A method for catalytically carbonizing polyolefins to produce high quality carbon nanotubes

Abstract

This invention belongs to the field of polyolefin recycling technology and relates to a method for preparing high-quality carbon nanotubes by catalytic carbonization of polyolefins. In this invention, polyolefins are degraded at high temperature to generate small-molecule hydrocarbons. Subsequently, a Ni-Mo-Al catalyst undergoes a reduction reaction to generate elemental nickel. The elemental nickel catalyzes the degradation products to undergo dehydrogenation, cyclization, and aromatization reactions, followed by carbon deposition on the surface of the elemental nickel, which gradually grows into carbon nanotubes. Furthermore, the Ni-Mo-Al catalyst can reduce the formation of amorphous carbon, thereby increasing the activity of elemental nickel and reducing its ability to dissolve carbon, thus preparing carbon nanotubes with smaller diameters and uniform morphology. This invention has the advantages of inexpensive and readily available carbon sources and a simple preparation method, realizing the upgraded chemical recycling of waste polyolefins and the controllable preparation of carbon nanotubes, effectively solving the problem of reusing urban and industrial waste polyolefins.

Description

Technical Field

[0001] This invention belongs to the field of polyolefin recycling technology, and more specifically, relates to a method for preparing high-quality carbon nanotubes by catalytic carbonization of polyolefins. Background Technology

[0002] Carbon nanotubes are hollow tubular structures formed by hexagonal graphite sheets spiraling around a central axis at a specific angle. With their large specific surface area, high electrical conductivity, high mechanical strength, and excellent thermal and electromagnetic properties, along with unique potential applications in various fields, carbon nanotubes have become a research focus in chemistry, physics, and other disciplines, attracting widespread attention from the scientific community. In recent years, plastics, as a major polymer material, have been widely used in chemistry, construction, electronics, and medicine. However, the widespread use of plastic products inevitably generates a large amount of waste plastic. Due to their good chemical stability and resistance to degradation, waste plastics can persist for a long time under natural environmental conditions, causing serious damage to the ecological environment. Therefore, the recycling and reuse of waste plastics is an urgent issue. Polyolefin plastic products, as the largest category of plastic products, mainly include polyethylene and polypropylene. Polyolefins are mainly composed of carbon and hydrogen elements, with carbon content reaching as high as 85.7%. Therefore, using polyolefins as a carbon source to prepare high-value-added carbon nanotubes is a highly promising approach for recycling waste polyolefins.

[0003] The research group of Professor Tang Tao at the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, proposed a strategy for the carbonization of polyethylene to prepare carbon nanotubes using a combination catalyst of nickel oxide and organically modified montmorillonite. In this strategy, the organically modified montmorillonite acts as a degradation catalyst, promoting the degradation of polyethylene to generate a large number of small-molecule hydrocarbons and aromatic compounds; nickel oxide acts as a carbon-forming catalyst, catalyzing the carbonization of these small molecules to form carbon nanotubes, achieving a maximum yield of 56.5 wt% (Striking influence of chain structure of polyethylene on the formation of cup-stacked carbon nanotubes / carbon nanoofibers under the combined catalysis of CuBr and NiO. Applied Catalysis B: Environmental 2014, 147, 592-601). The disadvantages of this method are that the single nickel oxide catalysis effect is not ideal, the yield of carbon nanotubes is very low (<10wt%), and additional catalysts (e.g., organically modified montmorillonite) are required to assist nickel oxide in catalyzing the carbonization of polypropylene into carbon nanotubes; the post-processing is complex, requiring the use of a large amount of highly corrosive hydrofluoric acid to remove residual montmorillonite, generating a large amount of wastewater, and making it difficult to prepare carbon nanotubes in large quantities.

[0004] The Williams research group at the University of Leeds, UK, used a pyrolysis-reforming technique with a supported nickel catalyst to convert waste plastics into carbon nanotubes (Processing real-world waste plastics by pyrolysis-reforming for hydrogen and high-value carbon nanotubes. Environment Science & Technology 2014, 48, 819-826). The drawbacks of this method are low catalyst efficiency, easy deactivation, which is detrimental to carbon nanotube growth, and low yield (less than 40%) and poor morphology of the carbon nanotubes.

