A single-walled carbon nanotube and its purification process

Abstract

This invention discloses a purification process for single-walled carbon nanotubes. The process includes: wet pulverizing and drying coarse single-walled carbon nanotubes containing magnetic metal catalyst residues (Fe, Co, Ni); subjecting the resulting coarse powder to a first magnetic separation to at least partially remove released free magnetic metal particles; subjecting the material after the first magnetic separation to low-temperature oxidation treatment under an oxidizing atmosphere at 300-400°C to at least partially remove amorphous carbon and etch the carbon coating layer, exposing at least partially the metal particles coated by the carbon layer; performing a second magnetic separation to at least partially remove the newly exposed magnetic metal particles; and then acid washing with 0.5-2 mol / L hydrochloric acid to remove remaining metals and metal oxides. The purified product is obtained after solid-liquid separation, water washing, and drying. This invention, by removing magnetic metal impurities in different states in stages, helps to reduce the acid washing load and minimize damage to the single-walled carbon nanotube structure.

Description

Technical Field

[0001] This invention relates to the field of carbon nanomaterial purification technology, specifically to a staged purification process for crude single-walled carbon nanotubes containing magnetic metal catalyst residues. Background Technology

[0002] Single-walled carbon nanotubes possess excellent electrical, mechanical, and thermal properties, and have broad application prospects in electronic devices, composite materials, and energy storage. However, transition metals such as Fe, Co, and Ni are often used as catalysts in the preparation processes of chemical vapor deposition and arc discharge, resulting in the presence of residual metal catalyst particles and amorphous carbon impurities in the obtained crude materials.

[0003] These metal catalyst particles exist in different states. Some exist in a free or weakly implicated state within carbon nanotube aggregates, bundles, or powder pores. Once the aggregates are broken, they can be easily separated by physical means. Others are coated with amorphous carbon or graphitized carbon layers. Due to the obstruction of the carbon layers, acid solutions cannot directly contact them, and conventional magnetic separation cannot completely remove them.

[0004] While traditional high-concentration acid pickling processes can dissolve some metals, their effectiveness in removing metal particles coated with carbon layers is limited. Furthermore, prolonged high-temperature, strong acid treatment can damage the sidewall structure of single-walled carbon nanotubes, introducing oxygen-containing functional groups or structural defects. Simple oxidation treatment, if the temperature is too high or the time is too long, may lead to the self-depletion or structural damage of the carbon nanotubes.

[0005] Therefore, a purification process is still needed that can treat metal catalyst residues in different states in stages, so as to effectively remove various metal impurities while reducing acid washing load and minimizing damage to the single-walled carbon nanotube structure. Summary of the Invention

[0006] The present invention aims to provide a purification process for single-walled carbon nanotubes to reduce metal catalyst residue under mild conditions, while taking into account low acid consumption and structural integrity.

[0007] To achieve the above objectives, the process provided by this invention allocates the removal of magnetic metal impurities in different states to different processing stages. Specifically, the process includes: First, the coarse single-walled carbon nanotubes containing residual magnetic metal catalysts such as Fe, Co, and Ni are subjected to wet pulverization and drying to break up the carbon nanotube aggregates and release the free magnetic metal particles trapped within, thus obtaining coarse single-walled carbon nanotube powder.

[0008] Subsequently, the coarse powder is subjected to a first magnetic separation to at least partially remove the free magnetic metal particles released after wet crushing, thereby obtaining the material after the first magnetic separation.

[0009] Next, the material after the first magnetic separation is subjected to low-temperature oxidation treatment in an oxidizing atmosphere (such as air) at 300~400℃. This step is used to remove at least some of the amorphous carbon and etch the carbon layer covering the surface of the metal particles, so that the metal particles originally covered by the carbon layer are at least partially exposed, resulting in the oxidized material.

[0010] Then, the oxidized material is subjected to a second magnetic separation to at least partially remove the newly exposed magnetic metal particles after the low-temperature oxidation treatment, resulting in the material after the second magnetic separation.

[0011] Finally, the material after the second magnetic separation was acid-washed with a 0.5~2 mol / L hydrochloric acid solution to remove at least some of the remaining metals and metal oxides. After solid-liquid separation, water washing and drying, purified single-walled carbon nanotubes were obtained.

