Catalyst for carbon nanotubes and preparation method therefor, and preparation method for carbon nanotubes

Abstract

The present invention relates to the preparation of carbon nanotubes. Disclosed is a catalyst for preparing carbon nanotubes. The catalyst comprises a molten halogen salt, wherein the halogen salt comprises a first halogen salt, a second halogen salt and a third halogen salt. The first halogen salt is calcium chloride; the second halogen salt is potassium chloride; and the third 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. On the basis of the total molar weight of the catalyst, the content of the first halogen salt is 20-80 mol%, the content of the second halogen salt is 0-10 mol%, and the content of the third halogen salt is 20-80 mol%. In the present invention, using the molten salt as a catalyst for preparing carbon nanotubes can effectively reduce metal pollution in a carbon product, and is more environmentally friendly than acid pickling in a metal catalytic system

Description

Catalyst for carbon nanotubes and preparation method thereof, and preparation method of carbon nanotubes

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of Chinese patent application 202311801426.2 filed on December 25, 2023, the contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to the technical field of catalysts for carbon nanotubes, and in particular to a catalyst for carbon nanotubes, a preparation method thereof, and a preparation method of carbon nanotubes. Background Art

[0004] Carbon nanotubes, due to their unique lattice structure, possess exceptional mechanical, thermal, and electrical properties, and are widely used in chemical, electronic, biological, and medical fields. Over the past few decades, the development and application of new energy materials such as carbon nanotubes have significantly fueled scientific progress. In recent years, market demand and scale for carbon nanotubes have experienced rapid growth. However, low production volumes and high prices have hindered their further widespread adoption. Developing technologies for the high-volume, low-cost production of carbon nanotubes has become one of the most challenging areas in the field of carbon nanomaterials, a crucial area of ​​intense competition among scientists worldwide.

[0005] The current method for preparing carbon nanotubes is mainly chemical vapor deposition, although this process has the characteristics of simple equipment and controllable carbon material structure. However, during the preparation process, the methane concentration of the raw gas is required to be extremely low, so it is necessary to specially equip it with inert gas to dilute it, which increases the investment cost. Furthermore, the introduction of inert gas significantly reduces the concentration of hydrogen generated by cracking, and the cost of hydrogen separation increases significantly. On the other hand, the vapor deposition method usually uses metal catalysts as the growth base of carbon materials, such as iron-based catalysts, copper-based catalysts, nickel-based catalysts, etc. It is worth noting that the use of metal catalysts will inevitably cause metal contamination in the generated carbon materials, making the separation and purification of carbon materials more difficult, and requiring the introduction of acid washing, which increases the complexity of the operation and also causes greater pollution to the environment.

[0006] CN114933296A discloses a method for producing carbon nanotubes using molten copper. To maintain the copper melt, the reaction temperature is controlled at 1000-1500°C, which inevitably increases system energy consumption and greenhouse gas emissions. Furthermore, due to the splashing and evaporation of the molten metal, the produced carbon material inevitably contains some small metal particles, which require acid washing for purification, increasing costs and causing significant environmental pollution.

[0007] CN115005494B proposes a method and application for lowering the cracking temperature of sugar compounds, which uses a metal salt or metal oxide solid catalyst to achieve a cracking reaction of monosaccharides and disaccharides at low temperature to produce aroma-causing substances. The catalyst design and scope of application are significantly different from those of the present invention.

[0008] CN109573983B discloses a transition metal-filled carbon nanotube, a preparation method, and an application thereof. The carbon nanotube is prepared by uniformly mixing a transition metal salt, melamine, and a molten salt in an ethanol solvent, and then placing the mixture in a tubular furnace and subjecting the mixture to high-temperature carbonization, acid washing, water washing, and drying. The preparation process is relatively complex, and the catalyst design is significantly different from that of the present invention.

[0009] Summary of the Invention

[0010] The purpose of the present invention is to solve the problem that the catalyst used in the prior art for preparing carbon nanotubes easily causes metal contamination and is difficult to separate, and to provide a green, low-cost catalyst for carbon nanotubes and a preparation method thereof.