[0005] The research group of Song Rongjun at Northeast Forestry University used Ni-Mo-Mg as a catalyst to catalyze the carbonization of waste plastics to prepare carbon nanotubes. Although the yield was significantly improved, reaching up to 58%, the morphology of the carbon nanotubes was not uniform. In addition to the traditional straight carbon nanotubes, helical carbon nanotubes were also generated as a byproduct (Synthesis of carbon nanotubes from polypropylene in the presence of Ni / Mo / MgO catalysts via combustion. Chemistry Letter 2011, 40, 1110-1112). Furthermore, they added carbon black as a co-catalyst to further improve the carbon nanotube yield (Universal Ni-Mo-Mg catalysts combined with carbon blacks for the preparation of carbon nanotubes from polyolefins. Journal Applied Polymer Science. 2017, 56, 11734-11744). A drawback of this method is the difficulty in separating the carbon black from the carbon nanotube products to obtain high-purity carbon nanotubes.

[0006] In summary, although various catalysts have been reported for the carbonization of waste plastics to prepare carbon nanotubes, the yield of carbon nanotubes is low. Furthermore, the process requires the addition of auxiliary catalysts and the use of hydrofluoric acid to remove the catalyst, leading to cumbersome processes; or the carbon nanotubes exhibit uneven morphology, and helical carbon nanotubes may remain as byproducts; or other forms of carbon products may remain. Therefore, developing novel and efficient catalysts to achieve efficient carbonization of waste polyolefins and prepare high-quality carbon nanotubes with uniform morphology is extremely important. Summary of the Invention

[0007] To address the shortcomings and problems of previously reported technologies, this invention provides a method for preparing high-quality carbon nanotubes using polyolefins as a carbon source and Ni-Mo-Al as a catalyst. This invention involves mixing waste polyolefins with a Ni-Mo-Al catalyst, then heating the mixture to induce polyolefin degradation and carbonization of the degradation products to generate carbon nanotubes. Specifically, the polyolefins degrade at high temperatures to generate small-molecule hydrocarbons. Subsequently, the Ni-Mo-Al catalyst undergoes a reduction reaction to generate elemental nickel. The elemental nickel catalyzes the degradation products to undergo dehydrogenation, cyclization, and aromatization reactions, followed by carbon deposition on the surface of the elemental nickel, which gradually grows into carbon nanotubes. Furthermore, the Ni-Mo-Al catalyst reduces the formation of amorphous carbon, thereby increasing the activity of elemental nickel and reducing its ability to dissolve carbon, thus producing carbon nanotubes with smaller diameters and uniform morphology. This invention offers advantages such as inexpensive and readily available carbon sources and a simple preparation method, enabling the upgraded chemical recycling of waste polyolefins and the controllable preparation of carbon nanotubes, effectively solving the problem of reusing urban and industrial waste polyolefins.

[0008] According to the present invention, a method for preparing carbon nanotubes by catalytic carbonization of polyolefins is provided, comprising the following steps:

[0009] (1) After thoroughly mixing nickel salt, molybdenum salt and aluminum salt, add organic alcohol, grind thoroughly and then heat to obtain catalyst;

[0010] (2) After mixing the catalyst obtained in step (1) with polyolefin, the mixture is heated to degrade the polyolefin into small molecule hydrocarbons. Then, the catalyst catalyzes the small molecule hydrocarbons to undergo dehydrogenation, cyclization and aromatization reactions, so that carbon is deposited on the surface of the catalyst and gradually grows into carbon nanotubes.

[0011] Preferably, the nickel salt is nickel nitrate, nickel carbonate, or nickel oxalate; the molybdenum salt is ammonium molybdate; and the aluminum salt is aluminum nitrate, aluminum carbonate, or aluminum oxalate.

[0012] Preferably, the organic alcohol is polyethylene glycol or ethylene glycol.

[0013] Preferably, the polyolefin is polypropylene, polyethylene, polybutene, or polyisobutylene.

[0014] Preferably, in step (1), the heating temperature is 200℃~400℃ and the time is 15min~60min.

[0015] Preferably, in step (2), the heating temperature is 600℃~950℃.

[0016] Preferably, in step (1), the molar ratio of nickel, molybdenum and aluminum in the nickel salt, molybdenum salt and aluminum salt is (4-8):(0.1-0.8):(0.5-3).

[0017] Preferably, in step (2), the mass ratio of the polyolefin to the catalyst is (80-95):(5-20).

[0018] Preferably, in step (2), the polyolefin is waste polyolefin.

