A method for preparing single-walled carbon nanotubes

Abstract

This invention discloses a method for preparing single-walled carbon nanotubes, belonging to the technical field of single-walled carbon nanotubes. In preparing single-walled carbon nanotubes, chemical vapor deposition is used to grow them. The grown single-walled carbon nanotubes are then acid-washed, followed by sorting and purification using a polymer dispersant. Finally, the polymer dispersant is removed to obtain the single-walled carbon nanotubes. The polymer dispersant is obtained by reacting a fluorene-terminated surfactant with diaminobipyridine. The fluorene-terminated surfactant is obtained by polymerizing 9,9-di-n-octylfluorene-2,7-diboronate dipinal ester and a quaternary ammonium salt containing 2,7-dibromofluorene, followed by end-capping with 5-aldehyde-2-thiophene borate pinalol ester. The single-walled carbon nanotubes obtained by this invention have high purity and high conversion rate.

Description

Technical Field

[0001] This invention relates to a method for preparing single-walled carbon nanotubes. Background Technology

[0002] Single-walled carbon nanotubes (SUVs) have broad application prospects in nanoelectronic devices, composite materials, energy storage, and biomedicine due to their unique structure and excellent electrical, mechanical, and thermal properties. With the development of related technologies, the demand for high-purity, high-performance SUVs is increasing, especially in precision electronic devices where materials with extremely high structural integrity and chemical purity are required.

[0003] Currently, the main methods for preparing single-walled carbon nanotubes include arc discharge, laser ablation, and chemical vapor deposition. Among them, CVD is considered the most suitable technology for industrial-scale preparation of single-walled carbon nanotubes due to its mild reaction conditions, low cost, high yield, easy control of growth parameters, and ability to achieve continuous and large-scale production.

[0004] However, despite the significant advantages of CVD in terms of yield and scalability, the products generally contain a large number of impurities, including amorphous carbon, multi-walled carbon nanotubes, graphite particles, and metal catalyst particles (especially iron-based catalysts). These impurities are extremely similar to single-walled carbon nanotubes in physicochemical properties, particularly the tight bonding between iron catalysts and single-walled carbon nanotubes, making separation extremely difficult. Therefore, effectively removing these impurities and improving the purity and conversion rate of the final product have become key bottlenecks restricting the preparation of high-quality single-walled carbon nanotubes by CVD and enabling their large-scale application.

[0005] It is worth noting that existing purification techniques have achieved good results in processing multi-walled carbon nanotubes. Through a combination of processes including oxidation, acid washing, centrifugation, and filtration, the purity of multi-walled carbon nanotubes can be increased to over 99%. However, due to the larger specific surface area, more fragile structure, and stronger interaction with iron-based catalysts, traditional purification methods often cause severe damage to the tubes or loss of a large amount of effective product while removing impurities, resulting in a significant reduction in the final product conversion rate.

[0006] Studies have shown that while existing purification techniques can increase the purity of single-walled carbon nanotubes prepared by arc discharge to over 99%, the conversion rate of the final product is typically below 10%, resulting in significant resource waste. For CVD products, which have greater industrial potential, the effectiveness of existing purification processes is even more limited due to higher catalyst loading and more complex impurities: the purity of purified single-walled carbon nanotubes generally only reaches about 90%, far from meeting the demands of high-end applications, while the conversion rate is also below 10%, severely restricting their practical application value and economic feasibility.

[0007] Therefore, there is an urgent need to develop a method for preparing single-walled carbon nanotubes to solve the above problems, so as to achieve high purity and high conversion rate of single-walled carbon nanotubes. Summary of the Invention

[0008] The purpose of this invention is to provide a method for preparing single-walled carbon nanotubes to solve the technical problems mentioned in the background section.

[0009] The technical solution to achieve the objective of this invention is:

[0010] This invention provides a method for preparing single-walled carbon nanotubes, comprising the following preparation steps:

[0011] (1) Single-walled carbon nanotubes were grown by chemical vapor deposition.

[0012] (2) The single-walled carbon nanotubes obtained in step (1) are acid-leached to remove the residual metal catalyst in the single-walled carbon nanotubes;

[0013] (3) The single-walled carbon nanotubes treated in step (2) are dispersed using a polymer dispersant, and then the carbon nanotube dispersion is centrifuged and the supernatant is collected.

[0014] (4) Adjust the pH of the supernatant from step (3) to acidic, and then separate it using a two-phase aqueous system after sonication to obtain single-walled carbon nanotubes.

[0015] Furthermore, the catalyst used in step (1) chemical vapor deposition is a transition metal layered double hydroxide catalyst, and the carbon source is carbon monoxide.

[0016] Furthermore, the transition metals in the transition metal-based double hydroxide catalyst include iron and cobalt.

[0017] Furthermore, the polymer dispersant is obtained by reacting a fluorene-based surfactant with an aldehyde-terminated group with diaminobipyridine; the volume ratio of the polymer dispersant to single-walled carbon nanotubes is 1–8:1; and the concentration of the polymer dispersant is 0.2–2 mg / mL.

