A method for removing carbon impurities and metal impurities from a single-walled carbon nanotube crude

Abstract

The application provides a method for removing carbon impurities and metal impurities from a single-walled carbon nanotube crude product, and relates to the technical field of separation and purification of carbon nanotube materials. The method comprises the following steps: performing calcination treatment on the single-walled carbon nanotube under a carbon dioxide atmosphere, then performing toluene reflux, adding polyvinylpyrrolidone and acid, and obtaining purified single-walled carbon nanotubes. The method removes excess amorphous carbon by calcination under the condition of carbon dioxide gas, realizes high separation of the single-walled carbon nanotubes by refluxing in toluene, simultaneously adds concentrated acid and polyvinylpyrrolidone to remove metal impurities, and finally performs high-temperature calcination to decompose the residual polyvinylpyrrolidone, thereby improving the purity of the single-walled carbon nanotubes. The purification method has the characteristics of high purity, high efficiency and high-quality products. The method has relatively excellent purification efficiency, and the purified single-walled carbon nanotubes have relatively high purity.

Description

Technical Field

[0001] This invention relates to the field of carbon nanotube material separation and purification technology, and in particular to a method for removing carbon impurities and metallic impurities from crude single-walled carbon nanotubes. Background Technology

[0002] Single-walled carbon nanotubes (SWC nanotubes) have been widely used in nanoelectronics, biosensors, and energy devices due to their excellent physical and chemical properties. Currently, SWC nanotubes are typically prepared using methods such as arc discharge, laser evaporation, and catalytic pyrolysis. However, in the production process of SWC nanotubes, besides the imperfections in large-scale preparation processes, the purification process is also a key factor affecting their application. SWC nanotube samples usually contain a large number of impurities, among which carbon impurities and metallic impurities are the two most common types. Carbon impurities include graphite particles, carbon nanoparticles, and amorphous carbon formed during the growth of carbon nanotubes. Metallic impurities originate from transition metal catalysts. The presence of these impurities seriously affects the properties and applications of SWC nanotubes. Therefore, developing efficient purification methods is crucial for obtaining high-purity SWC nanotube products. During the purification process, impurities can be removed using chemical, physical, and thermal treatment methods. For example, acid treatment can remove carbon impurities from the surface of carbon nanotubes, while acid washing and high-temperature treatment can remove metallic catalysts.

[0003] Given the significant impact of metallic impurities on the properties and applications of carbon nanotubes (SNTs), various methods are currently employed to purify SNTs. First, physical purification methods leverage the differences in physical properties between metallic impurities and SNTs, improving SNT purity through separation and screening. Physical purification offers rapid separation but is incomplete, leaving significant metal residue. Second, chemical purification utilizes the ease with which metallic impurities dissolve in acids or sublimate at high temperatures, purifying SNTs by oxidizing or dissolving these impurities. Compared to physical purification, chemical purification is more thorough but still requires further improvement. Finally, comprehensive purification methods combine multiple purification techniques to enhance both the purity and efficiency of SNTs. However, the purification reagents and conditions for comprehensive purification require further determination and selection to maximize carbon utilization of crude SNTs while minimizing metal and carbon impurities. During the purification process, it is crucial to prevent oxidation and ensure the high efficiency of the method.

[0004] Current research indicates that the most effective method for removing metal impurities is acidification of carbon nanotubes. Acidification requires lower temperatures and less sophisticated equipment, and is more economical and scalable than high-temperature purification methods. However, acidification cannot deeply dissolve the metal particles encapsulated within the inner walls of carbon nanotubes, necessitating other auxiliary methods to reduce impurity levels. Therefore, developing more suitable purification methods is essential to thoroughly disrupt the carbon layer on the surface of metal particles to achieve complete dissolution while maintaining the structural integrity of single-walled carbon nanotubes, thereby significantly improving the purity of the single-walled carbon nanotubes.

[0005] To overcome the shortcomings of the above purification processes, this invention provides a method for removing carbon and metal impurities from crude single-walled carbon nanotubes, effectively removing carbon and metal impurities from carbon nanotubes, improving the purity of single-walled carbon nanotubes, and thus enhancing their properties and application potential. Summary of the Invention

[0006] The purpose of this invention is to provide a method for removing carbon and metal impurities from crude single-walled carbon nanotubes, so as to solve the problems existing in the prior art.

[0007] To achieve the above objectives, the present invention provides the following solution:

[0008] This invention provides a method for removing carbon and metal impurities from crude single-walled carbon nanotubes, comprising the following steps:

[0009] The crude single-walled carbon nanotubes were calcined under a carbon dioxide atmosphere, and then refluxed with toluene, followed by the addition of polyvinylpyrrolidone and acid to obtain purified single-walled carbon nanotubes.

