JP3934089B2 - Carbon nanotube purification method - Google Patents

Description

【００１７】
第１表から、実施例１では、得られた精製試料は、未精製試料に比べて、アモルファスカーボン及び金属成分が除去され、カーボンナノチューブの純度が大幅に向上すると共に、比表面積も大きく増大していることが分かる。比較例１の精製試料は、実施例１の精製試料に比べて、金属成分の除去が十分ではなく、蟻酸のみによる精製処理では、カーボンナノチューブの純度及び比表面積が劣ることが分かる。比較例２の精製試料は、実施例１の精製試料に比べて、アモルファスカーボン及び金属成分の除去が十分ではなく、過酸化水素水のみによる精製処理では、カーボンナノチューブの純度及び比表面積が劣ることが分かる。比較例３の精製試料は、実施例１の精製試料に比べて、アモルファスカーボン及び金属成分の除去が十分ではなく、過酸化水素水処理に次いで蟻酸処理する二段階による精製処理では、カーボンナノチューブの純度及び比表面積が劣ることが分かる。
すなわち、比較例１〜３の結果から、蟻酸と過酸化水素水の共存下での１段処理（実施例１）の有効性が確認できる。
【００１８】
【発明の効果】
本発明によれば、アモルファスカーボンや金属成分などの不純物を含む未精製カーボンナノチューブを、簡単なプロセスで、加熱に伴うエネルギーを要することなく、効率よくかつ安全に精製し、ガス貯蔵材料を始め、従来の炭素材料が使用されている用途での代替炭素材料として期待できるカーボンナノチューブを工業的に有利に精製する方法を提供することができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for purifying carbon nanotubes. More specifically, the present invention relates to a method for purifying unpurified carbon nanotubes containing impurities such as amorphous carbon and metal components efficiently by a simple process without requiring energy associated with heating. The present invention relates to a method for purifying carbon nanotubes that can be expected as an alternative carbon material in applications where conventional carbon materials are used, including storage materials.
[0002]
[Prior art]
In recent years, carbon nanotubes, which are cylindrical carbon materials with diameters ranging from several angstroms to several tens of nanometers, include, for example, molecular devices that can be highly integrated, occlusion materials for various gases such as hydrogen, and field emission displays (FEDs). It has been attracting attention as a functional material such as a member for use in molding and an additive for resin molded products.
This carbon nanotube was found in a lump of carbon deposited on the cathode of an arc discharge method by Iijima et al. In 1991 (see, for example, Non-Patent Document 1), and since then it has been actively researched. It is synthesized by various methods such as a laser beam irradiation method and a gas phase synthesis method by thermal decomposition.
There are multi-walled carbon nanotubes (MWCNT) and single-walled carbon nanotubes (SWCNT). In particular, single-walled carbon nanotubes are expected to be used as storage materials for various gases including hydrogen. . However, the development of a method for efficiently removing and purifying impurities such as amorphous carbon and metal components used as a catalyst contained in the produced carbon nanotubes is an issue for use in various applications including these. It has become.
[0003]
As methods for purifying carbon nanotubes, various methods have been proposed so far as described below.