[0018] Furthermore, the fluorene-terminated surfactant is obtained by polymerizing 9,9-di-n-octylfluorene-2,7-diboronic acid dipina ester and a quaternary ammonium salt containing 2,7-dibromofluorene, followed by end-capping with 5-aldehyde-2-thiopheneboronic acid pina ester.

[0019] Furthermore, the preparation steps of the polymer dispersant are as follows:

[0020] An aldehyde-terminated fluorene surfactant was mixed with diaminobipyridine in equimolar amounts. Under nitrogen protection, a mixed solvent of 300–400 times the mass of diaminobipyridine was added and the mixture was heated to obtain a polymer dispersant. The mixed solvent was prepared by mixing mesitylene, dioxane, and an aqueous solution of acetic acid in a volume ratio of 19:1:2.

[0021] Furthermore, the preparation method of the fluorene-terminated surfactant is as follows:

[0022] Under nitrogen protection, a quaternary ammonium salt containing 2,7-dibromofluorene, potassium phosphate solution, and catalyst were mixed, and then a solvent was added and mixed. Next, a solvent mixture containing 9,9-di-n-octylfluorene-2,7-diboronic acid dipinal ester was added dropwise, and the mixture was heated under reflux. Then, a solvent mixture containing 5-aldehyde-2-thiophene borate dipinal ester was added and the mixture was stirred and reacted to obtain an aldehyde-terminated fluorene surfactant.

[0023] Further, the molar ratio of the quaternary ammonium salt containing 2,7-dibromofluorene, dipina 9,9-di-n-octylfluorene-2,7-diboronic acid dipina ester, and 5-aldehyde-2-thiophene borate dipina ester is (2-2.2):1:(1-1.4); the concentration of the potassium phosphate solution is 1.8-2.2 mol / L; the solvent is toluene, and the volume ratio of the solvent to the potassium phosphate solution is 3-5:1-3; the catalyst is obtained by combining palladium acetate, o-methyltriphenylphosphine, and tetrabutylammonium bromide in a molar ratio of 1:(0.8-1.2):(0.8-1.2); the amount of catalyst added is 1-2 wt%; and the molar ratio of 9,9-di-n-octylfluorene-2,7-diboronic acid dipina ester to toluene is 1 mol:(15-25) mL.

[0024] Further, the preparation steps of the quaternary ammonium salt containing 2,7-dibromofluorene are as follows: 2,7-dibromo-9,9-di(6-bromohexyl)fluorene, triethylamine, acetone, potassium carbonate, and potassium iodide are mixed, heated under nitrogen protection, and subjected to reflux and light-protected reaction to obtain the quaternary ammonium salt containing 2,7-dibromofluorene; wherein, the mass ratio of 2,7-dibromo-9,9-di(6-bromohexyl)fluorene, triethylamine, acetone, potassium carbonate, and potassium iodide is 2.3:(3-4):(31-32):(3-4):(0.8-1.2).

[0025] By adopting the above technical solution, the present invention has the following beneficial effects:

[0026] (1) The method for preparing single-walled carbon nanotubes of the present invention firstly prepares and grows single-walled carbon nanotubes by chemical vapor deposition, then removes the residual metal catalyst in the single-walled carbon nanotubes by acid leaching, then disperses the single-walled carbon nanotubes by polymer dispersant, centrifuges the dispersion and takes the supernatant; finally, adjusts the pH of the supernatant to acidic, sonicates and separates it by a two-phase system, and the purity of the obtained single-walled carbon nanotubes is >99%, and the conversion rate of the finished product is at least 70%.

[0027] (2) The chemical vapor deposition method of the present invention uses a transition metal-based layered double hydroxide catalyst as the catalyst, and carbon monoxide as the carbon source; the transition metal in the transition metal-based layered double hydroxide catalyst includes iron and cobalt; compared with traditional metal catalysts, the acid leaching purification process mainly faces two key problems: first, the acid consumption is large and the metal residue is difficult to completely remove. Since the metal particles are often wrapped by amorphous carbon or carbon layers, the strong acid must first corrode these carbonaceous coatings before reacting with the internal metal. This process not only leads to high acid consumption, but may also cause deep metal residue due to insufficient reaction; second, the single-walled carbon nanotube structure is easily damaged, and excessive acid washing will destroy its sp 2 The hybrid carbon skeleton introduces defects, reducing the integrity and electrical performance of the tube. The transition metal layered double hydroxide catalyst of this invention effectively avoids the above problems through a unique structural design: First, its layered structure is composed of metal hydroxide plates rich in hydroxyl bonds, which can be rapidly dissolved under weakly acidic conditions and removed preferentially before the carbon layer, thereby exposing the Fe / Co active metal particles uniformly dispersed between the layers. This process does not rely on strong acid to penetrate the carbon layer, significantly reducing the need for high-concentration acid solutions. Second, during the growth of carbon nanotubes, the Fe / Co metal is confined between the double hydroxide plates, and the formed metal particles are not tightly wrapped by the carbon layer. Therefore, they can be directly exposed and react with acid during acid leaching to generate soluble salts, achieving efficient removal and effectively increasing the conversion rate of single-walled carbon nanotubes.