[0011] To achieve the above-mentioned object, the present invention provides a catalyst for preparing carbon nanotubes in a first aspect, wherein the catalyst comprises a molten halogen salt, wherein the halogen salt comprises a first halogen salt, a second halogen salt and a third halogen salt, wherein the first halogen salt is calcium chloride, the second halogen salt is potassium chloride, and the third 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; wherein, based on the total molar amount of the catalyst, the content of the first halogen salt is 20-80 mol%, the content of the second halogen salt is 0-10 mol%, and the content of the third halogen salt is 20-80 mol%.

[0012] The second aspect of the present invention provides a method for preparing the catalyst for carbon nanotubes according to the first aspect, which comprises the following steps: mixing the first halide salt, the second halide salt and the third halide salt, and heating them to obtain a molten halide salt catalyst.

[0013] The third aspect of the present invention provides a method for preparing carbon nanotubes, 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 or the catalyst prepared using the method described in the second aspect of the present invention.

[0014] Through the above technical solution, the beneficial effects of the present invention are:

[0015] 1. For the first time, carbon nanotubes and hydrogen were prepared using molten salt as a catalyst. Compared with metal catalytic systems, salt costs are lower.

[0016] 2. By utilizing the physical properties of the molten liquid catalyst, that is, the density difference between the molten liquid catalyst and the carbon nanotubes, the generated carbon nanotubes can spontaneously float on the surface of the catalyst, effectively solving the problem of rapid deactivation of traditional solid catalysts due to carbon accumulation on the surface.

[0017] 3. The excellent thermal conductivity and uniform catalytic capacity of the liquid catalytic medium ensure more uniform heating and contact between the feed gas and the catalytic medium, improving cracking efficiency. Furthermore, the feed gas in the reactor must pass through the liquid layer, and the viscous resistance exerted in the vertical direction by the liquid medium provides sufficient residence time for methane decomposition, allowing it to fully react.

[0018] 4. Compared with metal catalytic systems (solid or molten), using molten salt as a catalyst for carbon nanotube preparation can effectively reduce micron- or nanoscale metal contamination in carbon nanotubes. This is because molten salt is highly soluble in water, and pure carbon nanotubes can be obtained by simple water washing. Moreover, the water washing in the present invention is more environmentally friendly than the acid washing in the metal catalytic system.

[0019] 5. The carbon nanotube preparation method provided in the present invention has no special requirements for the methane concentration in the mixed gas, which can break the limitation of low methane concentration raw gas in the traditional carbon nanotube preparation method, help to increase the hydrogen concentration in the output gas, reduce the subsequent hydrogen separation and purification costs, and also help to reduce the cost of additional inert gas supply.

[0020] Based on the above, the present invention has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a schematic diagram of a methane cracking unit;

[0022] FIG2a is a scanning electron microscopy image of the carbon nanotubes prepared in Example 1;

[0023] FIG2b is a scanning electron microscopy image of the carbon material prepared in Comparative Example 1;

[0024] FIG2c is a scanning electron microscopy image of the carbon material prepared in Comparative Example 3;

[0025] FIG2 d is a scanning electron microscopy image of the carbon material obtained in Comparative Example 5;

[0026] FIG3 is a scanning electron microscopy image of the carbon nanotubes prepared in Example 4;

[0027] FIG4 is the Raman analysis results of the carbon nanotubes prepared in Example 1 and the carbon materials prepared in Comparative Examples 1 and 3.

[0028] Explanation of the accompanying reference numerals: 1. Heating furnace, 2. Air inlet, 3. Catalyst, 4. Corundum tube, 5. Air outlet, 6. Carbon discharge groove, 7. Collection chamber, 8. Carbon material, 9. Valve, 10. Bubble. DETAILED DESCRIPTION

[0029] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0030] A first aspect of the present invention provides a catalyst for preparing carbon nanotubes, wherein the catalyst comprises a molten halogen salt, wherein the halogen salt comprises a first halogen salt, a second halogen salt and a third halogen salt, wherein the first halogen salt is calcium chloride, the second halogen salt is potassium chloride, and the third 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; wherein, based on the total molar amount of the catalyst, the content of the first halogen salt is 20-80 mol%, the content of the second halogen salt is 0-10 mol%, and the content of the third halogen salt is 20-80 mol%.