For example, (1) a carbon nanotube containing carbon impurities is finely pulverized, and the finely pulverized product is dispersed in a liquid and centrifuged to form a supernatant containing fine particles, and the supernatant is filtered. (2) crushed carbon nanotube crude product, stirred in solution, and centrifuged or flotated to obtain amorphous carbon Or graphite, and further dissolved in an acid or passed through a magnetic field to remove metal impurities and improve purity (for example, see Patent Document 2), (3) a carbon nanotube crude product, Impurities are removed through three steps: purification using an aqueous acid solution, purification using a mixture of acetone and acid, and purification using a purification gas. (4) For example, Patent Document 3), (4) A step of refluxing a crude product of carbon nanotubes in an acid or alkali to remove metal particles, heat treatment at a temperature of 200 to 700 ° C., and amorphous carbon After the two steps of the process of vaporizing carbon, the method for improving the purity of carbon nanotubes to remove impurities (see, for example, Patent Document 4), (5) After the carbon nanotube crude product is mixed with an acid treatment step and a sublimation substance , A method for improving the purity of carbon nanotubes (see, for example, Patent Document 5), which removes impurities through three stages of a porous treatment step and an oxidation treatment step in which a sublimable substance is removed from the sample by heating; Wet oxidation process in which the product is wet-oxidized in an aqueous solution of hydrogen peroxide to remove amorphous carbon particles. Carbon nanotubes that remove impurities through three steps: a sedimentation process that repeats dispersion and sedimentation separation to separate and remove nanoparticles and graphite, and an acid treatment process that removes metal particles by washing with an acid solution such as hydrochloric acid. (7) For example, Patent Document 6), (7) A process for removing a metal by immersing a crude carbon nanotube product in an acidic (sulfuric acid) solution, or in an acidic solution and applying an electric field, amorphous A method for improving the purity of carbon nanotubes (see, for example, Patent Document 7), which removes impurities through two stages of the carbon heat removal step, or (8) heating the carbon nanotube crude product at a temperature of 350 to 600 ° C., A first step consisting of a wet oxidation treatment in hydrogen peroxide water and a reflux treatment step in nitric acid, wet oxidation in hydrogen peroxide water A method for improving the purity of carbon nanotubes (see, for example, Patent Document 8) is disclosed in which impurities are removed through a second step consisting of a treatment step and a reflux treatment step in hydrochloric acid.
[0004]
The carbon nanotube crude product usually contains amorphous carbon and a metal component used as a catalyst as impurities, and the type and composition depend on the production method. It is effective to remove these impurities by different methods. In general, amorphous carbon is removed by oxidation treatment, and metal components are removed by dissolution treatment with acid.
Therefore, the carbon nanotube refining method according to the above (1) to (8) requires a plurality of stages of processing steps and has the disadvantage of poor efficiency.
In addition, a method for improving the purity of carbon nanotubes is disclosed in which impurities are removed by heat-treating a crude product of carbon nanotubes in a mixed solution of hydrogen peroxide and acid (see, for example, Patent Document 9). However, in this method, an inorganic acid is used as the acid, and the purity is improved by heating a mixed solution of the acid and hydrogen peroxide, and energy is required for the heat treatment. There are problems such as not necessarily a safe processing method.
[0005]
[Patent Document 1]
JP-A-7-48111 [Patent Document 2]
JP-A-8-198611 [Patent Document 3]
JP 2001-26410 A [Patent Document 4]
JP 2001-335310 A [Patent Document 5]
Japanese Patent Laid-Open No. 2002-121014 [Patent Document 6]
JP 2002-265209 A [Patent Document 7]
JP 2002-308610 A [Patent Document 8]
JP 2003-81616 A [Patent Document 9]
JP 2003-89510 A [Non-Patent Document 1]
“Nature”, 354, 56-58 (1991)
[0006]
[Problems to be solved by the invention]
The present invention has been made under such circumstances, and it is possible to efficiently and safely process unpurified carbon nanotubes containing impurities such as amorphous carbon and metal components by a simple process without requiring energy associated with heating. The purpose of the present invention is to provide a method for purifying and industrially advantageously purifying carbon nanotubes that can be expected as alternative carbon materials in applications where conventional carbon materials are used, including gas storage materials. .
[0007]
[Means for Solving the Problems]
As a result of intensive research to achieve the above object, the present inventor has used a mixed solution of organic acid and hydrogen peroxide, and has obtained unpurified carbon nanotubes, preferably at room temperature or without external heating. It has been found that the object can be achieved by bringing it into contact and purifying it in one step. The present invention has been completed based on such findings.
That is, the present invention
(1) A method for purifying carbon nanotubes, wherein the unpurified carbon nanotubes are brought into contact with a mixed solution of an organic acid and hydrogen peroxide and purified in one step.
