. Subsequently, the dried mixture of catalyst precursor powder is calcined in an air atmosphere at 500 °C for 6 hours to remove the template agent. Then the catalyst is reduced in a hydrogen atmosphere at 450 °C for 2 hours. The theoretical metal loading of the obtained catalyst is 2.5 wt%, and the XRD comparison diagram of it and WO3 is shown in Figure 1 .

[0032] Example 2: Explore the influence of different metal loadings on the performance of the catalyst

[0033] Weigh 1.0626 g of ammonium metatungstate hydrate, add 6 ml of deionized water and heat with stirring until all ammonium metatungstate is dissolved. Then add 0.0499, 0.0755, 0.1011, 0.127 g of nickel nitrate hexahydrate respectively and stir evenly. Weigh 3 g of template agent, impregnate the above mixed solution evenly on the template agent. Then dry at 60 °C for 12 hours to ensure complete removal of moisture. Subsequently, the dried mixture is calcined in an air atmosphere at 500 °C for 6 hours. Then the catalyst is reduced in a hydrogen atmosphere at 450 °C for 2 hours. The theoretical metal loadings of the obtained catalysts are 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%.

[0034] Weigh 0.4 g of the above catalysts with different metal loadings respectively and mix them with 4.0 g of polyethylene plastic and load them into a high-pressure reactor. React at a reaction condition of 3 MPa and 250 °C for 6 hours. After the reaction is completed and cooled to room temperature, collect the gas-phase products with an air bag, and separate the generated liquid and catalyst by centrifugation. The gas-phase and liquid-phase products are further analyzed by gas chromatography. The results show that the conversion rates are 62.5%, 83.27%, 90.63%, 97.98% respectively. It can be obtained that when the nickel loading in the catalyst is only 1 wt%, it can show high catalytic activity at a reaction temperature of 250 °C, and a liquid product with a mass yield of about 50% can be obtained. The detailed results are shown in Figure 2 .

[0035] Example 3: Explore the influence of different active metals on the performance of the catalyst

[0036] Weigh 1.0626 g of ammonium metatungstate hydrate, add 6 ml of deionized water, and heat and stir until the ammonium metatungstate is completely dissolved. Then add 0.4224 g of copper nitrate, 0.5487 g of cobalt nitrate, 0.5505 g of nickel nitrate, and 0.2044 g of ammonium molybdate by mass of the metal respectively. Weigh 3 g of the template agent, and uniformly impregnate the above mixed solution on the template agent. Then dry it at 60 °C for 12 hours to ensure that all the moisture is removed. Subsequently, calcine the dried mixture in an air atmosphere at 500 °C for 6 hours. Then reduce the catalyst in a hydrogen atmosphere at 400 °C for 2 hours. The theoretical metal loading of the obtained catalyst is 10 wt%.

[0037] Weigh 0.4 g of the above catalysts with different metal loadings respectively and mix them with 4.0 g of polyethylene plastic, then load them into a high-pressure reactor and react under the reaction conditions of 3 MPa and 300 °C for 8 hours. After the reaction is completed and cooled to room temperature, collect the gas-phase products with an air bag, and separate the generated liquid and the catalyst by centrifugation. Further analyze the gas-phase and liquid-phase products by gas chromatography. The results show that the conversion rates are 85.51%, 70.38%, 99.57%, and 99.56% respectively. Although the catalysts with these metal components have a high conversion rate of waste plastics, most of them are converted into waxes with a relatively large molecular weight. The catalyst loaded with nickel metal shows the highest liquid product yield. For the detailed results, see Figure 3 。

[0038] Example 4: Explore the influence of different supported metals on the performance of the catalyst

[0039] Weigh 1.0626 g of ammonium metatungstate hydrate, 7.3555 g of aluminum nitrate nonahydrate, and 3.484 g of zirconium nitrate hexahydrate, add 6 ml of deionized water, and heat and stir until completely dissolved. Then add 0.127 g of nickel nitrate hexahydrate. Weigh 3 g of the template agent, and uniformly impregnate the above mixed solution on the template agent. Then dry it at 60 °C for 12 hours to ensure that all the moisture is removed. Subsequently, calcine the dried mixture in an air atmosphere at 500 °C for 6 hours. Then reduce the catalyst in a hydrogen atmosphere at 450 °C for 2 hours. The theoretical metal loading of the obtained catalyst is 2.5 wt%.

[0040] Weigh 0.4 g of the above catalysts with different metal loadings respectively and mix them with 4.0 g of polyethylene plastic, then load them into a high-pressure reactor and react under the reaction conditions of 3 MPa and 250 °C for 6 hours. After the reaction is completed and cooled to room temperature, collect the gas-phase products with an air bag, and separate the generated liquid and the catalyst by centrifugation. Further analyze the gas-phase and liquid-phase products by gas chromatography. The results show that the conversion rates are 97.98%, 68.42%, and 96.35% respectively. The interaction between the loaded metal and the support makes the catalyst exhibit excellent catalytic performance.