[0031] In the present invention, the composition content of each component in the catalyst satisfies that the sum of the total molar content is 100 mol%.

[0032] Among them, in the present invention, the inventors used molten salt as a catalyst for the first time to prepare carbon nanotubes and hydrogen by catalyzing a gas carbon source. Compared with solid metal catalytic systems, metal salts have lower costs. Molten halogen salts are liquid, and there is a density difference between them and carbon nanotubes. The generated carbon nanotubes can spontaneously float on the surface of the catalyst, which can effectively solve the problem of rapid deactivation of traditional solid metal catalysts due to carbon accumulation on the surface. Using molten salt as a catalyst can also effectively reduce metal contamination in carbon nanotubes. This is because molten salt is highly soluble in water, and pure carbon nanotubes can be obtained by simple water washing. Moreover, water washing is more environmentally friendly than acid washing in metal catalytic systems.

[0033] In some specific embodiments of the present invention, preferably, based on the total molar amount of the catalyst, the content of the first halide salt is 30-60 mol%, the content of the second halide salt is 0-10 mol%, and the content of the third halide salt is 40-70 mol%.

[0034] Among them, in the present invention, when the content of each component in the catalyst is within the preferred range defined above, the catalyst has higher catalytic activity and better catalytic effect in the preparation of carbon nanotubes, can more effectively prepare high-quality carbon nanotubes, and more effectively reduce metal contamination in carbon nanotubes.

[0035] In some specific embodiments of the present invention, preferably, the third halogen salt is selected from one or more of barium chloride, chromium chloride, gallium chloride, cobalt chloride, nickel chloride, and manganese chloride.

[0036] Among them, in the present invention, when the third halogen salt is within the above preferred range, the catalyst can have higher catalytic activity and better effect in the preparation of carbon nanotubes, can more effectively prepare high-quality carbon nanotubes, and more effectively reduce metal contamination in carbon nanotubes.

[0037] In some specific embodiments of the present invention, preferably, the third halogen salt is further selected from one or more of sodium bromide, potassium bromide, calcium bromide, magnesium bromide, barium bromide, lithium bromide, manganese bromide, nickel bromide, copper bromide, chromium bromide, cobalt bromide, gallium bromide, and iron bromide.

[0038] That is, in the present invention, the third halogen salt can be 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, aluminum chloride, sodium bromide, potassium bromide, calcium bromide, magnesium bromide, barium bromide, lithium bromide, manganese bromide, nickel bromide, copper bromide, chromium bromide, cobalt bromide, gallium bromide, and ferric bromide.

[0039] In some specific embodiments of the present invention, preferably, the third halogen salt is selected from one or more of barium chloride, chromium chloride, gallium chloride, cobalt chloride, nickel chloride, manganese chloride, copper chloride, barium bromide, cobalt bromide, nickel bromide, manganese bromide, chromium bromide, and copper bromide.

[0040] The second aspect of the present invention provides a method for preparing the catalyst for preparing carbon nanotubes described in the first aspect of the present invention, which includes the following steps: mixing the first halide salt, the second halide salt and the third halide salt, and heating them to obtain a molten halide salt catalyst.

[0041] In some specific embodiments of the present invention, preferably, the heating is performed under an inert atmosphere; wherein, the inert atmosphere is preferably a nitrogen atmosphere.

[0042] The third aspect of the present invention provides a method for preparing carbon nanotubes, 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, or a catalyst prepared by the method described in the second aspect of the present invention.

[0043] 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, for example, 20 mL / min, 40 mL / min, 60 mL / min, 90 mL / min, 110 mL / min, 150 mL / min, 200 mL / min, 300 mL / min, 500 mL / min, 700 mL / min, 800 mL / min, 1000 mL / min, and any value therebetween, relative to 1 L of the molten catalyst. Alternatively, in the present invention, based on a molten catalyst with a liquid phase depth of 22 cm, the flow rate of the mixed gas containing the gaseous carbon source is 20-1000 mL / min, preferably 40-120 mL / min, and more preferably 80-100 mL / min.

[0044] In some specific embodiments of the present invention, the gas 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.