[0019] In summary, compared with the prior art, the above-described technical solutions conceived by this invention mainly possess the following technical advantages:

[0020] (1) The Ni-Mo-Al catalyst prepared in this invention incorporates molybdenum and aluminum to improve the yield of carbon nanotubes prepared by the carbonization of polyolefins using nickel catalysts, achieving a yield of up to >80%. Compared to existing technologies that use undoped nickel oxide as a catalyst to prepare carbon nanotubes, where the catalyst is easily poisoned and loses its activity, the presence of molybdenum and aluminum in the Ni-Mo-Al catalyst in this invention not only promotes the degradation of polyolefins to generate easily carbonizable small molecule degradation products, but also reduces the generation of amorphous carbon and improves the activity of elemental nickel.

[0021] (2) In this invention, carbon nanotubes are prepared by uniformly mixing Ni-Mo-Al catalyst and polyolefin, and then carbonizing the mixture at 600℃ to 950℃. Compared with previously reported carbon nanotubes, which have large diameters and uncontrollable morphology, the carbon nanotubes prepared in this invention have controllable morphology, high aspect ratio, high purity, and good quality. This is because the doping of nickel catalyst with molybdenum and aluminum reduces the ability of elemental nickel to dissolve carbon, thereby preparing carbon nanotubes with smaller diameters and greater uniformity. This invention effectively solves the problem of recycling and reusing urban and industrial waste polyolefins while also preparing high-value-added carbon nanotubes, and has broad application prospects.

[0022] (3) Preferably, in this invention, nickel nitrate, aluminum nitrate, ammonium molybdate, and polyethylene glycol are thoroughly mixed and ground using a simple physical-mechanical method. The mixture is then calcined in a muffle furnace, the sample is collected, and ground into powder to obtain the Ni-Mo-Al catalyst. Compared to the preparation of carbon nanotubes using combined catalysts, this invention features simple equipment, convenient operation, and low cost, and does not require the addition of additional co-catalysts to assist in the carbonization of polyolefins into carbon nanotubes. Furthermore, the post-treatment process is simple, eliminating the need for highly corrosive hydrofluoric acid to remove residual co-catalysts and preventing the generation of large amounts of wastewater.

[0023] (4) In this invention, carbon nanotubes are prepared by uniformly mixing Ni-Mo-Al catalyst and polyolefin and then carbonizing at 600℃ to 950℃. Previously reported methods using Ni-Mo-Mg catalyst to prepare carbon nanotubes from waste plastics achieved a significantly higher yield of 56%, but this resulted in 5% helical carbon nanotubes. In contrast, the carbon nanotubes prepared by this method do not contain other forms of carbon nanotubes, produce fewer carbon nanotube byproducts, and exhibit a high degree of graphitization. Attached Figure Description

[0024] Figure 1 The table shows the characterization results of the Ni-Mo-Al catalyst and carbon nanotube products in Example 1. In the table, a is a scanning electron microscope image of the carbon nanotubes, b is a transmission electron microscope image of the carbon nanotubes, c is the nitrogen adsorption-desorption curve of the carbon nanotubes, and d is the X-ray diffraction pattern of the carbon nanotubes.

[0025] Figure 2 The table shows the characterization results of the Ni-Mo-Al catalyst and carbon nanotube products in Example 2. In the table, a is the X-ray diffraction pattern of the Ni-Mo-Al catalyst, b is the Raman spectrum of the carbon nanotubes, c is the thermogravimetric curve of the carbon nanotubes, and d is the differential thermogravimetric curve of the carbon nanotubes.

[0026] Figure 3 The image shows the X-ray diffraction pattern of the Ni-Mo-Al catalyst in Example 3.

[0027] Figure 4 The image shows the X-ray diffraction pattern of the Ni-Mo-Al catalyst in Example 4.

[0028] Figure 5 The table shows the characterization results of the Ni-Mo-Al catalyst and carbon nanotube products in Example 5. In the table, a is the X-ray diffraction pattern of the Ni-Mo-Al catalyst, b is a scanning electron microscope image of the carbon nanotubes, c is the nitrogen adsorption-desorption curve of the carbon nanotubes, and d is the pore size distribution map of the carbon nanotubes.

[0029] Figure 6 The image shows the X-ray diffraction pattern of the Ni-Mo-Al catalyst in Example 6.