[0028] (3) The present invention utilizes a polymer dispersant to disperse single-walled carbon nanotubes; the polymer dispersant is prepared by reacting a fluorene-terminated surfactant with diaminobipyridine; wherein, the fluorene-terminated surfactant is obtained by polymerizing 9,9-di-n-octylfluorene-2,7-diboronic acid dipina ester and a quaternary ammonium salt containing 2,7-dibromofluorene, and then reacting it with 5-aldehyde-2-thiophene borate pina ester for end capping; the quaternary ammonium salt containing 2,7-dibromofluorene is generated by reacting the bromhexyl side chain of 2,7-dibromo-9,9-di(6-bromohexyl)fluorene with triethylamine;

[0029] First, starting with 2,7-dibromo-9,9-di(6-bromohexyl)fluorene, its bromohexyl side chain undergoes a nucleophilic substitution reaction with triethylamine to introduce a quaternary ammonium salt cation, endowing the polymer with good water solubility and interfacial activity. Subsequently, using Suzuki coupling polymerization, 9,9-di-n-octylfluorene-2,7-diboronic acid dipinara ester is combined with the aforementioned quaternary ammonium salt monomer to construct a conjugated backbone structure. This rigid fluorene backbone possesses highly conjugated properties and can interact with the sp(s) of single-walled carbon nanotubes. 2 The hybrid surface generates strong π-π interactions, ensuring that the dispersant is firmly adsorbed on the surface of the single-walled carbon nanotubes;

[0030] Furthermore, the polymer chain is capped with 5-aldehyde-2-thiophene borate pinacol ester to introduce terminal aldehyde groups, which not only provide active sites for subsequent reactions with diaminobipyridine but also enhance the electronic coupling between the polymer and single-walled carbon nanotubes. Finally, the fluorene polymer with terminal aldehyde groups reacts with diaminobipyridine to form a stable imine link, giving the polymer an additional π-conjugated system and nitrogen atom coordination sites, thereby enhancing the interaction with single-walled carbon nanotubes. The fluorene backbone provides a rigid conjugated framework and π-π stacking ability; the quaternary ammonium salt side chain imparts water solubility and electrostatic stability; the thiophene end group enhances electronic coupling; and the bipyridine unit provides selective recognition sites. The polymer dispersant obtained in this invention has a significant selective dispersion effect on single-walled carbon nanotubes.

[0031] Specifically, the strong π-π stacking interaction between the fluorene units in the polymer backbone and the walls of single-walled carbon nanotubes ensures that the dispersant molecules are not easily detached from the surface of single-walled carbon nanotubes during ultrasonic treatment and centrifugation. Simultaneously, the positive charges generated by the ionization of quaternary ammonium salt groups on the polymer side chains in aqueous solution form an electric double layer on the surface of the single-walled carbon nanotubes, generating electrostatic repulsion and effectively preventing aggregation between single-walled carbon nanotubes, thus achieving stable dispersion. Furthermore, the long-chain side groups in the polymer extend outward from the surface of the single-walled carbon nanotubes, forming a thick spatial barrier that prevents direct contact between the single-walled carbon nanotubes, further preventing aggregation. Finally, the introduction of diaminobipyridine units provides the dispersant with selective recognition capabilities, effectively enabling selective separation of single-walled carbon nanotubes and improving their purity.

[0032] (4) The polymer dispersant of the present invention is prepared by reacting a fluorene-based surfactant with an aldehyde-terminated group with diaminobipyridine. After dispersion in step (3), the single-walled carbon nanotubes enter step (4). In step (4), the supernatant of step (3) is adjusted to pH acidity, and after sonication, it is separated by an aqueous two-phase system. During this process, the polymer dispersant on the surface of the single-walled carbon nanotubes is dissociated by acid hydrolysis to form a fluorene-based surfactant with an aldehyde-terminated group and diaminobipyridine. The fluorene-based surfactant with an aldehyde-terminated group and diaminobipyridine are then separated by an aqueous two-phase system to remove them, resulting in high-purity single-walled carbon nanotubes. In step (4), the supernatant is adjusted to acidic conditions. Under acid catalysis, the imine bonds in the main chain of the polymer dispersant undergo hydrolytic breakage—that is, "acid hydrolysis," which causes the polymer to dissociate and regenerate in situ into the original components: end Aldehyde-terminated fluorene surfactants and diaminobipyridine are used. Ultrasonic treatment further promotes the desorption of pyrolysis products from the surface of single-walled carbon nanotubes. Subsequently, an aqueous two-phase system, such as a PEG / salt system, is introduced to utilize the differences in the distribution of different components between the two phases for efficient separation: single-walled carbon nanotubes, due to their high density and strong hydrophobicity, are mainly enriched at the interface or in the salt phase; aldehyde-terminated fluorene surfactants, due to their aromatic structure and certain hydrophobicity, tend to enter the PEG phase; while diaminobipyridine, which is highly hydrophilic and may carry a positive charge, preferentially enters the salt phase. Through multi-step extraction and phase transfer operations, small molecule impurities generated during dissociation can be gradually separated and removed from the single-walled carbon nanotubes. Ultimately, single-walled carbon nanotubes with high purity, low organic residue, and intact structure are obtained. Simultaneously, the pyrolysis products are expected to be recycled and reused, achieving a green and efficient purification process for single-walled carbon nanotubes. Detailed Implementation