(2) The above (1) wherein the organic acid is at least one selected from aliphatic monocarboxylic acids having 1 to 3 carbon atoms, halogens and / or hydroxyl substituents thereof, and benzoic acids, halogens and / or hydroxyl substituents thereof ( 1) Carbon nanotube purification method,
(3) The carbon nanotube according to (2), wherein the organic acid is at least one selected from formic acid, acetic acid, monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, trifluoroacetic acid, benzoic acid, m-chlorobenzoic acid, and salicylic acid. And (4) the carbon nanotube purification method of (1) to (3) above, wherein the purification treatment is carried out at room temperature or without external heating,
Is to provide.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
The unpurified carbon nanotube used in the carbon nanotube purification method of the present invention is not particularly limited, and is, for example, MWCNT or SWCNT manufactured by an arc discharge method, a laser method, a CVD method (chemical vapor deposition method), or the like. . These carbon nanotube immature products usually contain fine particles such as amorphous carbon, graphite, catalytic metals such as Fe, Co, Ni, Y, and metal oxides as impurities.
In the method of the present invention, a mixed liquid of an organic acid and hydrogen peroxide is used to remove impurities in the unpurified carbon nanotubes. When using a mixture of this organic acid and hydrogen peroxide, the reaction between the organic acid and hydrogen peroxide produces an organic peracid having a strong oxidizing power as shown by the following formula, which is contained in the carbon nanotube. Oxidizes and decomposes amorphous carbon.
R—COOH + H ₂ O ₂ → R—COOOH + H ₂ O
R-COOOH + (impurity component in carbon nanotube)
→ R-COOH + (impurity oxide)
As a result, the metal component that has been blocked from contact with the organic acid by the amorphous carbon is exposed, and is easily dissolved and removed in the organic acid.
[0009]
In the present invention, the organic acid is at least one selected from aliphatic monocarboxylic acids having 1 to 3 carbon atoms, halogen and / or hydroxyl group substituents thereof, and benzoic acid, halogen and / or hydroxyl group substituents thereof. Can be used. Here, examples of the aliphatic monocarboxylic acid having 1 to 3 carbon atoms include formic acid, acetic acid, propionic acid, and the halogen and / or hydroxyl group substituent thereof include, for example, monochloroacetic acid, dichloroacetic acid, and trichloroacetic acid. , Trifluoroacetic acid, glycolic acid and the like. Examples of the halogen and / or hydroxyl group-substituted benzoic acid include o-chlorobenzoic acid, m-chlorobenzoic acid, p-chlorobenzoic acid, salicylic acid (o-hydroxybenzoic acid), m-hydroxybenzoic acid, p -Hydroxybenzoic acid, 4-chloro-2-hydroxybenzoic acid, 2-chloro-4-hydroxybenzoic acid, 3-chloro-4-hydroxybenzoic acid and the like. Among these organic acids, formic acid, acetic acid, monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, trifluoroacetic acid, benzoic acid, m-chlorobenzoic acid and salicylic acid are particularly preferable. These organic acids may be used individually by 1 type, and may be used in combination of 2 or more type.
[0010]
When the organic acid is solid at room temperature, it is dissolved in a solvent that dissolves the organic acid, for example, water, formic acid, acetic acid, or an organic solvent inert to hydrogen peroxide, and used as a solution. It is desirable.
In the present invention, the amount of the organic acid used is preferably 10 times mol or more, more preferably 50 times mol or more, and particularly preferably 100 times mol with respect to the number of moles of the metal component contained in the unpurified carbon nanotube. That's it. Further, the amount of hydrogen peroxide used depends on the type and amount of amorphous carbon of the unpurified carbon nanotubes, but is usually 5 times or more, preferably 10 to 40 times the mass of the unpurified carbon nanotubes, more preferably Is selected in the range of 15 to 30 times the mass. Furthermore, the use ratio of the organic acid and hydrogen peroxide is usually 0.1 or more, preferably 1 or more, more preferably 1.5 to 10 or more in terms of the organic acid / hydrogen peroxide molar ratio.
In addition, before performing a refinement | purification processing operation, it is good to measure the component composition of a non-purified carbon nanotube beforehand, for example by EPMA or TG-DTA analysis, and to determine the usage-amount of an organic acid and hydrogen peroxide.