[0041] Example 5: Explore the influence of different calcination temperatures on the performance of the catalyst

[0042] Weigh 1.0626 g of ammonium metatungstate hydrate, add 6 ml of deionized water, heat and stir until completely dissolved, then add 0.127 g of nickel nitrate hexahydrate. After stirring evenly, impregnate the above mixed solution evenly on the template agent. Then dry at 60 °C for 12 hours to ensure that all the moisture is removed. Subsequently, calcine the dried mixture in an air atmosphere at 400 °C, 500 °C, 700 °C, and 800 °C for 6 hours. After that, treat the catalyst in a hydrogen atmosphere at 450 °C (when the calcination temperature is lower than 450 °C, the calcination temperature is the reduction temperature) for 2 hours.

[0043] Weigh 0.4 g of the above catalysts with different calcination temperatures respectively and mix them with 4.0 g of polyethylene plastic, then load them into a high-pressure reactor and react under the reaction conditions of 3 MPa and 250 °C for 6 hours. After the reaction is completed and cooled to room temperature, collect the gas-phase products with an air bag, and separate the generated liquid and the catalyst by centrifugation. The gas-phase and liquid-phase products are further analyzed by gas chromatography. The results show that the conversion rates are 78.55%, 97.98%, 53.16%, and 45.62% respectively.

[0044] Example 6: Explore the influence of different reduction temperatures on the performance of the catalyst

[0045] Weigh 1.0626 g of ammonium metatungstate hydrate, add 6 ml of deionized water, heat and stir until completely dissolved, then add 0.127 g of nickel nitrate hexahydrate. After stirring evenly, impregnate the above mixed solution evenly on the template agent. Then dry at 60 °C for 12 hours to ensure that all the moisture is removed. Subsequently, calcine the dried mixture in an air atmosphere at 500 °C for 6 hours. After that, treat the catalyst in a hydrogen atmosphere at 350 °C, 400 °C, and 450 °C for 2 hours.

[0046] Weigh 0.4 g of the above catalysts with different reduction temperatures respectively and mix them with 4.0 g of polyethylene plastic, then load them into a high-pressure reactor and react under the reaction conditions of 3 MPa and 250 °C for 6 hours. After the reaction is completed and cooled to room temperature, collect the gas-phase products with an air bag, and separate the generated liquid and the catalyst by centrifugation. The gas-phase and liquid-phase products are further analyzed by gas chromatography. The results show that the conversion rates are 35.92%, 77.20%, and 97.98% respectively.

[0047] By studying the factors affecting the performance of the catalyst, we select the Ni / WO3 catalyst with a mass fraction of 2.5 wt% and the optimal catalytic performance as the target catalyst for the subsequent Examples 7, 8, and 9.

[0048] Example 7: Explore the influence of different reaction pressures on the conversion of polyethylene waste plastics by the catalyst

[0049] Weigh 0.4 g of the catalyst and 4.0 g of polyethylene plastic separately, mix them and load them into a high-pressure reactor. React under the reaction conditions of 250 °C for 6 hours. Change the reaction pressure and mainly explore the influence of 1 MPa, 2 MPa, 3 MPa and 4 MPa on the reaction. After the reaction is completed and cooled to room temperature, collect the gas-phase products with an air bag. The generated liquid and the catalyst are separated by centrifugation. The gas-phase and liquid-phase products are further analyzed by gas chromatography. The results show that the conversion rates are 35.24%, 56.46%, 97.98% and 99.85% respectively. It shows that the greater the hydrogen pressure, the more conducive it is to the cracking of polyolefins by the catalyst.

[0050] Example 8: Explore the influence of different reaction times on the catalytic conversion of polyethylene waste plastics by the catalyst

[0051] Weigh 0.4 g of the catalyst and 4.0 g of polyethylene plastic separately, mix them and load them into a high-pressure reactor. Mainly explore the influence of the reaction time on the reaction. React under the reaction conditions of 250 °C and 3 MPa of hydrogen, change the reaction time, and explore the influence at 4 hours, 6 hours, 12 hours and 16 hours. After the reaction is completed and cooled to room temperature, collect the gas-phase products with an air bag. The generated liquid and the catalyst are separated by centrifugation. The gas-phase and liquid-phase products are further analyzed by gas chromatography. The results show that the conversion rates are 61.32%, 97.98%, 99.04% and 99.66% respectively. It shows that a long reaction time will not cause the catalyst to be poisoned and inactivated.