[0033] To better understand the above technical solution, the following will provide a detailed explanation of the technical solution in conjunction with specific implementation methods.

[0034] Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this invention.

[0035] The following embodiments are only used to illustrate the technical solutions of the present invention more clearly, and should not be used to limit the scope of protection of the present invention.

[0036] Example 1

[0037] A method for preparing single-walled carbon nanotubes includes the following preparation steps:

[0038] (1) Growth of single-walled carbon nanotubes by chemical vapor deposition: First, a quartz tube with an inner diameter of 40 mm was placed in the CVD equipment, and a transition metal layered double hydroxide catalyst was placed in a quartz boat and then transferred to the quartz tube; Argon gas was passed through at a rate of 300 sccm for 30 min to remove air; at the same time, the temperature zone slide rail furnace was heated to 600℃ at a rate of 15℃ / min; after the temperature stabilized, carbon monoxide gas was used instead of argon gas for 25 min to grow single-walled carbon nanotubes; finally, Ar gas was passed through at 300 sccm to remove carbon monoxide gas from the system, and after the reaction area cooled down naturally, the gas path was closed and the single-walled carbon nanotubes were removed; the entire CVD reaction process was carried out at atmospheric pressure.

[0039] (2) The product obtained in step (1) was soaked in 1M hydrochloric acid solution to remove the residual transition metal layered double hydroxide catalyst in the single-walled carbon nanotubes. After standing, the upper liquid was removed, and then the product was washed with deionized water until the pH of the washed liquid was neutral.

[0040] (3) Mix 4 mg of polymer dispersant with 3 mL of water, then sonicate for 5 min, add 0.5 mg of single-walled carbon nanotubes treated in step (2), sonicate for 40 min under ice bath conditions, centrifuge at 10000 rpm for 30 min at 10℃, and take the supernatant.

[0041] (4) Adjust the pH of the supernatant from step (3) to 3 with trifluoroacetic acid. After sonicating for 40 min under ice bath conditions, add 20% glucose 70k solution (twice the volume of the supernatant) and polyethylene glycol 6k solution (equal volume of the supernatant). Mix well and centrifuge at 10,000 rpm for 3 min at 20 °C. After phase separation, carbon nanotubes are enriched in the lower phase. Take the lower phase, filter, wash and dry to obtain single-walled carbon nanotubes.

[0042] The preparation steps of the transition metal layered double hydroxide catalyst are as follows: aluminum nitrate solution and 0.19 mol / L sodium carbonate solution are mixed at a volume ratio of 1:1, the pH is adjusted to ≥11 with 3M urea solution, heated to 100℃, and magnesium nitrate solution and cobalt nitrate solution are added dropwise while the pH is controlled to ≥11 with urea solution. The mixture is stirred and refluxed for 12 h, the suspension is stopped at 95℃ and allowed to stand and reflux for 12 h, then filtered, washed, freeze-dried, ground into powder, and calcined in a muffle furnace at 1000℃ for 4 h; wherein the initial molar ratio of cobalt, magnesium, and aluminum is 0.2:2:1.

[0043] The preparation steps of the polymer dispersant are as follows:

[0044] An aldehyde-terminated fluorene surfactant was mixed with diaminobipyridine in equal molar amounts. Under nitrogen protection, a mixed solvent of 360 times the mass of diaminobipyridine was added, and the mixture was reacted at 120°C for 3 days. After the reaction was completed, the mixture was cooled to room temperature, vacuum filtered, washed with acetone and tetrahydrofuran, and dried to obtain a polymer dispersant. The mixed solvent was prepared by mixing mesitylene, dioxane, and an aqueous solution of acetic acid in a volume ratio of 19:1:2.

