Catalyst for carbon nanotube, preparation method of catalyst and preparation method of carbon nanotube

Abstract

The present invention relates to the preparation of carbon nanotubes, and discloses a catalyst for the preparation of carbon nanotubes wherein the catalyst comprises a molten halogen salt, the halogen salt comprising calcium chloride, potassium chloride and other halogen salts, the other halogen salt is selected from one or more of zinc chloride, copper chloride, barium chloride, titanium chloride, chromium chloride, gallium chloride, cobalt chloride, nickel chloride, manganese chloride, ferric chloride, ferrous chloride and aluminum chloride, and based on the total amount of the catalyst, the content of calcium chloride is 20-80 mol%, the content of potassium chloride is 0-10 mol%, and the content of other halogen salt is 20-80 mol%. The catalyst prepared by using the molten salt as the carbon nanotube can effectively reduce metal pollution in a carbon product, and is more environment-friendly compared with acid pickling in a metal catalytic system.

Description

Technical Field

[0001] The present invention relates to the technical field of catalysts for carbon nanotubes, and specifically relates to a catalyst for carbon nanotubes, a preparation method thereof, and a preparation method of carbon nanotubes. Background Art

[0002] Due to its special lattice structure, carbon nanotubes are endowed with excellent mechanical, thermal, electrical and other properties, and are widely used in the fields of chemical industry, electronics, biology, and medicine. In the past few decades, the development and application of new energy materials such as carbon nanotubes have greatly promoted scientific progress. In recent years, the market demand and scale of carbon nanotubes are facing rapid growth. However, low yield and high price limit their further large-scale application. Developing technologies for batch and low-cost preparation of carbon nanotubes has become one of the most challenging directions in the field of carbon nanomaterials and a high point for scientific workers in various countries to compete for.

[0003] The current main method for preparing carbon nanotubes is chemical vapor deposition. Although this process has the characteristics of simple equipment and controllable carbon material structure. However, during the preparation process, it is required that the concentration of raw material gas methane is extremely low, so it is necessary to specially equip inert gas for dilution, increasing the input cost. Moreover, the introduction of inert gas significantly reduces the concentration of hydrogen generated by cracking, and the cost of hydrogen separation increases greatly. On the other hand, chemical vapor deposition usually uses metal catalysts as the growth base of carbon materials, such as iron-based catalysts, copper-based catalysts, nickel-based catalysts, etc. It should be noted that the use of metal catalysts will invariably cause metal pollution in the generated carbon materials, making the separation and purification of carbon materials more difficult. It is necessary to introduce pickling, increasing the operation complexity and also bringing greater pollution to the environment.

[0004] CN114933296A discloses a method for preparing carbon nanotubes by using molten metal copper. To keep the metal copper melted, the reaction temperature is controlled at 1000 - 1500 °C, which will inevitably increase the system energy consumption and the resulting greenhouse gas emissions. On the other hand, due to the splash and evaporation of the molten metal, there will surely be some metal small particles on the prepared carbon materials, which need to be purified by pickling, increasing the cost and also causing greater pollution to the environment.

[0005] CN115005494B proposes a method and application for reducing the cracking temperature of saccharide compounds, which realizes the cracking reaction of monosaccharides and disaccharides at low temperature to generate flavor substances by using metal salt or metal oxide solid catalysts. The catalyst design and applicable scope are significantly different from those of the present invention.

[0006] CN109573983B discloses a transition metal-filled carbon nanotube, a preparation method and an application thereof. It is prepared by uniformly mixing a transition metal salt, melamine and a molten salt in an ethanol solvent, and then placing it in a tubular furnace for high-temperature carbonization, pickling, water washing, drying and other processes. Its preparation process is relatively complex, and there are obvious differences in catalyst design compared with the present invention. Summary of the Invention

[0007] The purpose of the present invention is to overcome the problems that the catalyst used in the preparation of carbon nanotubes in the prior art causes metal pollution and is difficult to separate, and to provide a green and low-cost catalyst for carbon nanotubes, a preparation method thereof and a preparation method of carbon nanotubes.