[0045] In some specific embodiments of the present invention, the gas mixture includes a gaseous carbon source and an optional diluent gas, wherein the diluent gas is selected from one or more of nitrogen, helium, and argon, but is not limited to the foregoing gases. The volume concentration of the gaseous carbon source is 1-100%, preferably 30-60%, based on the total volume of the gas mixture. In the present invention, the gas mixture may contain a diluent gas to dilute the gaseous carbon source, or may not contain a diluent gas and directly produce carbon nanotubes using a pure gaseous carbon source as a raw material. Preferably, the gas mixture contains a diluent gas.

[0046] In some specific embodiments of the present invention, the protective gas is selected from one or more of nitrogen, argon, helium, and neon.

[0047] In some specific embodiments of the present invention, the catalytic cracking temperature is 600-1100°C, for example, 600°C, 700°C, 800°C, 900°C, 950°C, 1000°C, 1050°C, 1100°C, and any value therebetween. Preferably, the catalytic cracking temperature is 900-1000°C. Catalytic cracking temperatures within this range result in a higher catalytic activity of the catalyst, more efficiently producing high-quality carbon nanotubes, and more effectively reducing metal contamination in the carbon product.

[0048] In some specific embodiments of the present invention, the method further comprises: purifying the product obtained by the catalytic cracking to obtain carbon nanotubes and hydrogen; wherein the purification comprises: washing the product with water and drying it.

[0049] In this invention, the volatility of molten salt at high temperatures and the adhesiveness of the molten salt with the carbon nanotubes result in the catalytic cracking of carbon nanotubes mixed with halogen salts, requiring further purification. However, thanks to the high solubility of halogen salts in water, simple washing and drying can remove salt contamination, yielding carbon nanotubes with a purity of ≥99%.

[0050] In some specific embodiments of the present invention, preferably, the washing and drying operations include: first placing the carbon nanotubes mixed with the halogen salt in deionized water, soaking them at 70-90°C for 10-14 hours, and then filtering to obtain filtered powder; then adding the filtered powder to deionized water and soaking it for 1 hour, filtering it again, and repeatedly soaking and rinsing it with deionized water and anhydrous ethanol for 2-4 times to obtain carbon powder; finally, drying the obtained carbon powder in an oven at 100-105°C to obtain carbon nanotubes with a purity of 99-99.9%.

[0051] In some specific embodiments of the present invention, preferably, before conducting the catalytic cracking reaction, the reaction tube is heated at a heating rate of 5-30°C / min, preferably 8-15°C / min. Heating within this range is more conducive to improving the mixing uniformity of the catalyst, thereby achieving a better catalytic effect of the catalyst of the present invention.

[0052] The present invention will be described in detail below through examples.

[0053] In the following examples and comparative examples, if no specific conditions are specified, the experiments were carried out under conventional conditions or those recommended by the manufacturer. All reagents and instruments used, if no manufacturer is specified, are commercially available conventional products.

[0054] Example 1

[0055] As shown in Figure 1, 45 mol% calcium chloride, 5 mol% potassium chloride, and 50 mol% manganese chloride are placed in a corundum reaction tube 4 and mixed evenly. The dimensions of the corundum tube 4 are 10 cm in outer diameter, 8 cm in inner diameter, 2 cm in bottom thickness, and 30 cm in total height.

[0056] The assembled reaction tube 4 was heated with ventilation at a rate of 10°C / min. During the heating process, nitrogen was introduced to displace the air in the reaction tube 4, ensuring that no oxygen was detected at the outlet 5 of the reaction tube. When heated to 500°C, the material in the reaction tube 4 began to melt, forming a molten halide salt catalyst with a liquid phase depth of 22 cm. The temperature was continued to rise while nitrogen was introduced until the desired reaction temperature of 1000°C was reached, and the temperature was maintained unchanged.