[0011]
Although there is no restriction | limiting in particular about the method of making an unpurified carbon nanotube contact with the liquid mixture of an organic acid and hydrogen peroxide, and refine | purifying processing, For example, the method shown below is employable.
First, unpurified carbon nanotubes are added to an organic acid solution, about 5 to 30% by mass of hydrogen peroxide solution is added thereto, and the mixture is stirred for about 1 to 7 days at room temperature or without external heating. While doing the reaction. There is no restriction | limiting in particular about the addition rate of the said hydrogen peroxide solution. Moreover, as stirring, the method using the mechanical stirring, ultrasonic stirring, the flow in the liquid by the gas generated from hydrogen peroxide, the method of using these together, etc. can be used.
After the purification treatment is performed in this manner, the carbon nanotubes are separated into solid and liquid by known means such as filtration (natural filtration, suction filtration), decantation, and centrifugation. Next, the solid-liquid separated carbon nanotubes are sufficiently washed with water such as ion exchange water. This washing with water is usually performed using about 20 to 100 mL of water per 1 g of the solid-liquid separated carbon nanotubes until the pH of the washing water is close to 7. Thereafter, about 20 to 100 mL of lower alcohol such as methanol or ethanol is used for 1 g of the carbon nanotube, and the carbon nanotube is washed about 1 to 3 times. The washed carbon nanotubes are dried at normal temperature or reduced pressure (about 1.3 × 10 ⁻² Pa) at a temperature of room temperature to about 100 ° C. The drying treatment time is usually about 1 to 12 hours, although it depends on the properties of the object to be treated. There is no restriction | limiting in particular about a temperature increase rate.
In this way, high-purity carbon nanotubes can be efficiently purified from unpurified carbon nanotubes containing impurities such as amorphous carbon and metal components.
[0012]
【Example】
EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited at all by these examples.
Example 1
A flask was charged with 65 g (HCOOH: 1.27 mol) of a 90% by weight formic acid aqueous solution, and while stirring, 1 g of carbon nanotubes [manufactured by CarboLex, trade name “CarboLex-AP”] powder, which was a commercial unpurified product, was added thereto. added. Next, 150 mL of 15% by mass hydrogen peroxide prepared by adding 75 mL of ion exchange water to 75 mL of 30% by mass hydrogen peroxide (H ₂ O ₂ : about 0.66 mol) Was gradually added to the mixture and stirring was continued at room temperature. The metal component molar ratio in HCOOH / unpurified carbon nanotubes was 165.6, and the HCOOH / H ₂ O ₂ molar ratio was 1.92.
After stirring for 4 days, the stirring was stopped, the mixture was allowed to stand, and the supernatant was removed by decantation. Add 300 mL of ion exchange solution to the residue, apply to the centrifuge for about 10 minutes at 3,500 rpm, remove the supernatant by decantation, add 300 mL of ion exchange water to the remaining solid phase, and centrifuge again. It was. This operation was repeated, and when the pH of the supernatant liquid became close to 7, 200 mL of methanol was added to the remaining solid phase and centrifuged.
The methanol in the supernatant is removed by decantation, and the solid phase is taken out. This is dried in a vacuum dryer under conditions of 1.3 × 10 ⁻⁴ Pa and 27 ° C. for 5 hours. Obtained.
About this sample, while performing TG-DTA analysis and measuring a component composition, the specific surface area was measured by BET method. The results are shown in Table 1.
[0013]
Comparative Example 1
A flask was charged with 65 g of a 90% by mass aqueous formic acid solution and 130 mL of ion-exchanged water, mixed and stirred. To this, 1 g of carbon nanotube “CarboLex-AP” (supra) powder, which is a commercially available unrefined product, was added and stirred at room temperature. After stirring for 4 days, stirring was stopped and the mixture was allowed to stand. Thereafter, the same operation as in Example 1 was performed.
About the obtained sample, the TG-DTA analysis was performed like Example 1, and while measuring a component composition, the specific surface area was measured. The results are shown in Table 1.