[0008] In order to achieve the above object, in the first aspect of the present invention, a catalyst for preparing carbon nanotubes is provided, wherein the catalyst comprises a molten halogen salt, and the halogen salt comprises calcium chloride, potassium chloride and other halogen salts, and the other halogen salts are selected from one or more of zinc chloride, copper chloride, barium chloride, titanium chloride, chromium chloride, gallium chloride, cobalt chloride, nickel chloride, manganese chloride, iron chloride, ferrous chloride and aluminum chloride. Based on the total amount of the catalyst, the content of calcium chloride is 20-80 mol%, the content of potassium chloride is 0-10 mol%, and the content of other halogen salts is 20-80 mol%.

[0009] In the second aspect of the present invention, a preparation method of the catalyst for carbon nanotubes according to the first aspect is provided, which includes the following steps: mixing the calcium chloride, potassium chloride and other halogen salts and heating to obtain a molten halogen salt catalyst.

[0010] In the third aspect of the present invention, a preparation method of carbon nanotubes is provided, which includes the following steps: under the protection of a gas and in the presence of a catalyst, catalytic cracking of a mixed gas containing a gaseous carbon source is carried out to obtain carbon nanotubes and hydrogen; wherein the catalyst is the catalyst described in the first aspect.

[0011] Through the above technical solutions, the beneficial effects of the present invention are as follows:

[0012] 1. For the first time, carbon nanotubes and hydrogen are prepared with molten salts, and the cost of salts is lower than that of the metal catalytic system.

[0013] 2. Utilizing the physical property of the molten liquid catalyst, that is, the density difference between the molten liquid catalyst and the carbon material, enables the generated carbon material to spontaneously float on its surface, effectively avoiding the problem that the catalyst quickly deactivates due to surface carbon deposition in traditional solid-state catalytic cracking.

[0014] 3. The good thermal conductivity and uniform catalytic ability of the liquid-phase catalytic medium enable the feed gas to be heated and contact with the catalytic medium more evenly, improving the cracking efficiency. Moreover, the feed gas in the reactor needs to pass through the liquid phase layer, and the viscous resistance exerted in the vertical direction by the liquid-phase medium provides sufficient residence time for the decomposition of methane, allowing it to react fully.

[0015] 4. Compared with metal catalytic systems (solid state, molten state), using molten salt as the catalyst for carbon nanotube preparation can effectively reduce micron-level or nano-level metal contamination in carbon products. Since it is highly soluble in water, pure carbon materials can be obtained through simple water washing, which is more environmentally friendly than acid washing in metal catalytic systems.

[0016] 5. The catalyst of the present invention can effectively solve the limitation of the traditional preparation of carbon nanotubes that requires low-concentration feed gas, helping to increase the hydrogen concentration in the product gas, reduce the subsequent hydrogen separation and purification costs, and also helping to reduce the cost of additional supply of inert gas. Based on the above, the present invention has good application prospects. Description of the Drawings

[0017] Figure 1 It is a schematic diagram of a methane cracking device;

[0018] Figure 2a It is a scanning electron microscope result diagram of the carbon material prepared in Example 1;

[0019] Figure 2b It is a scanning electron microscope result diagram of the carbon material prepared in Comparative Example 1;

[0020] Figure 2c It is a scanning electron microscope result diagram of the carbon material prepared in Comparative Example 3;

[0021] Figure 2d It is a scanning electron microscope result diagram of the carbon material prepared in Comparative Example 5;

[0022] Figure 3 It is a scanning electron microscope result diagram of the carbon material prepared in Example 4;

[0023] Figure 4 It is the Raman analysis result of the carbon materials prepared in Example 1, Comparative Example 1, and Comparative Example 3.

[0024] Description of the reference numerals:

[0025] 1. Heating furnace, 2. Air inlet hole, 3. Catalyst, 4. Corundum tube, 5. Air outlet hole, 6. Carbon discharge tank, 7. Collection bin, 8. Carbon material, 9. Valve, 10. Bubble. Detailed Embodiments

[0026] The endpoints and any values in the ranges disclosed herein are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0027] The first aspect of the present invention provides a catalyst for preparing carbon nanotubes. Among them, the catalyst includes a molten halogen salt, and the halogen salt includes calcium chloride, potassium chloride and other halogen salts. Among them, the other halogen salts are selected from one or more of zinc chloride, copper chloride, barium chloride, titanium chloride, chromium chloride, gallium chloride, cobalt chloride, nickel chloride, manganese chloride, iron chloride, ferrous chloride and aluminum chloride. Based on the total amount of the catalyst, the content of calcium chloride is 20-80 mol%, the content of potassium chloride is 0-10 mol%, and the content of other halogen salts is 20-80 mol%.