[0030] Figure 7 The image shows the X-ray diffraction pattern of the Ni-Mo-Al catalyst in Example 7.

[0031] Figure 8 The table shows the characterization results of the Ni-Mo-Al catalyst and carbon nanotube products in Example 8. In the table, a is the X-ray diffraction pattern of the Ni-Mo-Al catalyst, b is a scanning electron microscope image of the carbon nanotubes, c is the nitrogen adsorption-desorption curve of the carbon nanotubes, and d is the pore size distribution map of the carbon nanotubes.

[0032] Figure 9 These are low-magnification and high-magnification scanning electron microscope images of the carbon material product in Comparative Example 1.

[0033] Figure 10 These are low-magnification and high-magnification scanning electron microscope images of the carbon material products in Comparative Example 2. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0035] This invention discloses a method for preparing carbon nanotubes by catalytic carbonization of polyolefins, comprising the following steps:

[0036] (1) After thoroughly mixing nickel salt, molybdenum salt and aluminum salt, add organic alcohol, grind thoroughly and then heat to obtain catalyst;

[0037] (2) After mixing the catalyst obtained in step (1) with polyolefin, the mixture is heated to degrade the polyolefin into small molecule hydrocarbons. Then, the catalyst catalyzes the small molecule hydrocarbons to undergo dehydrogenation, cyclization and aromatization reactions, so that carbon is deposited on the surface of the catalyst and gradually grows into carbon nanotubes.

[0038] In some embodiments, the present invention provides a method for preparing high-quality carbon nanotubes by catalytic carbonization of waste polyolefins, comprising the following steps:

[0039] (1) Preparation of Ni-Mo-Al catalyst

[0040] Nickel nitrate, ammonium molybdate, and aluminum nitrate were mixed evenly in a certain molar ratio, then polyethylene glycol was added, and the mixture was ground thoroughly. After that, it was calcined in a muffle furnace. The product was cooled to room temperature and then ground into powder to obtain the Ni-Mo-Al catalyst.

[0041] (2) Preparation and purification of carbon nanotubes

[0042] Waste polyolefin and Ni-Mo-Al catalyst powder were mixed in a certain mass ratio, the mixture was placed in a crucible, and then heated for carbonization. After the reaction was completed, the product was cooled to room temperature and then purified, washed and dried to obtain high-quality carbon nanotubes.

[0043] In some embodiments, the molar ratio of nickel, molybdenum and aluminum in the catalyst described in step (1) is 4-8:0.1-0.8:0.5-3.

[0044] In some embodiments, the temperature used in the heating process in step (1) is 200℃~400℃, and the holding time is 15min~60min.

[0045] In some embodiments, the polyolefin in step (2) is one or more of polypropylene, low-density polyethylene, high-density polyethylene, polybutene, or polyisobutylene.

[0046] In some embodiments, the mass ratio of waste polyolefin to catalyst in step (2) is (80-95):(5-20).

[0047] In some embodiments, the carbonization temperature in step (2) is 600°C to 950°C.

[0048] The following are specific embodiments.

[0049] Example 1

[0050] Nickel nitrate, ammonium molybdate, and aluminum nitrate were mixed evenly in a molar ratio of 4:0.1:0.5. Polyethylene glycol was then added, and the mixture was thoroughly ground. The mixture was then calcined in a muffle furnace at 200°C for 15 minutes, cooled to room temperature, and ground into powder to obtain the Ni-Mo-Al catalyst. 5.00 g of waste polypropylene and 1.25 g of Ni-Mo-Al catalyst powder were mixed at a mass ratio of 80%:20%. The mixture was placed in a crucible and carbonized at 600°C. After the reaction was complete, the product was cooled to room temperature, purified with nitric acid, washed, and dried to obtain 3.72 g of carbon nanotubes, with a yield of 86.8%.

[0051] Figure 1 In the image, 'a' represents a scanning electron microscope image of carbon nanotubes. Figure 1 In the image, b is a transmission electron microscope image of carbon nanotubes. Figure 1 In the figure, c represents the nitrogen adsorption-desorption curve of carbon nanotubes. Figure 1 In the diagram, 'd' represents the X-ray diffraction pattern of the carbon nanotubes. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that the carbon nanotubes have a diameter of 15–20 nm and a length of several micrometers. The nitrogen adsorption and desorption curves reveal a distinct adsorption and desorption lag ring, and the specific surface area of ​​the carbon nanotubes is 174.6 m². 2 / g. The crystal structure of carbon nanotubes can be confirmed by X-ray diffraction patterns.