[0014]
Comparative Example 2
To the flask, 75 mL of 30% by mass hydrogen peroxide solution and 75 mL of ion exchange water were added and stirred to prepare 150 mL of 15% by mass hydrogen peroxide solution. To this, 1 g of carbon nanotube “CarboLex-AP” (supra) powder, which is a commercially available unrefined product, was added and stirred at room temperature. After stirring for 4 days, stirring was stopped and the mixture was allowed to stand. Thereafter, the same operation as in Example 1 was performed.
About the obtained sample, the TG-DTA analysis was performed like Example 1, and while measuring a component composition, the specific surface area was measured. The results are shown in Table 1.
[0015]
Comparative Example 3
To the flask, 75 mL of 30% by mass hydrogen peroxide solution and 75 mL of ion exchange water were added and stirred to prepare 150 mL of 15% by mass hydrogen peroxide solution. To this, 1 g of carbon nanotube “CarboLex-AP” (supra) powder, which is a commercially available unrefined product, was added and stirred at room temperature.
After stirring for 4 days, the stirring was stopped, the mixture was allowed to stand, and the supernatant was removed by decantation. Add 300 mL of ion exchange solution to the residue, apply to the centrifuge for about 10 minutes at 3,500 rpm, remove the supernatant by decantation, add 300 mL of ion exchange water to the remaining solid phase, and centrifuge again. It was. This operation was repeated, and the washing with water was terminated when the pH of the supernatant liquid was close to 7.
Separately, 65 g of a 90% by mass aqueous formic acid solution and 130 mL of ion-exchanged water were charged into a flask, mixed and stirred. Next, the solid phase after the above water washing was added thereto and stirred at room temperature. After stirring for 4 days, stirring was stopped and the mixture was allowed to stand. Thereafter, the same operation as in Example 1 was performed.
About the obtained sample, the TG-DTA analysis was performed like Example 1, and while measuring a component composition, the specific surface area was measured. The results are shown in Table 1.
[0016]
[Table 1]

[0017]
From Table 1, in Example 1, the obtained purified sample has amorphous carbon and metal components removed, and the purity of the carbon nanotubes is greatly improved and the specific surface area is greatly increased as compared with the unpurified sample. I understand that Compared with the purified sample of Example 1, the purified sample of Comparative Example 1 does not sufficiently remove the metal component, and it can be seen that the purification process using only formic acid is inferior in the purity and specific surface area of the carbon nanotubes. Compared with the purified sample of Example 1, the purified sample of Comparative Example 2 does not sufficiently remove amorphous carbon and metal components, and the purification treatment using only hydrogen peroxide solution is inferior in the purity and specific surface area of carbon nanotubes. I understand. Compared with the purified sample of Example 1, the purified sample of Comparative Example 3 does not sufficiently remove amorphous carbon and metal components, and in the two-stage purification process in which the formic acid treatment is performed after the hydrogen peroxide solution treatment, It turns out that purity and a specific surface area are inferior.
That is, from the results of Comparative Examples 1 to 3, the effectiveness of the one-stage treatment (Example 1) in the presence of formic acid and hydrogen peroxide water can be confirmed.
[0018]
【The invention's effect】
According to the present invention, unpurified carbon nanotubes containing impurities such as amorphous carbon and metal components can be purified efficiently and safely by a simple process, without the energy associated with heating, starting with gas storage materials, It is possible to provide a method for industrially advantageously purifying carbon nanotubes that can be expected as an alternative carbon material in applications where conventional carbon materials are used.

Claims (3)

A method for purifying carbon nanotubes, comprising: bringing unpurified carbon nanotubes into contact with a mixed solution of an organic acid and hydrogen peroxide in a state of room temperature or without external heating, and performing purification in one step.   2. The organic acid is at least one selected from aliphatic monocarboxylic acids having 1 to 3 carbon atoms, halogen and / or hydroxyl group-substituted products thereof, and benzoic acid, halogen and / or hydroxyl group-substituted products thereof. Of carbon nanotube purification.   The method for purifying carbon nanotubes according to claim 2, wherein the organic acid is at least one selected from formic acid, acetic acid, monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, trifluoroacetic acid, benzoic acid, m-chlorobenzoic acid and salicylic acid. .