[0028] In the present invention, the composition of each component in the catalyst satisfies 100 mol%.

[0029] In the present invention, for the first time, carbon nanotubes and hydrogen are prepared using molten salts. Compared with the metal catalytic system, the salt cost is lower. Utilizing the physical property of the molten liquid catalyst, that is, the density difference between the molten liquid catalyst and the carbon material, enables the generated carbon material to spontaneously float on its surface, effectively avoiding the problem of rapid deactivation of the catalyst due to surface carbon deposition in traditional solid-state catalytic cracking. Using molten salts as the catalyst can effectively reduce metal contamination in carbon products because it is highly soluble in water, and pure carbon materials can be obtained through simple water washing, which is more environmentally friendly than pickling in the metal catalytic system.

[0030] In some specific embodiments of the present invention, preferably, based on the total amount of the catalyst, the content of calcium chloride is 30-60 mol%, the content of potassium chloride is 0-10 mol%, and the content of other metal salts is 40-70 mol%. The catalyst component content within the above range has better effects in the preparation of carbon nanotubes, can more effectively prepare high-quality carbon nanotubes, more effectively reduce metal contamination in carbon products, and has higher catalytic activity.

[0031] In some specific embodiments of the present invention, preferably, the other halogen salts are selected from one or more of barium chloride, chromium chloride, gallium chloride, cobalt chloride, nickel chloride and manganese chloride. The catalyst components within the above range have better effects in the preparation of carbon nanotubes, can more effectively prepare high-quality carbon nanotubes, more effectively reduce metal contamination in carbon products, and have higher catalytic activity.

[0032] In the second aspect of the present invention, a method for preparing a catalyst for preparing carbon nanotubes is provided, which includes the following steps: mixing calcium chloride, potassium chloride and other halogen salts and heating to obtain a molten halogen salt catalyst.

[0033] In the third aspect of the present invention, a method for preparing carbon nanotubes is provided, which includes the following steps: catalytically cracking a mixed gas containing a gaseous carbon source in the presence of a protective gas and a catalyst to obtain carbon nanotubes and hydrogen; wherein, the catalyst is the catalyst described in the first aspect.

[0034] In some specific embodiments of the present invention, the flow rate of the mixed gas containing the gaseous carbon source is 20 - 1000 mL / min relative to 1 L of the molten catalyst.

[0035] In some specific embodiments of the present invention, the gaseous carbon source is a C1 - C5 hydrocarbon gas, selected from one or more of methane, ethane, ethylene, acetylene, propane, propylene, propyne, butane, butene, butyne, pentane, pentene and pentyne.

[0036] In some specific embodiments of the present invention, the mixed gas includes a gaseous carbon source and a diluent gas to achieve the effect of diluting the gaseous carbon source. The diluent gas is selected from one of nitrogen, helium, argon, but is not limited to the above gases. Based on the total amount of the mixed gas, the volume concentration of the gaseous carbon source is 1 - 100%, preferably 30 - 60%.

[0037] In some specific embodiments of the present invention, the protective gas is selected from one or more of nitrogen, argon, helium, neon.

[0038] In some specific embodiments of the present invention, the temperature of the catalytic cracking is 600 - 1100 °C. Preferably, the temperature of the catalytic cracking is 900 - 1000 °C. The temperature of the catalytic cracking within the above range enables the preparation of carbon nanotubes to have better effects, enables the catalyst of the present invention to have better effects, can more effectively prepare high-quality carbon nanotubes, more effectively reduce metal contamination in carbon products, and has more effective catalytic activity.