[0052] Example 2

[0053] Nickel nitrate, ammonium molybdate, and aluminum nitrate were mixed evenly in a molar ratio of 4:0.2:0.5. Polyethylene glycol was then added, and the mixture was thoroughly ground. The mixture was then calcined in a muffle furnace at 250°C for 20 minutes. After cooling to room temperature, the product was ground into powder to obtain the Ni-Mo-Al catalyst. 5 g of waste polyolefin and 0.88 g of Ni-Mo-Al catalyst powder were mixed at a mass ratio of 85%:15%. The mixture was placed in a crucible and carbonized at 600°C. After the reaction was complete, the product was cooled to room temperature, purified with nitric acid, washed, and dried to obtain 3.65 g of carbon nanotubes, with a yield of 78.0%.

[0054] Figure 2 In the diagram, 'a' represents the X-ray diffraction pattern of the Ni-Mo-Al catalyst. Figure 2 In the image, b represents the Raman spectrum of the carbon nanotube. Figure 2 In the figure, c represents the thermogravimetric curve of carbon nanotubes. Figure 2 In the figure, d represents the differential thermogravimetric curve of carbon nanotubes. The X-ray diffraction pattern shows the characteristic absorption peaks of elemental Ni and NiO. The Raman spectrum shows the peak at 1340 cm⁻¹. -1 The D peak and 1583 cm -1 The G peak is a typical peak for carbon nanotubes. The thermogravimetric curve of the carbon nanotubes shows that the purity is 96%; the differential thermogravimetric curve shows that the temperature of the maximum weight loss rate is 617℃.

[0055] Example 3

[0056] Nickel nitrate, ammonium molybdate, and aluminum nitrate were mixed evenly in a molar ratio of 5:0.3:1. Polyethylene glycol was then added, and the mixture was thoroughly ground. The mixture was then calcined in a muffle furnace at 300°C for 25 minutes. After cooling to room temperature, the product was ground into powder to obtain the Ni-Mo-Al catalyst. 5.00 g of waste low-density polyethylene and 0.56 g of Ni-Mo-Al catalyst powder were mixed at a mass ratio of 90%:10%. The mixture was placed in a crucible and carbonized at 650°C. After the reaction was complete, the product was cooled to room temperature, purified with nitric acid, washed, and dried to obtain 2.82 g of carbon nanotubes, with a yield of 65.8%.

[0057] Figure 3 The image shows the X-ray diffraction pattern of the Ni-Mo-Al catalyst. The X-ray diffraction pattern reveals the characteristic absorption peaks of elemental Ni and NiO.

[0058] Example 4

[0059] Nickel nitrate, ammonium molybdate, and aluminum nitrate were mixed evenly in a molar ratio of 5:0.4:1.5. Polyethylene glycol was then added, and the mixture was thoroughly ground. The mixture was then calcined in a muffle furnace at 350°C for 30 minutes. After cooling to room temperature, the product was ground into powder to obtain the Ni-Mo-Al catalyst. 5.00 g of waste low-density polyethylene and 0.26 g of Ni-Mo-Al catalyst powder were mixed at a mass ratio of 95%:5%. The mixture was placed in a crucible and carbonized at 650°C. After the reaction was complete, the product was cooled to room temperature, purified with nitric acid, washed, and dried to obtain 2.51 g of carbon nanotubes, with a yield of 58.6%.

[0060] Figure 4 The image shows the X-ray diffraction pattern of the Ni-Mo-Al catalyst. The X-ray diffraction pattern reveals the characteristic absorption peaks of elemental Ni and NiO.

[0061] Example 5

[0062] Nickel nitrate, ammonium molybdate, and aluminum nitrate were mixed evenly in a molar ratio of 6:0.5:2. Polyethylene glycol was then added, and the mixture was thoroughly ground. The mixture was then calcined in a muffle furnace at 400°C for 35 minutes. After cooling to room temperature, the product was ground into powder to obtain the Ni-Mo-Al catalyst. 5.00 g of waste high-density polyethylene and 0.56 g of Ni-Mo-Al catalyst powder were mixed at a mass ratio of 90%:10%. The mixture was placed in a crucible and carbonized at 700°C. After the reaction was complete, the product was cooled to room temperature, purified with nitric acid, washed, and dried to obtain 3.11 g of carbon nanotubes, with a yield of 72.6%.