[0012] In the above process, by primarily allocating the removal of free metal and carbon-coated metal to the first and second magnetic separations, respectively, the metal impurity load entering the acid washing step is reduced, allowing subsequent final purification to be completed using only low-concentration hydrochloric acid. Simultaneously, the use of low-temperature oxidation (300-400℃) and low-concentration hydrochloric acid treatment helps minimize damage to the sidewall structure of single-walled carbon nanotubes, preserving their intrinsic properties. In the examples, the Raman spectrum IG / ID ratio of the purified product can reach 90, and the residual metal content can be controlled below 3000 ppm. Attached Figure Description

[0013] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0014] Figure 1 This is a schematic diagram of a single-walled carbon nanotube purification process according to one embodiment of the present invention, wherein the first magnetic separation is used to remove free magnetic metal particles released after wet pulverization, and the second magnetic separation is used to remove magnetic metal particles exposed after low-temperature oxidation.

[0015] Figure 2 The thermogravimetric analysis (TGA) curves are shown in Example 1; where (a) is the crude material before purification, (b) is the material after the first magnetic separation, (c) is the material after the second magnetic separation, and (d) is the purified product after acid washing.

[0016] Figure 3 (a) is the Raman spectrum of the purified product obtained in Example 1; (b) is the Raman spectrum of the purified product obtained in Comparative Example 1.

[0017] Figure 4 These are transmission electron microscope (TEM) images of the material before and after purification in Example 1; where (a) is the crude material before purification and (b) is the product after purification. Detailed Implementation

[0018] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] The present invention will be further illustrated below with reference to specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in this technical field.

[0020] In this invention, the terms used are defined as follows: The term "free magnetic metal particles" refers to Fe, Co, and Ni metal particles that are not completely covered by a continuous carbon layer, can be at least partially released from single-walled carbon nanotube aggregates, bundles, or powder pores after wet pulverization, and can be captured by a magnetic field.

[0021] The "magnetic metal particles exposed after low-temperature oxidation treatment" refers to Fe, Co, and Ni metal particles that are at least partially covered by an amorphous carbon or graphitized carbon layer before low-temperature oxidation treatment, and whose outer carbon layer is at least partially etched after low-temperature oxidation treatment, thereby exposing at least part of the surface and making them easier to remove by magnetic separation.

[0022] The above definition does not require a specific particle size or morphology, but is used to describe the relative existence and removability of particles in each step of the process of the present invention.

[0023] Example 1

[0024] Take 100 g of single-walled carbon nanotubes prepared by chemical vapor deposition, wherein the metal catalyst is mainly Fe with a content of about 30 wt%, and contains amorphous carbon.

[0025] (1) Wet grinding and drying: Add the coarse material to deionized water, shear and grind for 30 minutes, filter, and dry in an oven at 100℃ for 12 hours to obtain coarse powder of single-walled carbon nanotubes.

[0026] (2) First magnetic separation: The coarse powder is passed through a high gradient dry magnetic separator with a magnetic field strength of 1.0 T to remove the magnetic particles captured by the magnetic field and collect the material after the first magnetic separation.

[0027] (3) Low-temperature oxidation: The material after the first magnetic separation is spread in a quartz boat, placed in a tube furnace, heated to 350°C in a flowing air atmosphere, and kept at a constant temperature for 2 hours to obtain the oxidized material.

[0028] (4) Second magnetic separation: The oxidized material is passed through a magnetic separator again with a magnetic field strength of 1.0 T to remove exposed magnetic metal particles and collect the material after the second magnetic separation.

[0029] (5) Pickling: Disperse the material after the second magnetic separation in 5 L of hydrochloric acid solution with a concentration of 1.0 mol / L, and stir and reflux for 4 hours in a water bath at 60℃.

[0030] (6) Post-treatment: After the reaction is completed, filter the solution and wash it with deionized water until the pH of the filtrate is 7. Dry the solution at 100°C to obtain purified single-walled carbon nanotubes.