[0039] In some specific embodiments of the present invention, due to the volatility of the molten salt at high temperature and the adhesiveness of the molten salt to the carbon material, the generated carbon nanotubes are contaminated with a certain amount of salt and need further purification treatment. Benefiting from the characteristic that salt is highly soluble in water, the salt contamination can be removed by simple water washing and drying to obtain relatively pure carbon nanotubes. The method further includes: purifying the product obtained by the catalytic cracking, and the purification includes: washing the product with water and drying.

[0040] In some specific embodiments of the present invention, before the catalytic cracking reaction, the reaction tube is first heated at a heating rate of 5 - 30 °C / min, preferably 8 - 15 °C / min. Within the above range, it is more conducive to obtaining a uniformly mixed catalyst, making the catalyst of the present invention have better catalytic effects.

[0041] The present invention will be described in detail below through examples.

[0042] For those not specifying specific conditions in the following examples and comparative examples, they are carried out according to conventional conditions or conditions recommended by the manufacturer. For reagents or instruments without indicating the manufacturer, they are all conventional products that can be obtained through commercial channels.

[0043] Example 1

[0044] As Figure 1 shown, it is prepared according to 45 mol% calcium chloride, 5 mol% potassium chloride, and 50 mol% manganese chloride, and placed in a corundum reaction tube 4 and mixed evenly. The corundum tube 4 has an outer diameter of 10 cm, an inner diameter of 8 cm, a bottom thickness of 2 cm, and a total height of 30 cm.

[0045] The loaded reaction tube 4 is heated while ventilating, and the heating rate is controlled at 10 °C / min. During the heating process, nitrogen is introduced to displace the air in the reaction tube 4 to ensure that no oxygen is detected at the outlet end 5 of the reaction tube. When heated to a certain temperature of 500 °C, the materials in the reaction tube 4 start to melt, forming a molten halogen salt catalyst with a liquid phase depth of 22 cm. Continue heating and keep nitrogen gas flowing in. When the required reaction temperature of 1000 °C is reached, the temperature is maintained constant.

[0046] And after simultaneously meeting the required reaction temperature for cracking, a mixture of methane and nitrogen is introduced at a flow rate of 90 mL / min, where the volume concentration of methane is 50%. The gas bubbles out in the form of bubbles 10 in the molten catalyst. Methane starts to undergo a cracking reaction under high temperature and the action of the liquid-phase catalyst, generating hydrogen and carbon atoms. The carbon atoms are orderly assembled on the surface of the bubbles to form high-value carbon nanotubes 8. Since the density of the generated carbon nanotubes is much lower than that of the liquid-phase catalytic medium, they spontaneously float on the surface, solving the problem of rapid deactivation of the catalyst due to carbon deposition, achieving long-cycle and long-life stable operation, and contributing to the mass production of carbon nanotubes and the reduction of preparation costs. When the generated carbon nanotubes accumulate to a certain height, they flow into the carbon material collection bin 7 through the discharge carbon groove 6 at the right opening of the reaction tube 4. After a certain amount accumulates in the collection bin 7, the valve 9 is opened to take out the carbon material. The outlet end 5 of the reaction tube is connected to a gas chromatograph to detect the concentration of the gas products after the reaction in real time, and then calculate the conversion rate of the raw material gas.

[0047] The obtained product was further purified as follows: (1) The obtained carbon nanotubes mixed with molten salt were placed in a beaker, and the beaker was filled with deionized water and then placed in an oven at 80 °C for about 12 h; (2) The salt solution containing carbon nanotubes was filtered by suction to obtain carbon powder, and then deionized water was added again and soaked for 1 h; (3) Filtered by suction again, and repeatedly soaked and rinsed with deionized water and absolute ethanol for 2 - 4 times; (4) Finally, the carbon powder obtained by suction filtration was dried in an oven at 103 °C to obtain a carbon material, and the obtained carbon material was characterized by scanning electron microscopy, and the results are shown in Fig. 2(a).

[0048] Example 2

[0049] According to the method of Example 1, as Figure 1 shown, prepared with 45 mol% calcium chloride and 55 mol% manganese chloride, placed in a corundum reaction tube 4, and mixed evenly. After melting, the liquid phase depth of the liquid phase catalyst was 22 cm. The methane cracking reaction was carried out under the same reaction conditions, and the obtained carbon material was subjected to the above purification treatment to obtain a carbon material.