[0063] Figure 5 In the diagram, 'a' represents the X-ray diffraction pattern of the Ni-Mo-Al catalyst. Figure 5 In the image, b is a scanning electron microscope image of carbon nanotubes. Figure 5 In the figure, c represents the nitrogen adsorption-desorption curve of carbon nanotubes. Figure 5 In the diagram, 'd' represents the pore size distribution of the carbon nanotubes. X-ray diffraction patterns show characteristic absorption peaks for elemental Ni and NiO. Scanning electron microscopy reveals that the carbon nanotubes have a diameter of 20–30 nm and a length of several micrometers. Nitrogen adsorption and desorption curves and the pore size distribution diagram show a distinct adsorption and desorption lag ring, and the specific surface area of ​​the carbon nanotubes is 215.5 m². 2 / g, and has abundant micropores (<2nm), mesopores (2-50nm) and macropores (>50nm).

[0064] Example 6

[0065] Nickel nitrate, ammonium molybdate, and aluminum nitrate were mixed evenly in a molar ratio of 7:0.6:2.5. Polyethylene glycol was then added, and the mixture was thoroughly ground. The mixture was then calcined in a muffle furnace at 400°C for 40 minutes. After cooling to room temperature, the product was ground into powder to obtain the Ni-Mo-Al catalyst. 5.00 g of waste high-density polyethylene and 0.88 g of Ni-Mo-Al catalyst powder were mixed at a mass ratio of 85%:15%. The mixture was placed in a crucible and carbonized at 750°C. After the reaction was complete, the product was cooled to room temperature, purified with nitric acid, washed, and dried to obtain 3.66 g of carbon nanotubes, with a yield of 85.4%.

[0066] Figure 6 The image shows the X-ray diffraction pattern of the Ni-Mo-Al catalyst. The characteristic absorption peaks of elemental Ni and NiO can be observed from the X-ray diffraction pattern.

[0067] Example 7

[0068] Nickel nitrate, ammonium molybdate, and aluminum nitrate were mixed evenly in a molar ratio of 7:0.7:3. Polyethylene glycol was then added, and the mixture was thoroughly ground. The mixture was then calcined in a muffle furnace at 350°C for 45 minutes. After cooling to room temperature, the product was ground into powder to obtain the Ni-Mo-Al catalyst. 5.00 g of waste polybutene and 1.25 g of Ni-Mo-Al catalyst powder were mixed at a mass ratio of 80%:20%. The mixture was placed in a crucible and carbonized at 850°C. After the reaction was complete, the product was cooled to room temperature, purified with nitric acid, washed, and dried to obtain 3.85 g of carbon nanotubes, with a yield of 89.8%.

[0069] Figure 7 This is the X-ray diffraction pattern of the Ni-Mo-Al catalyst. The characteristic absorption peaks of elemental Ni and NiO can be seen from the X-ray diffraction pattern.

[0070] Example 8

[0071] Nickel nitrate, ammonium molybdate, and aluminum nitrate were mixed evenly in a molar ratio of 8:0.8:0.1. Polyethylene glycol was then added, and the mixture was thoroughly ground. The mixture was then calcined in a muffle furnace at 300°C for 60 minutes. After cooling to room temperature, the product was ground into powder to obtain the Ni-Mo-Al catalyst. 5.00 g of waste polyisobutylene and 0.88 g of Ni-Mo-Al catalyst powder were mixed at a mass ratio of 85%:15%. The mixture was placed in a crucible and then carbonized in a muffle furnace at 850°C. After the reaction was complete, the product was cooled to room temperature, purified with nitric acid, washed, and dried to obtain 3.73 g of carbon nanotubes, with a yield of 87%.

[0072] Figure 8 In the diagram, 'a' represents the X-ray diffraction pattern of the Ni-Mo-Al catalyst. Figure 8In the image, b is a scanning electron microscope image of carbon nanotubes. Figure 8 In the figure, c represents the nitrogen adsorption-desorption curve of carbon nanotubes. Figure 8 In the diagram, 'd' represents the pore size distribution of the carbon nanotubes. X-ray diffraction patterns show characteristic absorption peaks for elemental Ni and NiO. Scanning electron microscopy reveals that the carbon nanotubes have a diameter of 15–25 nm and a length of several micrometers. Nitrogen adsorption-desorption curves and the pore size distribution diagram indicate the presence of a significant adsorption-desorption lag ring, and the specific surface area of ​​the carbon nanotubes is 167.2 m². 2 / g, and has abundant micropores (<2nm), mesopores (2-50nm) and macropores (>50nm).