[0010] As a further preferred embodiment of the present invention, the carbon dioxide flow rate is 800 sccm, the calcination temperature is 600-900°C, and the calcination time is 3-6 hours. More preferably, the calcination temperature is 600-750°C, and the calcination time is 3-6 hours; even more preferably, the calcination temperature is 800-900°C, and the calcination time is 3-6 hours.

[0011] This invention involves contacting crude single-walled carbon nanotubes with carbon dioxide to oxidize the amorphous carbon at the tips. Simultaneously, under sufficient temperature and time, a preliminary treatment of the crude product is performed to avoid severe damage to the sidewalls. The method includes preparing crude single-walled carbon nanotube material with amorphous carbon at the tips, contacting the single-walled carbon nanotubes with carbon dioxide, and continuously reacting at 600-900℃ for 3-6 hours to oxidize the amorphous carbon at the tips. The appropriate oxidation time ensures the removal of the carbon cap at the tips of the single-walled carbon nanotubes. If the oxidation time is too long, the structure of the sidewalls of the single-walled carbon nanotubes will be significantly damaged. Therefore, during the preliminary treatment of the crude single-walled carbon nanotubes, excess amorphous carbon is oxidized to carbon monoxide to remove the tip carbon cap, amorphous carbon, and graphitic carbon.

[0012] As a further preferred embodiment of the present invention, the temperature of the toluene reflux is 120-150°C, more preferably 120°C; the pressure of the toluene reflux is 0.5-2 MPa, more preferably 0.5 MPa; and the time of the toluene reflux is 16 h.

[0013] This invention involves reflux in toluene to remove fullerenes and soluble impurities from single-walled carbon nanotubes (SUVs). After liquid-phase oxidation of amorphous carbon and reflux of fullerenes and soluble impurities, the mixture is washed with distilled water until neutral. Reflux in toluene for 16 hours removes other types of carbon materials from the SUVs. Reflux in toluene improves reaction efficiency. This reflux technique allows the crude SUVs in the reaction system to be maintained at a specific temperature and pressure for continuous reaction, effectively increasing reaction efficiency. During reflux, a condenser cools the generated gases or volatiles into liquid, which are then refluxed back into the reaction system, preventing the loss of crude SUVs. Reflux technology allows for the control and optimization of reaction conditions such as temperature and pressure.

[0014] A mixture containing carbon nanotubes and impurities is heated to the boiling point of toluene, causing the toluene to evaporate. Toluene has a lower boiling point than other components in the mixture, so it evaporates first. The evaporated toluene enters a reflux condenser, where its temperature is lowered by a coolant or cold water, causing it to reliquefy. The liquid toluene is then reinjected into the reactor or separation equipment via a reflux device. This increases the concentration of toluene in the mixture while also refluxing impurities and other impurities carried by the toluene. Repeating this toluene reflux cycle gradually removes impurities and other impurities from the surface of the carbon nanotubes, achieving purification. The principle of toluene reflux is to separate carbon nanotubes from impurities through continuous circulation, evaporation, and condensation, thereby improving the purity and quality of the carbon nanotubes.

[0015] The reasons for choosing toluene as the reflux medium in this invention are as follows:

[0016] 1) Solubility: Toluene can dissolve carbon nanotubes well, helping to effectively disperse and remove impurities and impurities on the surface of carbon nanotubes.

[0017] 2) Evaporation temperature: Toluene has a high boiling point (110.63℃), which means that a relatively high temperature can be used in the purification process to speed up the evaporation and improve the purification efficiency.

[0018] 3) Solvent properties: Toluene has moderate polarity, which allows it to interact with carbon nanotubes and help remove surface impurities.

[0019] 4) Benzene has a boiling point of 80.1℃, ethanol has a boiling point of 78.4℃, and toluene has a boiling point of 110.63℃. Based on the boiling points, the following order can be derived: toluene > benzene > ethanol.

[0020] In comparison, ethanol and benzene have lower solubility and solvent properties than toluene, so ethanol and benzene are not chosen for reflux.

[0021] As a further preferred embodiment of the present invention, the addition of polyvinylpyrrolidone and acid is performed simultaneously.

[0022] As a further preferred embodiment of the present invention, the amount of polyvinylpyrrolidone used is 1-10% of the mass of the single-walled carbon nanotubes, more preferably 1%.