[0052] Example 9: Explore the influence of the catalyst on the catalytic conversion of different types of waste plastics

[0053] Weigh 0.4 g of the catalyst and mix it with 4.0 g of polyethylene plastic, polypropylene plastic and polystyrene plastic separately, and load them into a high-pressure reactor. React under the reaction conditions of 300 °C and 3 MPa of hydrogen for 8 hours. After the reaction is completed and cooled to room temperature, collect the gas-phase products with an air bag. The generated liquid and the catalyst are separated by centrifugation. The gas-phase and liquid-phase products are further analyzed by gas chromatography. The detailed results are shown in Figure 4 。

[0054] Comparative Example 1: Explore the influence of different preparation methods on the catalytic performance of the Ni / WO3 catalyst

[0055] Weigh 1.0626 g of ammonium metatungstate hydrate, add 6 ml of deionized water, and heat and stir until completely dissolved. Then add 0.127 g of nickel nitrate hexahydrate. After stirring evenly, 1) Immerse the above mixed solution evenly on the templating agent; 2) Without using the templating agent, directly stir the mixed solution in a 60 °C water bath until it becomes viscous; 3) Impregnate the nickel nitrate solution in equal volume onto WO3 obtained by calcining the ammonium metatungstate solution, and then dry it at 60 °C for 12 hours to ensure complete removal of moisture. Subsequently, calcine the dried mixture in an air atmosphere at 500 °C for 6 hours. Then treat the catalyst in a hydrogen atmosphere at 450 °C for 2 hours. The theoretical metal loading of the obtained catalyst is 2.5 wt%.

[0056] Weigh 0.4 g of the catalysts prepared by the three methods respectively and mix them with 4.0 g of polyethylene plastic, and then load them into a high-pressure reactor. React under the reaction conditions of 3 MPa and 250 °C for 6 hours. After the reaction is completed and cooled to room temperature, collect the gas-phase products with an air bag, and separate the generated liquid and the catalyst by centrifugation. Further analyze the gas-phase and liquid-phase products by gas chromatography. The results show that the conversion rates are 97.98%, 13.16%, and 0% respectively. The templating agent causes an interaction between the loaded metal of the catalyst and the support to form an alloy, promoting the performance improvement of the catalyst in plastic hydrocracking.

Claims

1. A method for hydrogenolysis of waste plastics using a non-noble metal catalyst, characterized in that, The steps are as follows (1)Disperse the active metal salt and the support metal salt into deionized water respectively, and stir at 15-45 °C until the solids are completely dissolved to obtain transparent solution A and transparent solution B respectively; (2)Mix transparent solution A and transparent solution B evenly to obtain precursor solution C; (3)Impregnate precursor solution C evenly onto the template agent to obtain intermediate D; (4)After the intermediate D is completely dried, calcine it in air to obtain intermediate E; (5)Reduce intermediate E under certain conditions to obtain a non-noble metal catalyst.

2. The preparation method according to claim 1, characterized in that In step (1), the active metal salt includes one or more mixtures of the corresponding hydrates of iron nitrate, cobalt nitrate, nickel nitrate, ammonium molybdate, zinc nitrate, copper nitrate, manganese nitrate, aluminum nitrate, and yttrium nitrate.

3. The preparation method according to claim 1, characterized in that, In step (1), the support metal salt is one of the corresponding hydrates of zirconium nitrate, aluminum nitrate, and ammonium metatungstate; the mass ratio of the active metal salt to the support metal salt is 1:(5-50).

4. The preparation method according to claim 1, wherein, In step (2), the concentration of precursor solution C is 0.1-30 mol / L.

5. The preparation method according to claim 4, characterized in that, In step (3), the template agent is one of C3N4, graphite, SBA-15, carbon black, and silica; the mass ratio of the template agent to precursor solution C is 1:(5-20).

6. The preparation method according to claim 1, characterized in that, In step (4), the drying temperature is 60-100 °C, the calcination temperature is 400-800 °C, and the calcination time is 4-7 hours.

7. The preparation method according to claim 1, characterized in that, In step (5), the reduction temperature is 300-500 °C, and the reduction treatment time is 1-4 hours.

8. A method for using the non-noble metal catalyst obtained by the preparation method according to any one of claims 1-7 in the hydrocracking of waste plastics, characterized in that, The mass ratio of the non-noble metal catalyst to the waste plastic is 0.05-0.5; the hydrogen pressure charged at room temperature is 0-4 MPa; the reaction temperature is 200-300 °C; the reaction time is 4-16 hours.

9. The method according to claim 8, wherein The waste plastic is one or more mixtures of polyethylene, polypropylene, and polystyrene.

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