[0050] Example 3

[0051] According to the method of Example 1, as Figure 1 shown, prepared with 40 mol% calcium chloride, 5 mol% potassium chloride, 40 mol% manganese chloride, and 15 mol% cobalt chloride, placed in a corundum reaction tube 4, and mixed evenly. After melting, the liquid phase depth of the liquid phase catalyst was 22 cm. The methane cracking reaction was carried out under the same reaction conditions, and the obtained carbon material was subjected to the above purification treatment to obtain a carbon material.

[0052] Example 4

[0053] According to the method of Example 1, the catalyst type, loading amount, and reaction conditions were exactly the same. The difference was that the volume concentration of the supplied raw material gas (methane) was increased from 50% to 100%. The obtained carbon material was subjected to the above purification treatment to obtain a carbon material.

[0054] Example 5

[0055] According to the method of Example 1, as Figure 1 shown, prepared with 25 mol% calcium chloride, 5 mol% potassium chloride, and 70 mol% manganese chloride, placed in a corundum reaction tube 4, and mixed evenly. After melting, the liquid phase depth of the liquid phase catalyst was 22 cm. The methane cracking reaction was carried out under the same reaction conditions, and the obtained carbon material was subjected to the above purification treatment to obtain a carbon material.

[0056] Example 6

[0057] According to the method of Example 1, as Figure 1As shown in the figure, it is prepared according to 45 mol% calcium chloride, 5 mol% potassium chloride, and 50 mol% copper chloride, and placed in a corundum reaction tube 4 and mixed evenly. After melting, the liquid phase depth of the liquid phase catalyst is 22 cm. Under the same reaction conditions, a methane cracking reaction is carried out, and the obtained carbon material is subjected to the purification treatment to obtain a carbon material.

[0058] Example 7

[0059] According to the method of Example 1, as Figure 1 shown in the figure, weigh 45 mol% calcium chloride, 5 mol% potassium chloride, and 50 mol% iron chloride, place them in a corundum reaction tube 4, and mix evenly. After melting, the liquid phase depth of the liquid phase catalyst is 22 cm. Under the same reaction conditions, a methane cracking reaction is carried out, and the obtained carbon material is subjected to the purification treatment to obtain a carbon material.

[0060] Comparative Example 1

[0061] According to the method of Example 1, as Figure 1 shown in the figure, charge a metal tin catalyst (Sn, melting point 232 °C, boiling point 2600 °C) with the same liquid phase height (22 cm) into the reaction tube 4, and keep other reaction conditions unchanged. It should be noted that due to the interaction between the molten metal and the carbon atoms being generated and the metal evaporation depositing on the surface of the solid carbon, the purity of the carbon material is not high, and further acidification and purification treatment are required. The specific operation steps are as follows:

[0062] (1) First, put the prepared carbon material sample into deionized water, ultrasonically vibrate for 1 h, and perform preliminary filtration to remove obvious large particle metals;

[0063] (2) Put the preliminarily screened carbon material sample into the prepared 1 mol / L iron chloride solution, and keep it in an oven at 80 °C for two days to corrode the remaining alloy particles;

[0064] (3) After the corrosion is completed, rinse and filter the filtered carbon material again with dilute hydrochloric acid with a concentration of about 10%;

[0065] (4) Finally, wash repeatedly with deionized water and absolute ethanol for 2 - 4 times;

[0066] (5) Put the washed carbon material into an oven at 80 °C to dry, and finally obtain a carbon material.

[0067] Comparative Example 2

[0068] According to the method of Comparative Example 1, as Figure 1As shown, the types of catalysts, the loading amounts, and the reaction conditions are exactly the same. The difference lies in that the concentration of the supplied raw material gas (methane) is increased from 50% to 100%. The obtained carbon material is purified (Comparative Example 1), and then the carbon material is obtained.

[0069] Comparative Example 3

[0070] According to the method of Example 1, as Figure 1 shown, a blank experiment is carried out without adding any catalyst, and other reaction conditions remain unchanged. Since there is no interference from impurities such as catalysts, the obtained carbon material has a high purity and does not require further purification.