[0057] A methane and nitrogen mixture is then introduced at a flow rate of 90 mL / min, with a methane volume concentration of 50%. The gas emerges from the molten catalyst as bubbles 10. Under the high temperature and the action of the molten catalyst, methane undergoes a cracking reaction, generating hydrogen and carbon atoms. The carbon atoms then assemble in an orderly manner on the surface of the bubbles, forming high-value-added carbon nanotubes 8. Because the density of the generated carbon nanotubes is lower than that of the molten catalyst, they can spontaneously float on the surface of the molten catalyst, eliminating the problem of rapid catalyst deactivation due to carbon deposition. This enables stable operation over a long cycle and a long lifespan, helping to increase carbon nanotube production and reduce production costs. When the generated carbon nanotubes accumulate to a certain height, they flow from the right opening of the reaction tube 4 through the carbon discharge groove 6 into the carbon material collection chamber 7. Once a certain amount of carbon nanotubes has accumulated in the collection chamber 7, valve 9 is opened to remove the carbon nanotubes. The gas outlet 5 of the reaction tube is connected to a gas chromatograph for real-time monitoring of the concentration of the post-reaction gas products and calculation of the methane conversion rate in the gas mixture.

[0058] The carbon nanotubes collected in the carbon material collection chamber 7 are mixed with halogen salts. The carbon nanotubes from the carbon material collection chamber 7 are further purified, specifically as follows: (1) the obtained carbon nanotubes mixed with halogen salts are placed in a beaker, after filling the beaker with deionized water, the beaker is placed in an oven at 80°C for about 12 hours to completely dissolve the chloride salt particles; (2) the salt solution containing the carbon nanotubes is filtered to obtain filtered powder, and then the filtered powder is added to deionized water and soaked for 1 hour; (3) the solution is filtered again, and the solution is repeatedly soaked and rinsed with deionized water and anhydrous ethanol for 2-4 times; (4) finally, the carbon powder obtained by filtration is dried in an oven at 103°C to obtain carbon nanotubes with a purity of 99.5%. The obtained carbon nanotubes with a purity of 99.5% are characterized by scanning electron microscopy, and the results are shown in Figure 2(a).

[0059] Example 2

[0060] The method of Example 1 was followed, except that 35 mol% calcium chloride, 60 mol% manganese chloride, and 5 mol% barium bromide were placed in a corundum reaction tube 4 and mixed uniformly. The melted liquid catalyst had a liquid phase depth of 22 cm. Methane cracking and purification were performed under the same reaction conditions, yielding carbon nanotubes with a purity of 99.4%.

[0061] Example 3

[0062] The method of Example 1 was followed, except that 40 mol% calcium chloride, 5 mol% potassium chloride, 40 mol% manganese chloride, and 15 mol% cobalt chloride were prepared and placed in a corundum reaction tube 4 and mixed uniformly. The melted liquid catalyst had a liquid phase depth of 22 cm. Methane cracking and purification were performed under the same reaction conditions, yielding carbon nanotubes with a purity of 99.5%.

[0063] Example 4

[0064] The method of Example 1 is the same as that of Example 1, with the same catalyst type, loading amount and reaction conditions. The only difference is that the volume concentration of methane in the supply mixed gas is increased from 50% to 100%, and carbon nanotubes with a purity of 99.3% are obtained.

[0065] Example 5

[0066] The method of Example 1 was followed, except that 45 mol% calcium chloride, 5 mol% potassium chloride, and 50 mol% copper chloride were placed in a corundum reaction tube 4 and mixed uniformly. The melted liquid catalyst had a liquid phase depth of 22 cm. The reaction temperature was 900°C, and carbon nanotubes with a purity of 99.4% were obtained.

[0067] Comparative Example 1

[0068] According to the method of Example 1, as shown in FIG1 , a metal tin catalyst (Sn, melting point 232° C., boiling point 2600° C.) with the same liquid phase height (22 cm) was loaded into reaction tube 4, and other reaction conditions remained unchanged. It is worth noting that due to the interaction between the molten metal and the generated carbon atoms and the evaporation and deposition of the metal on the solid carbon surface, the purity of the carbon material is not high, and further acidification and purification treatment is required. The specific operation steps are as follows:

[0069] (1) The prepared carbon material sample was placed in deionized water, ultrasonically shaken for 1 h, and initially filtered to remove obvious large particles of metal;

[0070] (2) The carbon material sample after the initial screening was placed in a prepared 1 mol / L ferric chloride solution and kept in an oven at 80°C for two days to corrode the remaining alloy particles;

[0071] (3) After the corrosion is completed, the filtered carbon material is rinsed and filtered again using dilute hydrochloric acid with a concentration of about 10%;

[0072] (4) Finally, wash with deionized water and anhydrous ethanol alternately 2-4 times;

[0073] (5) The cleaned carbon material is placed in an oven at 80°C for drying to obtain the carbon material.