[0023] As a further preferred embodiment of the present invention, the acid is one of sulfuric acid, nitric acid, hydrochloric acid, aqua regia, and hydrofluoric acid.

[0024] As a further preferred embodiment of the present invention, the concentration of the acid is 3-10 mol / L, and the mass-to-volume ratio of the single-walled carbon nanotubes to the acid is 1 g: 50-100 mL, more preferably 1 g: 80 mL.

[0025] High-concentration acidic reagents and polyvinylpyrrolidone (PVP) are used to remove metal particles not protected by amorphous carbon, effectively dispersing and then removing transition metal nanoparticles capped with end carbon. Furthermore, PPVP disperses impurities in the metal catalyst, thereby removing graphite carbon-coated metal catalysts. After dispersion, acid treatment effectively removes impurities, residual short carbon nanotubes, residual multi-walled carbon nanotubes, amorphous carbon, and excess transition metal impurities, further improving the purity of single-walled carbon nanotubes.

[0026] The reason for choosing to add polyvinylpyrrolidone and acid simultaneously in this invention is as follows:

[0027] 1. Control of the reaction process: Adding polyvinylpyrrolidone and acid separately can lead to uneven dispersion or difficulty in mixing of the reactants, which limits the reaction and thus affects the yield. Simultaneously removing metal impurities with acid under conditions of polyvinylpyrrolidone dispersion will be more effective.

[0028] 2. Reaction Rate Difference: Polyvinylpyrrolidone (PVP) and single-walled carbon nanotubes (SUVs) exhibit π-π stacking interactions, electrostatic interactions, and hydrogen bonding. These non-covalent interactions lead to the formation of complexes between PPVP and SUVs, promoting the uniform dispersion of SUVs within PPVP. There is a difference in reaction rates between PPVP and acids; SUVs typically react quickly with acids but slowly with PPVP. When PPVP and acid are added simultaneously, some SUVs react with PPVP and disperse uniformly. The remaining SUVs react with the acid and then form complexes with PPVP, further promoting dispersion. Therefore, the simultaneous addition of PPVP and acid can, to some extent, improve the purification rate.

[0029] As a further preferred embodiment of the present invention, the addition of polyvinylpyrrolidone and acid further includes a secondary calcination step, wherein the secondary calcination is carried out in an argon atmosphere, the temperature of the secondary calcination is 600-900℃, more preferably 600℃, and the time of the secondary calcination is 10-16h, more preferably 10h.

[0030] The purpose of adding polyvinylpyrrolidone and acid and then calcining them at high temperature is to remove residual polyvinylpyrrolidone from the product.

[0031] As a further preferred embodiment of the present invention, the toluene reflux is further comprising a centrifugation step, more preferably, the centrifugation speed is 10000 rpm and the centrifugation time is 1 h; the addition of polyvinylpyrrolidone and acid and the subsequent calcination before the second calcination further comprises a washing to neutral and drying step, more preferably, the washing reagent is water, the drying temperature is 50-70℃, more preferably 60℃, and the drying time is 2-5 h, more preferably 3 h.

[0032] The refluxed suspension is centrifuged to remove toluene. After centrifugation, the crude single-walled carbon nanotubes settle to the bottom of the centrifuge tube, separating the crude single-walled carbon nanotubes from the toluene. Washing to neutrality after the simultaneous addition of polyvinylpyrrolidone and concentrated acid is to remove residual acidic substances to avoid adverse effects on subsequent operations and applications. Ensure the washing process is thorough and that no residual acidic substances remain.

[0033] As a further preferred embodiment of the present invention, the crude single-walled carbon nanotubes are crude single-walled carbon nanotubes containing carbon impurities and metal impurities, and more preferably, the purity of the crude single-walled carbon nanotubes is 70%.

[0034] The purification method of this invention was used to purify 5g of crude single-walled carbon nanotubes to obtain more than 2g of single-walled carbon nanotubes without obvious particulate impurities, with a yield of more than 40%.

[0035] The present invention discloses the following technical effects:

[0036] 1) This invention removes excess amorphous carbon through calcination under carbon dioxide gas. Then, reflux and filtration with toluene are used to achieve a high degree of separation of single-walled carbon nanotubes. Concentrated acid and polyvinylpyrrolidone are added simultaneously to remove metallic impurities. The nanotubes are further purified by washing with distilled water until neutral and filtering to remove excess acid. Finally, calcination at high temperature decomposes residual polyvinylpyrrolidone, thereby improving the purity of the single-walled carbon nanotubes.