[0071] Comparative Example 4

[0072] According to the method of Comparative Example 3, as Figure 1 shown, a blank experiment is carried out without adding any catalyst, and other conditions remain unchanged. Only the concentration of the supplied raw material gas (methane) is changed, increased from 50% to 100%. Since there is no interference from impurities such as catalysts, the obtained carbon material has a high purity and does not require further purification.

[0073] Comparative Example 5

[0074] According to the method of Example 1, as Figure 1 shown, it is prepared according to 10 mol% of calcium chloride, 75 mol% of potassium chloride, and 15 mol% of manganese chloride, and placed in a corundum reaction tube 4 and mixed evenly. The liquid phase depth of the molten liquid phase catalyst is 22 cm. The methane cracking reaction is carried out under the same reaction conditions, and the obtained carbon material is subjected to the purification treatment, and then the carbon material is obtained.

[0075] The methane conversion rate, selectivity, the hydrogen concentration in the product, and the purity of the carbon nanotubes were measured for the examples and comparative examples, and the results are shown in Table 1.

[0076] Table 1

[0077]

[0078]

[0079] Through the comparison of the above examples and comparative examples, it can be found that by catalytic cracking with the catalyst of the present invention, the conversion rate of the raw material gas and the hydrogen selectivity can be significantly improved. The high hydrogen selectivity indicates that the selectivity of the carbon material is also relatively high, with fewer side reactions and fewer by-products. At the same time, reducing the concentration of the raw material gas helps to improve the conversion rate of the raw material gas and the hydrogen selectivity, which can also indicate that the selectivity of the carbon material is relatively high. This is attributed to the fact that methane cracking (CH4→C+2H2) is a reaction with an increase in stoichiometry. Reducing the concentration of the raw material gas helps the reaction equilibrium to shift to the right, thereby improving the methane conversion rate.

[0080] On the other hand, as can be seen from Table 1, the molten salt catalytic system of the present invention has a higher methane conversion rate compared to the comparative examples (metal Sn system and blank system). The reason is that the molten liquid phase medium has homogeneous catalytic ability, and its active components are not confined to a specific area but are in a freely moving state, providing good catalytic and heat transfer characteristics. Moreover, the viscous resistance exerted by the molten liquid phase medium in the vertical reactor provides sufficient residence time for methane decomposition. The high conversion rate brings a high hydrogen yield. Therefore, the hydrogen concentration in Example 1 is the best. Compared with the molten metal Sn system, the molten salt catalyst of the present invention has higher activity.

[0081] In addition, as can be seen from Table 1, for this example, the salt pollution of the catalytic system can be removed by simple water washing, and the purity of the carbon material reaches more than 99%. In contrast, for the molten metal Sn system, water washing can only remove larger and more obvious metal particles, but it is difficult to remove micron-sized or nano-sized metals combined with the carbon material, and the purity of the carbon material is only about 58%. Further pickling is required for purification, but this will inevitably bring environmental pollution and cost increase. The present invention can achieve the purity of water washing and pickling in Comparative Example 1 only by water washing, with significantly better results.

[0082] Figures 2(a), 2(b), 2(c) and 2(d) respectively describe the scanning electron microscope characterization results of the carbon materials obtained in Example 1, Comparative Example 1, Comparative Example 3 and Comparative Example 5. It can be clearly seen that the carbon material obtained from the molten salt system of Example 1 has an elongated tubular structure, and it can be preliminarily determined that the carbon material is a carbon nanotube. In contrast, for the blank system (Comparative Example 3), the obtained carbon material has a spherical structure, and according to the publicly reported data, the carbon material can be preliminarily determined to be carbon black. For the molten metal Sn system, the generated carbon material is an amorphous deposit and cannot form carbon nanotubes. Although Comparative Example 5 uses the same catalyst components as Example 1, the component content far exceeds the specified value, and the prepared carbon material has obvious agglomeration phenomenon, the tubular structure is shorter and more solid, and cannot form carbon nanotubes.

[0083] By comparing Figures 2(a), Figure 3 , that is, the scanning electron microscope characterization results of the carbon materials obtained in Example 1 and Example 4, it can be clearly seen that the carbon nanotubes prepared from the low-concentration raw material gas (50% methane) are significantly better in quality than those from the high-concentration raw material gas (100% methane). The tubular structure of the carbon material is longer and more slender, and the distribution is relatively uniform, indicating that the volume concentration of the gas carbon source in the present application being 30-60% has significantly better effects.