[0031] TGA testing results showed that the residual mass percentage of the crude material after combustion in air at 900℃ was approximately 35% before purification. After the first magnetic separation, the residual mass percentage decreased to approximately 26%. After low-temperature oxidation, some metal particles were exposed, and the residual mass percentage was further reduced to approximately 20% after a second magnetic separation. The residual mass percentage of the product after acid washing was less than 2.0%. ICP-OES analysis showed that the metal content of the final product was 1500 ppm. Raman spectroscopy showed that the IG / ID ratio of the product was 90. TEM images showed that the surface of the purified tube bundle was clean, with no obvious impurity particles attached.

[0032] Example 2

[0033] The method used in Example 1 was the same, except that the low-temperature oxidation temperature was 400°C and the pickling temperature was 80°C. The final product was found to have a metal residue of 1000 ppm.

[0034] Example 3

[0035] The method was the same as in Example 1, except that the magnetic field strength for both the first and second magnetic separations was 0.5 T; the low-temperature oxidation temperature was 300℃; and the pickling was performed using 0.5 mol / L hydrochloric acid, with soaking and stirring at 25℃ for 10 hours. Testing showed that the metal content of the final product was controlled to be below 3000 ppm.

[0036] Implementation methods 4-10 The following implementation demonstrates the adjustable range of process parameters. Based on Example 1, the following parameters can be adjusted respectively: Implementation method 4: The magnetic field strength is set to 0.5 T.

[0037] Implementation method 5: The magnetic field strength is set to 2.0 T.

[0038] Implementation method 6: The low-temperature oxidation temperature is adjusted to 300℃.

[0039] Implementation method 7: The low-temperature oxidation temperature is adjusted to 400℃.

[0040] Implementation method 8: Adjust the hydrochloric acid concentration to 0.5 mol / L.

[0041] Implementation method 9: The hydrochloric acid concentration is adjusted to 2.0 mol / L.

[0042] Implementation method 10: The pickling temperature is adjusted to 90℃.

[0043] Comparative Example 1 (Traditional concentrated acid process) Take 100 g of single-walled carbon nanotube coarse material from the same batch as in Example 1, without performing the first and second magnetic separations. After wet crushing and drying the coarse material in deionized water, keep it at 350°C for 2 hours in air atmosphere, then add 5 L of concentrated hydrochloric acid (37 wt%), reflux at 60°C for 4 hours, then filter, wash with water until neutral, and dry. Figure 3 (b) Raman spectroscopy showed that the IG / ID ratio of the product was 60, lower than that of Example 1, indicating that the purity of the single-walled carbon nanotubes purified by the conventional concentrated acid process was worse than that of the one-walled carbon nanotubes purified by this process, with relatively more defects in the carbon nanotubes, and that the conventional concentrated acid process caused relatively greater damage to the single-walled carbon nanotubes. ICP-OES analysis showed that the metal residue in the obtained sample was 2500 ppm.

[0044] Comparative Example 2 (wet crushing and first magnetic separation omitted) 100 g of single-walled carbon nanotube coarse material from the same batch as in Example 1 was taken and placed directly into a tube furnace without wet grinding and a first magnetic separation. It was heated to 350°C and held at that temperature for 2 hours in air. The oxidized powder was then subjected to a single magnetic separation (magnetic field strength 1.0 T). The collected non-magnetic powder was then dispersed in 5 L of a 1.0 mol / L hydrochloric acid solution and stirred under reflux at 60°C for 4 hours. The mixture was then filtered, washed with water until neutral, and dried. The metal residue in the obtained sample was found to be 10,000 ppm.

[0045] Comparative Example 3 (dry grinding with the second magnetic separation omitted) 100 g of single-walled carbon nanotube coarse material from the same batch as in Example 1 was taken and dry-milled (200 rpm, 30 minutes), then dried at 100°C for 12 hours. The dry-milled powder was subjected to a first magnetic separation (magnetic field strength 1.0 T) to collect the non-magnetic powder. After the powder was kept at 350°C in air for 2 hours, without a second magnetic separation, it was directly dispersed in 5 L of 1.0 mol / L hydrochloric acid solution, stirred and refluxed at 60°C for 4 hours, filtered, washed with water until neutral, and dried. The metal residue in the obtained sample was found to be 6000 ppm. Since this comparative example changed both the milling method and whether a second magnetic separation was performed, the results are mainly used to evaluate the difference in overall purification effect between this simplified process route and the complete process route of Example 1.