[0074] Comparative Example 2

[0075] According to the method of comparative example 1, the catalyst type, loading amount and reaction conditions are exactly the same. The difference is that the concentration of the supplied raw gas (methane) is increased from 50% to 100%. The obtained carbon material is purified by the method of comparative example 1 to obtain the carbon material.

[0076] Comparative Example 3

[0077] According to the method of Example 1, as shown in Figure 1, a blank experiment was carried out without adding any catalyst, and other reaction conditions remained unchanged. Due to the absence of interference from impurities such as catalysts, the obtained carbon material has a high purity and does not require further purification.

[0078] Comparative Example 4

[0079] According to the method of Comparative Example 3, as shown in Figure 1, a blank experiment was carried out without adding any catalyst, and other conditions remained unchanged. Only the concentration of the supplied raw gas (methane) was changed from 50% to 100%. Since there was no interference from impurities such as catalysts, the obtained carbon material had a high purity and did not require further purification.

[0080] Comparative Example 5

[0081] According to the method of Example 1, as shown in Figure 1, 10 mol% calcium chloride, 75 mol% potassium chloride, and 15 mol% manganese chloride were placed in a corundum reaction tube 4 and mixed uniformly. After melting, the liquid phase depth of the liquid catalyst was 22 cm. A methane cracking reaction was carried out under the same reaction conditions, and the obtained carbon material was subjected to the purification treatment described above to obtain a carbon material.

[0082] The methane conversion rate and selectivity, hydrogen concentration in the product, and carbon nanotube purity in Examples 1-5 and Comparative Examples 1-5 were tested, and the results are shown in Table 1.

[0083] Table 1

[0084] As can be seen from Table 1, the method provided in the present invention can produce high-purity carbon nanotubes. By comparing Example 1 and Example 4, it can be seen that reducing the concentration of methane in the mixed gas helps to improve the methane conversion rate and hydrogen selectivity, which can explain that the selectivity of carbon nanotubes is also high. This may be attributed to the fact that methane cracking (CH4→C+2H2) is a stoichiometric increase reaction, and reducing the concentration of methane in the mixed gas helps to shift the reaction equilibrium to the right, thereby improving the methane conversion rate. On the other hand, from the comparison results of Example 1 and Example 4, the quality of carbon nanotubes produced by low-concentration raw gas is higher than that of carbon nanotubes produced by high-concentration raw gas, and the tube diameter is more uniform and slender. This is mainly due to the fact that under limited methane conversion (i.e., the number of carbon atoms produced is limited), carbon atoms can be better assembled in an orderly manner to produce lattice-ordered carbon nanotubes.

[0085] Comparison of Example 1 with Comparative Examples 1, 3, and 5 demonstrates that the presence or absence of a catalyst, as well as its specific composition, significantly influences the type of carbon produced (the carbon nanotubes produced in Examples 1-5 and the carbon materials produced in Comparative Examples 1-5 are collectively referred to as C). Under identical conditions, using a metallic tin catalyst alone, simply high-temperature treating methane, or using calcium chloride, potassium chloride, and manganese chloride compositions outside the specified ranges of this invention, while methane can be converted to carbon, it cannot be converted into carbon nanotubes.

[0086] Furthermore, as can be seen from Table 1, in the embodiments of the present invention, the catalyst can be removed by simple water washing, thus solving the problem of metal contamination caused by the catalyst, and the purity of the carbon material can reach over 99%. For the molten metal Sn system, water washing can only remove larger, more obvious metal particles, but it is difficult to remove the micron- or nano-scale metals bound to the carbon material. The purity of the carbon material is only about 58%, requiring further acid washing for purification, but this is bound to cause environmental pollution and increase costs. The present invention can achieve the purity of water washing and acid washing in Comparative Example 1 by only water washing, with significantly better results.