[0037] 2) The purification method of this invention features high purity, high efficiency, and high-quality products. Using this method to purify single-walled carbon nanotubes, the yield is greater than 40%, demonstrating excellent purification efficiency. The purified single-walled carbon nanotubes have a high purity, reaching 97.5%. Attached Figure Description

[0038] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0039] Figure 1 This is a roadmap of the method for removing carbon and metal impurities from crude single-walled carbon nanotubes according to the present invention.

[0040] Figure 2 This is a SEM image of the single-walled carbon nanotubes in Example 1 before purification;

[0041] Figure 3 This is a SEM image of the purified single-walled carbon nanotubes from Example 1.

[0042] Figure 4 This is a SEM image of the purified single-walled carbon nanotubes from Example 2.

[0043] Figure 5 This is a SEM image of the purified single-walled carbon nanotubes from Example 3.

[0044] Figure 6 This is a SEM image of the purified single-walled carbon nanotubes from Example 4.

[0045] Figure 7 Thermogravimetric curves of the purified single-walled carbon nanotubes in Example 4;

[0046] Figure 8 This is a SEM image of the purified single-walled carbon nanotubes from Example 5.

[0047] Figure 9 Here is a SEM image of the purified single-walled carbon nanotubes from Example 6;

[0048] Figure 10 This is a SEM image of the purified single-walled carbon nanotubes from Comparative Example 1.

[0049] Figure 11 This is a SEM image of the purified single-walled carbon nanotubes from Comparative Example 2. Detailed Implementation

[0050] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0051] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0052] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0053] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be readily apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0054] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0055] The technical solution of the present invention will be further described in detail below with reference to the embodiments.

[0056] The roadmap for the method of removing carbon and metal impurities from crude single-walled carbon nanotubes of the present invention is shown below. Figure 1 As shown.

[0057] SEM images of the crude single-walled carbon nanotubes containing carbon and metal impurities used in the following embodiments and comparative examples of this invention before purification are shown below. Figure 2 As shown.

[0058] The purity of the purified single-walled carbon nanotubes in the following embodiments and comparative examples of the present invention was calculated using the TGA test method.

[0059] Purity = (1 - weight of residue / weight of initial sample) × 100%. Assuming the initial sample weight is W0 and the residue weight is W1, the formula for calculating purity is: Purity = (1 - W1 / W0) × 100%.

[0060] Example 1

[0061] (1) 5g of crude single-walled carbon nanotubes containing carbon and metal impurities and with a purity of 70% were calcined in a tube furnace at 600℃ for 3h by passing carbon dioxide gas through at a flow rate of 800sccm.

[0062] (2) The single-walled carbon nanotubes were refluxed in toluene for 16 hours at a temperature of 120°C and a pressure of 0.5 MPa.

[0063] (3) Centrifuge the refluxed suspension at 10,000 rpm for 1 h and remove the supernatant;

[0064] (4) Add 0.05g of polyvinylpyrrolidone and 250mL of concentrated nitric acid with a concentration of 3mol / L at the same time. After the reaction is complete, wash with water until neutral, and then dry at 60℃ for 3h.

[0065] (5) The purified single-walled carbon nanotubes were obtained by calcining them for 10 hours at 600℃ under an argon atmosphere.

[0066] In this embodiment, 5g of crude single-walled carbon nanotubes were purified to obtain 3.95g of single-walled carbon nanotubes without obvious particulate impurities, with a purity of 80.6% and a yield of 79%.

[0067] The SEM image of the purified single-walled carbon nanotubes in this embodiment is shown below. Figure 3As shown, a comparison with the SEM images of the single-walled carbon nanotubes before purification reveals that the metal particle impurities in the single-walled carbon nanotubes are reduced.

[0068] Example 2

[0069] The only difference between this embodiment and Embodiment 1 is that the calcination temperature in step 1) and the secondary calcination temperature in step 5) are 900°C.

[0070] The SEM image of the purified single-walled carbon nanotubes in this embodiment is shown below. Figure 4 As shown, a comparison with the SEM images of single-walled carbon nanotubes before and after purification reveals that the single-walled carbon nanotubes contained more metal impurity particles before purification, while the metal impurities in the single-walled carbon nanotubes decreased after purification.

[0071] In this embodiment, 5g of crude single-walled carbon nanotubes were purified to obtain 4g of single-walled carbon nanotubes without obvious particulate impurities, with a purity of 92.5% and a yield of 80%.

[0072] Example 3

[0073] The only difference between this embodiment and Embodiment 1 is that the calcination temperature in step 1) is 700°C.

[0074] The SEM image of the purified single-walled carbon nanotubes in this embodiment is shown below. Figure 5 As shown.