[0046] Test case Thermogravimetric analysis: The TGA results of Example 1 are as follows Figure 2 As shown, the crude material residue is about 35%, about 20% after two magnetic separations, and less than 2.0% after acid washing.

[0047] Raman spectroscopy analysis: The Raman spectrum of the product in Example 1 is as follows Figure 3 As shown, the G band is significantly higher than the D band, and the IG / ID ratio is 90, indicating a lower product defect density.

[0048] Morphological characterization: TEM images of the product in Example 1 are shown below. Figure 4 As shown, the surface of the purified tube bundle is clean, with no obvious particles adhering to it.

[0049] Acid consumption comparison: Compared with Comparative Example 1, under the same treatment liquid volume (5 L), Example 1 uses 1.0 mol / L hydrochloric acid, while Comparative Example 1 uses 37 wt% concentrated hydrochloric acid (12.0 mol / L). The amount of HCl consumed by the process of this invention is significantly lower, which is beneficial to reducing the burden of waste liquid treatment.

[0050] In summary, this invention removes magnetic metal catalyst residues from single-walled carbon nanotube coarse materials in stages through a combination of wet crushing, first magnetic separation, low-temperature oxidation, second magnetic separation, and low-concentration hydrochloric acid washing. This is beneficial for obtaining single-walled carbon nanotubes with reduced metal residues under mild conditions.

Claims

1. A purification process for single-walled carbon nanotubes, characterized in that, Includes the following steps: S1. The coarse material of single-walled carbon nanotubes containing magnetic metal catalyst residue is subjected to wet pulverization and dried to obtain coarse powder of single-walled carbon nanotubes; the metal element of the magnetic metal catalyst residue includes one or more of Fe, Co and Ni. S2. Perform a first magnetic separation on the single-walled carbon nanotube coarse powder to at least partially remove the free magnetic metal particles released after the wet pulverization process, and obtain the material after the first magnetic separation. S3. The material after the first magnetic separation is subjected to low-temperature oxidation treatment in an oxidizing atmosphere of 300~400℃ to remove at least part of the amorphous carbon and at least part of the carbon layer coated on the surface of the metal particles, so that the metal particles coated by the carbon layer are at least partially exposed, and the oxidized material is obtained. S4. Perform a second magnetic separation on the oxidized material to at least partially remove the magnetic metal particles exposed after the low-temperature oxidation treatment, and obtain the material after the second magnetic separation. S5. The material after the second magnetic separation is acid-washed with 0.5~2 mol / L hydrochloric acid solution to remove at least some of the remaining metal and metal oxides; S6. The product after acid washing is subjected to solid-liquid separation, water washing and drying to obtain purified single-walled carbon nanotubes.

2. The process according to claim 1, characterized in that, In step S1, the dispersion medium used in the wet pulverization process is deionized water, ethanol, or a mixture of both.

3. The process according to claim 1, characterized in that, In step S1, the wet pulverization process is shear pulverization.

4. The process according to claim 1, characterized in that, In step S1, the drying temperature is 80~120℃.

5. The process according to claim 1, characterized in that, The magnetic field strengths of the first magnetic separation in step S2 and the second magnetic separation in step S4 are independently 0.5~2.0 T.

6. The process according to claim 1, characterized in that, In step S3, the oxidizing atmosphere is air.

7. The process according to claim 1, characterized in that, In step S3, the low-temperature oxidation treatment lasts for 0.5 to 10 hours, and the temperature is 350 to 400°C.

8. The process according to claim 1, characterized in that, In step S3, the low-temperature oxidation treatment is carried out at 350°C for 2 hours.

9. The process according to claim 1, characterized in that, In step S5, the pickling temperature is 25~90℃ and the pickling time is 1~10 hours.

10. A single-walled carbon nanotube product, characterized in that, It is prepared by any one of claims 1 to 9.

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