[0087] Figures 2(a), 2(b), 2(c) and 2(d) are scanning electron microscope characterization images of C obtained in Example 1 and Comparative Example 1, as well as Comparative Example 3 and Comparative Example 5, respectively. It can be clearly seen from Figures 2(a), 2(b), 2(c) and 2(d) that the C generated in Example 1 is in the form of an elongated tubular structure and can be preliminarily identified as a carbon nanotube. In comparison, the C generated in Comparative Example 3 is in the form of a spherical structure and can be preliminarily identified as carbon black. The C generated in Comparative Example 1 is in the form of a disordered accumulation 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. The C prepared therefrom has obvious agglomeration phenomenon, the tubular structure is shorter and smaller, and tends to be solid, and cannot form carbon nanotubes.

[0088] By comparing Figure 2(a) and Figure 3, i.e., the scanning electron microscopy characterization results of the carbon nanotubes obtained in Example 1 and Example 4, it can be clearly seen that the quality of the carbon nanotubes produced using a mixed gas with a low volume content of methane (50v% methane) is significantly better than that of the carbon nanotubes produced using a mixed gas with a high volume content of methane (100v% methane). The tubular structure of the carbon nanotubes in the former is more slender and more evenly distributed, indicating that the volume concentration of the gas carbon source of the present invention of 30-60% has a significantly better effect.

[0089] Figure 4 shows the Raman analysis results of C obtained in Example 1 and Comparative Examples 1 and 3. In the Raman spectrum of C, the D peak, G peak, and 2D peak mainly appear. The D peak represents the crystal defects or structural disorder of the C structure; the G peak represents the order of the C structure and is a characteristic peak of graphene and carbon nanotubes; the 2D peak represents the degree of defects or changes in the C structure. The intensity ratio of the D peak and the G peak is generally used to measure the disorder of C. For carbon nanotubes, the D peak generally appears at 1340 cm -1 Nearby, the G peak appears at 1572cm -1 Nearby, the 2D peak appears at 2659 cm -1 Obviously, no 2D peak is formed in Comparative Examples 1 and 3. As shown in Figure 4, the intensity ratio of the characteristic peak G to D peak of C produced in Example 1 is significantly higher than that of Comparative Examples 1 and 3. Combined with the scanning electron microscopy analysis results in Figure 2, it is shown that the C produced in Example 1 has better order and higher quality, and can produce carbon nanotubes, while Comparative Examples 1 and 3 cannot produce carbon nanotubes.

[0090] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection 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 a first halogen salt, a second halogen salt, and a third halogen salt. The first halogen salt is calcium chloride, the second halogen salt is potassium chloride, and the third 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, iron chloride, ferrous chloride, and aluminum chloride. Among them, based on the total molar amount of the catalyst, the content of the first halogen salt is 20 - 80 mol%, the content of the second halogen salt is 0 - 10 mol%, and the content of the third halogen salt is 20 - 80 mol%.

2. The catalyst according to claim 1, wherein, Based on the total molar amount of the catalyst, the content of the first halogen salt is 30 - 60 mol%, the content of the second halogen salt is 0 - 10 mol%, and the content of the third halogen salt is 40 - 70 mol%.

3. The catalyst according to claim 2, wherein, The third halogen salt is selected from one or more of barium chloride, chromium chloride, gallium chloride, cobalt chloride, nickel chloride, and manganese chloride.

4. The preparation method of the catalyst for preparing carbon nanotubes according to any one of claims 1-3, characterized in that, It includes the following steps: mixing and heating the first halogen salt, the second halogen salt, and the third halogen salt to obtain a molten halogen salt catalyst.

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

6. The preparation method according to claim 6, 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.

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

8. The preparation method according to claim 5, wherein Based on the total amount of the mixed gas, the volume concentration of the gaseous carbon source is 1 - 100%, preferably 30 - 60%.

9. The preparation method according to claim 5, wherein, The protective gas is selected from one or more of nitrogen, argon, helium, and neon.

10. The preparation method according to claim 5, wherein, The temperature of the catalytic cracking is 600 - 1100 °C.

11. The preparation method according to claim 10, wherein, The temperature of the catalytic cracking is 900 - 1000 °C.

12. The preparation method according to any one of claims 5-11, wherein, The method further includes: purifying the product obtained by the catalytic cracking.

13. The preparation method according to claim 12, wherein, The purification process includes: washing the product with water and drying it.

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