[0075] In this embodiment, 5g of crude single-walled carbon nanotubes were purified to obtain 4g of single-walled carbon nanotubes without obvious particulate impurities, with a purity of 90.5% and a yield of 80%.

[0076] Example 4

[0077] The only difference between this embodiment and Embodiment 1 is that the calcination temperature in step 1) is 750°C.

[0078] The SEM image of the purified single-walled carbon nanotubes in this embodiment is shown below. Figure 6 As shown.

[0079] The thermogravimetric curve of this embodiment is as follows: Figure 7 As shown. From Figure 7 It can be seen that the thermogravimetric purity decreased from 100% to 2.5%. The purity calculation is: Purity = (1 - 0.025 / 1) × 100% = 97.5%.

[0080] In this embodiment, 5g of crude single-walled carbon nanotubes were purified to obtain 4.3g of single-walled carbon nanotubes without obvious particulate impurities, with a purity of 97.5% and a yield of 86%.

[0081] Example 5

[0082] The only difference between this embodiment and Embodiment 1 is that the calcination temperature in step 1) is 800°C.

[0083] The SEM image of the purified single-walled carbon nanotubes in this embodiment is shown below. Figure 8 As shown.

[0084] In this embodiment, 5g of crude single-walled carbon nanotubes were purified to obtain 3.65g of single-walled carbon nanotubes without obvious particulate impurities, with a purity of 85.9% and a yield of 73%.

[0085] Example 6

[0086] The only difference between this embodiment and Embodiment 1 is that the calcination time in step 1) is 5 hours.

[0087] The SEM image of the purified single-walled carbon nanotubes in this embodiment is shown below. Figure 9 As shown.

[0088] In this embodiment, 5g of crude single-walled carbon nanotubes were purified to obtain 2.8g of single-walled carbon nanotubes with a purity of 90.5% and a yield of 56%.

[0089] Comparative Example 1

[0090] The only difference between this comparative example and Example 1 is that the reflux medium in step 2) is ethanol.

[0091] The SEM images of the purified single-walled carbon nanotubes in this comparative example are shown below. Figure 10 As shown.

[0092] In this comparative example, 5g of crude single-walled carbon nanotubes were purified to obtain 2.45g of single-walled carbon nanotubes with a purity of 80.6% and a yield of 49%.

[0093] Comparative Example 2

[0094] The only difference between this comparative example and Example 1 is that in step 4), the order of adding polyvinylpyrrolidone and concentrated nitric acid is that polyvinylpyrrolidone is added first, followed by concentrated nitric acid.

[0095] The SEM images of the purified single-walled carbon nanotubes in this comparative example are shown below. Figure 11 As shown.

[0096] In this comparative example, 5g of crude single-walled carbon nanotubes were purified to obtain 1.85g of single-walled carbon nanotubes with a purity of 85.9% and a yield of 37%.

[0097] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for removing carbon and metallic impurities from crude single-walled carbon nanotubes, characterized in that, Includes the following steps: The crude single-walled carbon nanotubes were calcined in a carbon dioxide atmosphere, and then refluxed with toluene, polyvinylpyrrolidone and acid were added, followed by a second calcination to obtain purified single-walled carbon nanotubes. The carbon dioxide flow rate is 800 sccm, the calcination temperature is 700-900℃, and the calcination time is 3-6h. The addition of polyvinylpyrrolidone and acid is described as the simultaneous addition of polyvinylpyrrolidone and acid; The secondary calcination is carried out in an argon atmosphere, at a temperature of 600-900℃, and for a duration of 10-16 hours.

2. The method according to claim 1, characterized in that, The toluene reflux temperature is 120-150℃, the toluene reflux pressure is 0.5-2MPa, and the toluene reflux time is 16h.

3. The method according to claim 1, characterized in that, The amount of polyvinylpyrrolidone used is 1-10% of the mass of the single-walled carbon nanotubes.

4. The method according to claim 1, characterized in that, The acid is one of sulfuric acid, nitric acid, hydrochloric acid, aqua regia, and hydrofluoric acid.

5. The method according to claim 1, characterized in that, The concentration of the acid is 3-10 mol / L, and the mass-to-volume ratio of the single-walled carbon nanotubes to the acid is 1 g: 50-100 mL.

6. The method according to claim 1, characterized in that, The process after toluene reflux includes a centrifugation step, and the process after adding polyvinylpyrrolidone and acid and before secondary calcination includes washing to neutrality and drying.

7. The method according to claim 1, characterized in that, The crude single-walled carbon nanotubes are crude single-walled carbon nanotubes containing carbon impurities and metal impurities.

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