[0073] The following is a comparative example

[0074] Comparative Example 1

[0075] Nickel nitrate and ammonium molybdate were mixed evenly at a molar ratio of 6:0.5, then polyethylene glycol was added, and the mixture was thoroughly ground. The mixture was then calcined in a muffle furnace at 400℃ for 35 min, cooled to room temperature, and the product was ground into powder to obtain the Ni-Mo catalyst. 5.00 g of waste high-density polyethylene and 0.56 g of Ni-Mo catalyst powder were mixed at a mass ratio of 90%:10%. The mixture was placed in a crucible and carbonized at 700℃. After the reaction was complete, the product was cooled to room temperature, purified with nitric acid, washed, and dried to obtain 0.98 g of carbon material product, with a yield of 22.9%.

[0076] Figure 9 The images show low-magnification and high-magnification scanning electron microscope (SEM) images of the carbon material products in Example 1 for comparison. The carbon material products consist of a small amount of carbon nanotubes and a large amount of amorphous carbon materials. The diameter of the carbon nanotubes is 30–45 nm.

[0077] Comparative Example 2

[0078] Nickel nitrate and aluminum nitrate were mixed evenly at a molar ratio of 6:2, then polyethylene glycol was added, and the mixture was thoroughly ground. The mixture was then calcined in a muffle furnace at 400℃ for 35 min, cooled to room temperature, and the product was ground into powder to obtain the Ni-Al catalyst. 5.00 g of waste high-density polyethylene and 0.56 g of Ni-Al catalyst powder were mixed at a mass ratio of 90%:10%. The mixture was placed in a crucible and carbonized at 700℃. After the reaction was complete, the product was cooled to room temperature, purified with nitric acid, washed, and dried to obtain 0.90 g of carbon material product, with a yield of 21.0%.

[0079] Figure 10The images show low-magnification and high-magnification scanning electron microscope (SEM) images of the carbon material products in Example 2 for comparison. The carbon material products consist of a small amount of carbon nanotubes and a large amount of amorphous carbon material. The diameter of the carbon nanotubes is 30–50 nm.

[0080] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing carbon nanotubes by catalytic carbonization of polyolefins, characterized in that, Includes the following steps: (1) After thoroughly mixing nickel salt, molybdenum salt and aluminum salt, polyethylene glycol is added, and the mixture is thoroughly ground and then heated. The heating temperature is 200 ℃~400 ℃ and the time is 15 min~60 min to obtain a catalyst. The molar ratio of nickel, molybdenum and aluminum elements in the nickel salt, molybdenum salt and aluminum salt is (4-8):(0.1-0.8):(0.5-3). (2) After mixing the catalyst obtained in step (1) with polyolefin, it is heated to a temperature of 600 ℃~950 ℃, so that the polyolefin is first degraded to generate small molecule hydrocarbons. Then the catalyst catalyzes the small molecule hydrocarbons to undergo dehydrogenation, cyclization and aromatization reactions, so that carbon is deposited on the surface of the catalyst and gradually grows into carbon nanotubes.

2. The method for preparing carbon nanotubes by catalytic carbonization of polyolefins as described in claim 1, characterized in that, The nickel salt is nickel nitrate, nickel carbonate, or nickel oxalate; the molybdenum salt is ammonium molybdate; and the aluminum salt is aluminum nitrate, aluminum carbonate, or aluminum oxalate.

3. The method for preparing carbon nanotubes by catalytic carbonization of polyolefins as described in claim 1, characterized in that, The polyolefin is polypropylene, polyethylene, polybutene, or polyisobutylene.

4. The method for preparing carbon nanotubes by catalytic carbonization of polyolefins as described in claim 1, characterized in that, In step (2), the mass ratio of the polyolefin to the catalyst is (80~95):(5~20).

5. The method for preparing carbon nanotubes by catalytic carbonization of polyolefins as described in claim 1, characterized in that, In step (2), the polyolefin is waste polyolefin.

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