[0045] The preparation method of the fluorene-terminated aldehyde surfactant is as follows: Under nitrogen protection, a quaternary ammonium salt containing 2,7-dibromofluorene, potassium phosphate solution, and catalyst are mixed, and then a solvent is added and mixed. Next, a solvent mixture containing 9,9-di-n-octylfluorene-2,7-diboronic acid dipinal ester is added dropwise at 1 drop / s. The mixture is heated to 80°C and refluxed for 24 hours. Then, a solvent mixture containing 5-aldehyde-2-thiophene borate dipinal ester is added and the mixture is stirred and reacted for another 24 hours to obtain the fluorene-terminated aldehyde surfactant; wherein, the quaternary ammonium salt containing 2,7-dibromofluorene, 9,9- The molar ratio of di-n-octylfluorene-2,7-diboronic acid dipinacol ester and 5-aldehyde-2-thiopheneboronic acid pinacol ester is 2:1:1; the concentration of the potassium phosphate solution is 1.8 mol / L; the solvent is toluene, and the volume ratio of solvent to potassium phosphate solution is 3:1; the catalyst is a composite of palladium acetate, o-methyltriphenylphosphine, and tetrabutylammonium bromide in a molar ratio of 1:0.8:0.8; the catalyst addition amount is 1 wt%; the molar ratio of 9,9-di-n-octylfluorene-2,7-diboronic acid dipinacol ester to toluene is 1 mol:15 mL.

[0046] The preparation steps of the quaternary ammonium salt containing 2,7-dibromofluorene are as follows: 2,7-dibromo-9,9-di(6-bromohexyl)fluorene, triethylamine, acetone, potassium carbonate, and potassium iodide are mixed, heated to 65°C under nitrogen protection, and refluxed in the dark for 16 hours. The mixture is filtered, and the filtrate is collected. The organic solvent and unreacted triethylamine are removed by vacuum distillation. The filtrate is diluted with dichloromethane and washed three times with deionized water. The organic layer is dried with anhydrous sodium sulfate to obtain the quaternary ammonium salt containing 2,7-dibromofluorene. The mass ratio of 2,7-dibromo-9,9-di(6-bromohexyl)fluorene, triethylamine, acetone, potassium carbonate, and potassium iodide is 2.3:3:31:3:0.8.

[0047] Example 2

[0048] A method for preparing single-walled carbon nanotubes includes the following preparation steps:

[0049] (1) Growth of single-walled carbon nanotubes by chemical vapor deposition: First, a quartz tube with an inner diameter of 40 mm was placed in the CVD equipment, and a transition metal layered double hydroxide catalyst was placed in a quartz boat and then transferred to the quartz tube; Argon gas was passed through at a rate of 300 sccm for 30 min to remove air; at the same time, the temperature zone slide rail furnace was heated to 600℃ at a rate of 15℃ / min; after the temperature stabilized, carbon monoxide gas was used instead of argon gas for 25 min to grow single-walled carbon nanotubes; finally, Ar gas was passed through at 300 sccm to remove carbon monoxide gas from the system, and after the reaction area cooled down naturally, the gas path was closed and the single-walled carbon nanotubes were removed; the entire CVD reaction process was carried out at atmospheric pressure.

[0050] (2) The product obtained in step (1) was soaked in 1M hydrochloric acid solution to remove the residual transition metal layered double hydroxide catalyst in the single-walled carbon nanotubes. After standing, the upper liquid was removed, and then the product was washed with deionized water until the pH of the washed liquid was neutral.

[0051] (3) Mix 4 mg of polymer dispersant with 3 mL of water, then sonicate for 5 min, add 0.5 mg of single-walled carbon nanotubes treated in step (2), sonicate for 40 min under ice bath conditions, centrifuge at 10000 rpm for 30 min at 10℃, and take the supernatant.

[0052] (4) Adjust the pH of the supernatant from step (3) to 3-4 with trifluoroacetic acid, sonicate for 40 min under ice bath conditions, add 20% glucose 70k solution (twice the volume of the supernatant) and polyethylene glycol 6k (equal volume of the supernatant), mix well, centrifuge at 10000 rpm for 3 min at 20℃, after phase separation, carbon nanotubes are enriched in the lower phase, take the lower phase, filter, wash and dry to obtain single-walled carbon nanotubes.

[0053] The preparation steps of the transition metal layered double hydroxide catalyst are as follows: aluminum nitrate solution and 0.19 mol / L sodium carbonate solution are mixed at a volume ratio of 1:1, the pH is adjusted to ≥11 with 3M urea solution, heated to 100℃, and magnesium nitrate solution and cobalt nitrate solution are added dropwise while the pH is controlled to ≥11 with urea solution. The mixture is stirred and refluxed for 12 h, the suspension is stopped at 95℃ and allowed to stand and reflux for 12 h, then filtered, washed, freeze-dried, ground into powder, and calcined in a muffle furnace at 1000℃ for 4 h; wherein the initial molar ratio of cobalt, magnesium, and aluminum is 0.2:2:1.

[0054] The preparation steps of the polymer dispersant are as follows:

[0055] An aldehyde-terminated fluorene surfactant was mixed with diaminobipyridine in equal molar amounts. Under nitrogen protection, a mixed solvent of 360 times the mass of diaminobipyridine was added, and the mixture was reacted at 120°C for 3 days. After the reaction was completed, the mixture was cooled to room temperature, vacuum filtered, washed with acetone and tetrahydrofuran, and dried to obtain a polymer dispersant. The mixed solvent was prepared by mixing mesitylene, dioxane, and an aqueous solution of acetic acid in a volume ratio of 19:1:2.