[0084] Such as Figure 4As shown, in the Raman spectrum of the carbon material, the D peak, G peak, and 2D peak mainly appear. Among them, the D peak represents the crystal defects or structural disorder of the carbon material structure; the G peak represents the structural order and is the characteristic peak of graphene and carbon nanotubes; the 2D peak represents the degree of structural defects or changes. Generally, the intensity ratio of the D peak and G peak is used to measure the degree of disorder of the carbon material. For carbon nanotubes, the D peak generally appears around 1340 cm -1 nearby, the G peak appears around 1572 cm -1 nearby, the 2D peak appears around 2659 cm -1 nearby, and no 2D peak is formed in Comparative Example 1 and Comparative Example 3. Figure 4 Table 1 describes the Raman analysis results of the carbon materials obtained in the examples and comparative examples. It can be clearly seen that for the carbon material obtained in the molten salt system of Example 1, the intensity ratio of the characteristic peak G peak to the D peak is significantly higher than that of the molten metal Sn system in Comparative Example 1 and the blank system in Comparative Example 3. Combining with the scanning electron microscope analysis results in Figure 2, it shows that the carbon material generated in Example 1 has better order and better quality, and carbon nanotubes can be prepared, while carbon nanotubes cannot be prepared in Comparative Example 1 and Comparative Example 3.

[0085] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the technical concept scope of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A catalyst for preparing carbon nanotubes, characterized in that, The catalyst includes molten halogen salts, and the halogen salts include calcium chloride, potassium chloride and other halogen salts. Among them, the other halogen salts are selected from one or more of zinc chloride, copper chloride, barium chloride, titanium chloride, chromium chloride, gallium chloride, cobalt chloride, nickel chloride, manganese chloride, iron chloride, ferrous chloride and aluminum chloride. Based on the total molar amount of the catalyst, the content of calcium chloride is 20-80 mol%, the content of potassium chloride is 0-10 mol%, and the content of other halogen salts is 20-80 mol%.

2. The catalyst according to claim 1, wherein Based on the total amount of the catalyst, the content of calcium chloride is 30-60 mol%, the content of potassium chloride is 0-10 mol%, and the content of other halogen salts is 40-70 mol%; Preferably, the other halogen salts are selected from one or more of barium chloride, chromium chloride, gallium chloride, cobalt chloride, nickel chloride and manganese chloride.

3. A method for preparing a catalyst for preparing carbon nanotubes, characterized in that, It includes the following steps: mixing calcium chloride, potassium chloride and other halogen salts and heating to obtain a molten halogen salt catalyst.

4. A method for preparing carbon nanotubes, characterized in that, It includes the following steps: catalytically cracking a gas mixture containing a gaseous carbon source in the presence of a protective gas and a catalyst to obtain carbon nanotubes and hydrogen; wherein, the catalyst is the catalyst described in Claim 1 or 2, or the catalyst prepared by the preparation method described in Claim 3.

5. The preparation method according to claim 4, wherein, The gaseous carbon source is a C1-C5 hydrocarbon gas, selected from one or more of methane, ethane, ethylene, acetylene, propane, propylene, propyne, butane, butene, butyne, pentane, pentene and pentyne.

6. The preparation method according to claim 4 or 5, wherein The gas mixture includes a gaseous carbon source and a diluent gas, and the diluent gas is selected from one of nitrogen, helium and argon.

7. The preparation method according to any one of claims 4-6, wherein Based on the total amount of the gas mixture, the volume concentration of the gaseous carbon source is 1-100%, preferably 30-60%.

8. The preparation method according to any one of claims 4-7, wherein The protective gas is selected from one or more of nitrogen, argon, helium and neon.

9. The preparation method according to any one of claims 4-8, wherein, The temperature of the catalytic cracking is 600-1100 °C, preferably 900-1000 °C.

10. The preparation method according to any one of claims 4-9, wherein, The method further includes: purifying the product obtained by the catalytic cracking; Preferably, the purification process includes: washing the product with water and drying.

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