[0056] The preparation method of the fluorene-terminated aldehyde surfactant is as follows: Under nitrogen protection, a quaternary ammonium salt containing 2,7-dibromofluorene, potassium phosphate solution, and catalyst are mixed, and then a solvent is added and mixed. Next, a solvent mixture containing 9,9-di-n-octylfluorene-2,7-diboronic acid dipinal ester is added dropwise at 1 drop / s. The mixture is heated to 80°C and refluxed for 24 hours. Then, a solvent mixture containing 5-aldehyde-2-thiophene borate dipinal ester is added and the mixture is stirred and reacted for another 24 hours to obtain the fluorene-terminated aldehyde surfactant; wherein, the quaternary ammonium salt containing 2,7-dibromofluorene, 9,9-dioctylfluorene-2,7-diboronic acid dipinal ester, and the catalyst are mixed. The molar ratio of di-n-octylfluorene-2,7-diboronic acid dipina ester to 5-aldehyde-2-thiopheneboronic acid pinacol ester is 2.1:1:1.2; the concentration of the potassium phosphate solution is 2 mol / L; the solvent is toluene, and the volume ratio of solvent to potassium phosphate solution is 4:2; the catalyst is a composite of palladium acetate, o-methyltriphenylphosphine, and tetrabutylammonium bromide in a molar ratio of 1:1:1; the catalyst addition amount is 2 wt%; the molar ratio of 9,9-di-n-octylfluorene-2,7-diboronic acid dipina ester to toluene is 1 mol:20 mL.

[0057] The preparation steps of the quaternary ammonium salt containing 2,7-dibromofluorene are as follows: 2,7-dibromo-9,9-di(6-bromohexyl)fluorene, triethylamine, acetone, potassium carbonate, and potassium iodide are mixed, heated to 65°C under nitrogen protection, and refluxed in the dark for 16 hours. The mixture is filtered, and the filtrate is collected. The organic solvent and unreacted triethylamine are removed by vacuum distillation. The filtrate is diluted with dichloromethane and washed three times with deionized water. The organic layer is dried with anhydrous sodium sulfate to obtain the quaternary ammonium salt containing 2,7-dibromofluorene. The mass ratio of 2,7-dibromo-9,9-di(6-bromohexyl)fluorene, triethylamine, acetone, potassium carbonate, and potassium iodide is 2.3:3.6:31.5:3.5:1.

[0058] Example 3

[0059] A method for preparing single-walled carbon nanotubes includes the following preparation steps:

[0060] (1) Growth of single-walled carbon nanotubes by chemical vapor deposition: First, a quartz tube with an inner diameter of 40 mm was placed in the CVD equipment, and a transition metal layered double hydroxide catalyst was placed in a quartz boat and then transferred to the quartz tube; Argon gas was passed through at a rate of 300 sccm for 30 min to remove air; at the same time, the temperature zone slide rail furnace was heated to 600℃ at a rate of 15℃ / min; after the temperature stabilized, carbon monoxide gas was used instead of argon gas for 25 min to grow single-walled carbon nanotubes; finally, Ar gas was passed through at 300 sccm to remove carbon monoxide gas from the system, and after the reaction area cooled down naturally, the gas path was closed and the single-walled carbon nanotubes were removed; the entire CVD reaction process was carried out at atmospheric pressure.

[0061] (2) The product obtained in step (1) was soaked in 1M hydrochloric acid solution to remove the residual transition metal layered double hydroxide catalyst in the single-walled carbon nanotubes. After standing, the upper liquid was removed, and then the product was washed with deionized water until the pH of the washed liquid was neutral.

[0062] (3) Mix 4 mg of polymer dispersant with 3 mL of water, then sonicate for 5 min, add 0.5 mg of single-walled carbon nanotubes treated in step (2), sonicate for 40 min under ice bath conditions, centrifuge at 10000 rpm for 30 min at 10℃, and take the supernatant.

[0063] (4) Adjust the pH of the supernatant from step (3) to 4 with trifluoroacetic acid. After sonicating for 40 min under ice bath conditions, add 20% glucose 70k solution (twice the volume of the supernatant) and polyethylene glycol 6k solution (equal volume of the supernatant). Mix well and centrifuge at 10,000 rpm for 3 min at 20 °C. After phase separation, carbon nanotubes are enriched in the lower phase. Take the lower phase, filter, wash and dry to obtain single-walled carbon nanotubes.

[0064] The preparation steps of the transition metal layered double hydroxide catalyst are as follows: aluminum nitrate solution and 0.19 mol / L sodium carbonate solution are mixed at a volume ratio of 1:1, the pH is adjusted to ≥11 with 3M urea solution, heated to 100℃, and magnesium nitrate solution and cobalt nitrate solution are added dropwise while the pH is controlled to ≥11 with urea solution. The mixture is stirred and refluxed for 12 h, the suspension is stopped at 95℃ and allowed to stand and reflux for 12 h, then filtered, washed, freeze-dried, ground into powder, and calcined in a muffle furnace at 1000℃ for 4 h; wherein the initial molar ratio of cobalt, magnesium, and aluminum is 0.2:2:1.

[0065] The preparation steps of the polymer dispersant are as follows:

[0066] An aldehyde-terminated fluorene surfactant was mixed with diaminobipyridine in equal molar amounts. Under nitrogen protection, a mixed solvent of 360 times the mass of diaminobipyridine was added, and the mixture was reacted at 120°C for 3 days. After the reaction was completed, the mixture was cooled to room temperature, vacuum filtered, washed with acetone and tetrahydrofuran, and dried to obtain a polymer dispersant. The mixed solvent was prepared by mixing mesitylene, dioxane, and an aqueous solution of acetic acid in a volume ratio of 19:1:2.

[0067] The preparation method of the aldehyde-terminated fluorene surfactant is as follows: Under nitrogen protection, a quaternary ammonium salt containing 2,7-dibromofluorene, potassium phosphate solution, and catalyst are mixed, and then a solvent is added and mixed. Next, a solvent mixture containing 9,9-di-n-octylfluorene-2,7-diboronic acid dipinal ester is added dropwise at 1 drop / s. The mixture is heated to 80°C and refluxed for 24 hours. Then, a solvent mixture containing 5-aldehyde-2-thiophene borate dipinal ester is added and the mixture is stirred and reacted for another 24 hours to obtain the aldehyde-terminated fluorene surfactant; wherein, the quaternary ammonium salt containing 2,7-dibromofluorene, 9,9-di-n-octylfluorene, and potassium phosphate solution are mixed. The molar ratio of dipina fluorene-2,7-diboronate to 5-aldehyde-2-thiopheneboronate pinacol ester is 2.2:1:1.4; the concentration of the potassium phosphate solution is 2 mol / L; the solvent is toluene, and the volume ratio of solvent to potassium phosphate solution is 3-5:1-3; the catalyst is a composite of palladium acetate, o-methyltriphenylphosphine, and tetrabutylammonium bromide in a molar ratio of 1:1.2:1.2; the catalyst addition amount is 2 wt%; the molar ratio of 9,9-di-n-octylfluorene-2,7-diboronate to toluene is 1 mol:25 mL.

[0068] The preparation steps of the quaternary ammonium salt containing 2,7-dibromofluorene are as follows: 2,7-dibromo-9,9-di(6-bromohexyl)fluorene, triethylamine, acetone, potassium carbonate, and potassium iodide are mixed, heated to 65°C under nitrogen protection, and refluxed in the dark for 16 hours. The mixture is filtered, and the filtrate is collected. The organic solvent and unreacted triethylamine are removed by vacuum distillation. The filtrate is diluted with dichloromethane and washed three times with deionized water. The organic layer is dried with anhydrous sodium sulfate to obtain the quaternary ammonium salt containing 2,7-dibromofluorene. The mass ratio of 2,7-dibromo-9,9-di(6-bromohexyl)fluorene, triethylamine, acetone, potassium carbonate, and potassium iodide is 2.3:4:32:4:1.2.

[0069] Comparative Example 1

[0070] The difference between Comparative Example 1 and Example 2 is that iron powder is used as the catalyst in the chemical vapor deposition method, and the concentration of hydrochloric acid solution in step (2) is 6M. The remaining steps and components are the same as in Example 2.

[0071] Comparative Example 2

[0072] The difference between Comparative Example 2 and Example 2 lies in the polymer dispersant. The polymer dispersant in Comparative Example 2 is prepared by reacting fluorene with a terminal aldehyde group with diaminobipyridine. The terminal aldehyde group fluorene is obtained by reacting 9,9-dioctyl-2,7-dibromofluorene with pinacol ester of 5-aldehyde-2-thiophene borate. The remaining steps and components are the same as in Example 2.

[0073] Example of effect

[0074] The single-walled carbon nanotubes obtained in the examples and comparative examples were calcined at 800°C for 30 minutes in an air atmosphere with an air flow rate of 400 mL / min. After cooling, the residual ash was weighed, and the purity of the single-walled carbon nanotubes and the conversion rate of the finished product were tested. The conversion rate of the finished product (%) = (mass of single-walled carbon nanotubes after calcination / mass of the initial crude product obtained in step (1)) × 100%.

[0075] Table 1 below shows the test results of the single-walled carbon nanotubes prepared in the examples and comparative examples:

[0076] Table 1

[0077] purity(%) Finished product conversion rate (%) Example 1 99.2 69.3 Example 2 99.4 70.9 Example 3 99.3 70.2 Comparative Example 1 99.2 18.0 Comparative Example 2 96.9 65.7

[0078] As shown in Table 1, the single-walled carbon nanotubes obtained in Examples 1-3 have high purity and high conversion rate.

[0079] The difference between Comparative Example 1 and Example 2 is that the chemical vapor deposition method uses a traditional iron powder catalyst, resulting in a low conversion rate of the single-walled carbon nanotube product.

[0080] The difference between Comparative Example 2 and Example 2 lies in the polymer dispersant. The polymer dispersant in Comparative Example 2 was prepared by reacting fluorene with a terminal aldehyde group with diaminobipyridine. The terminal aldehyde group fluorene was obtained by reacting 9,9-dioctyl-2,7-dibromofluorene with pinacol ester of 5-aldehyde-2-thiophene borate. The purity of the resulting single-walled carbon nanotubes was slightly lower because there were more small molecule pyrolysis products of the residual polymer dispersant on the surface of the single-walled carbon nanotubes, and the conversion rate of the finished single-walled carbon nanotubes was lower.

[0081] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing single-walled carbon nanotubes, characterized in that, The preparation steps include the following: (1) Single-walled carbon nanotubes were grown by chemical vapor deposition. (2) The single-walled carbon nanotubes obtained in step (1) are acid-leached to remove the residual metal catalyst in the single-walled carbon nanotubes; (3) The single-walled carbon nanotubes treated in step (2) were dispersed using a polymer dispersant, and then the carbon nanotube dispersion was centrifuged and the supernatant was collected. (4) Adjust the pH of the supernatant from step (3) to acidic, and then separate it using a two-phase aqueous system after sonication to obtain single-walled carbon nanotubes. The polymer dispersant is obtained by reacting a fluorene-based surfactant with an aldehyde-terminated group with diaminobipyridine. The volume ratio of the polymer dispersant to single-walled carbon nanotubes is 1~8:1; the concentration of the polymer dispersant is 0.2~2 mg / mL. The preparation method of the fluorene-terminated surfactant is as follows: Under nitrogen protection, a quaternary ammonium salt containing 2,7-dibromofluorene, potassium phosphate solution, and catalyst are mixed, and then a solvent is added and mixed. Next, a solvent mixture containing 9,9-di-n-octylfluorene-2,7-diboronic acid dipinal ester is added dropwise, and the mixture is heated under reflux. Then, a solvent mixture containing 5-aldehyde-2-thiophene borate dipinal ester is added and the mixture is stirred and reacted to obtain the fluorene-terminated surfactant. The molar ratio of the quaternary ammonium salt containing 2,7-dibromofluorene, dipina 9,9-di-n-octylfluorene-2,7-diboronic acid dipina ester, and 5-aldehyde-2-thiophene borate dipina ester is (2~2.2):1:(1~1.4); the concentration of the potassium phosphate solution is 1.8~2.2 mol / L; the solvent is toluene, and the volume ratio of the solvent to the potassium phosphate solution is 3~5:1~3; the catalyst is obtained by combining palladium acetate, o-methyltriphenylphosphine, and tetrabutylammonium bromide in a molar ratio of 1:(0.8~1.2):(0.8~1.2); the amount of catalyst added is 1~2 wt%; the molar volume ratio of 9,9-di-n-octylfluorene-2,7-diboronic acid dipina ester to toluene is 1 mol:(15~25) mL.

2. The method for preparing single-walled carbon nanotubes according to claim 1, characterized in that, The catalyst used in step (1) of the chemical vapor deposition method is a transition metal layered double hydroxide catalyst, and the carbon source is carbon monoxide.

3. The method for preparing single-walled carbon nanotubes according to claim 2, characterized in that, The transition metals in the transition metal-based double hydroxide catalyst include iron and cobalt.

4. The method for preparing single-walled carbon nanotubes according to claim 1, characterized in that, The fluorene-terminated surfactant is obtained by polymerizing 9,9-di-n-octylfluorene-2,7-diboronic acid dipina ester and a quaternary ammonium salt containing 2,7-dibromofluorene, followed by end-capping with 5-aldehyde-2-thiopheneboronic acid pina ester.

5. The method for preparing single-walled carbon nanotubes according to claim 1, characterized in that, The preparation steps of the polymer dispersant are as follows: An aldehyde-terminated fluorene surfactant was mixed with diaminobipyridine in equimolar amounts. Under nitrogen protection, a mixed solvent of 300-400 times the mass of diaminobipyridine was added and the mixture was heated to obtain a polymer dispersant. The mixed solvent was prepared by mixing mesitylene, dioxane, and an aqueous acetic acid solution in a volume ratio of 19:1:

2.

6. The method for preparing single-walled carbon nanotubes according to claim 1, characterized in that, The preparation steps of the quaternary ammonium salt containing 2,7-dibromofluorene are as follows: 2,7-dibromo-9,9-di(6-bromohexyl)fluorene, triethylamine, acetone, potassium carbonate, and potassium iodide are mixed, heated under nitrogen protection, and subjected to reflux and light-protected reaction to obtain the quaternary ammonium salt containing 2,7-dibromofluorene; wherein, the mass ratio of 2,7-dibromo-9,9-di(6-bromohexyl)fluorene, triethylamine, acetone, potassium carbonate, and potassium iodide is 2.3:(3~4):(31~32):(3~4):(0.8~1.2).