JP2010173874A - Method for obtaining carbon nanotube aggregate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for obtaining a CNT aggregate by efficiently aggregating carbon nanotubes (CNTs) by a simple treatment from a dispersion of the CNTs regardless of various methods for synthesizing CNTs. <P>SOLUTION: The method for obtaining a CNT aggregate without increasing the concentration of impurities includes adding an acid as a CNT flocculant to a CNT dispersion to thereby increase the hydrogen ion concentration of the dispersion. A CNT aggregate obtained by using the method for obtaining a CNT aggregate is also provided. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method of aggregating carbon nanotubes from a carbon nanotube dispersion, and more particularly to a method of aggregating carbon nanotubes from a carbon nanotube dispersion in which carbon nanotubes having functional groups are dispersed.

  Carbon nanotubes (hereinafter abbreviated as CNT) were first widely reported in 1991. A CNT has a shape formed by winding a single surface of graphite into a cylindrical shape. A single-walled CNT is a single-walled CNT and a multi-walled CNT is a multi-walled one. Among multi-walled CNTs, those wound in two layers are called double-walled CNTs. CNTs themselves have excellent intrinsic conductivity and are expected to be used as conductive materials.

  Known methods for producing CNTs include arc discharge, laser evaporation, and chemical vapor deposition. Among them, catalytic chemical vapor deposition is a method for inexpensively producing high-quality CNTs with few defects in the graphite layer. It has been known. In the catalytic chemical vapor deposition method, a method in which a catalyst is supported on a carrier is known.

  CNTs produced by these methods contain impurities such as amorphous carbon and catalysts. In order to obtain high-purity CNT by removing impurities from crude CNT containing impurities, for example, in Patent Document 1, after contacting oxygen at a high temperature, an operation of contacting an acidic aqueous solution is performed to obtain purified CNT. It is disclosed. Of these, a solid-liquid separation operation is required in the process of contacting the acidic aqueous solution. For example, after performing the solid-liquid separation operation by filtration, a part of the washing is performed with water to remove the acidic aqueous solution. CNT passes through the filtration membrane and moves to the filtrate side. It is economically important to collect dispersed CNTs present on the filtrate side because CNTs are generally expensive. In addition, it is not preferable to discard the filtrate in which CNTs are dispersed because disposal costs are much required and there is a concern about environmental impact.

  Until now, several methods for dispersing CNTs have been disclosed, such as those using ultrasonic treatment in an acidic solution (for example, Non-Patent Document 1) and those using a high-speed vibration grinding method (for example, Patent Document 2). Has been. However, only a method for recovering CNT from a CNT dispersion is reported in Patent Document 3 for CNT synthesized by an arc discharge method. This method is intended for CNT synthesized by an arc discharge method among various CNT synthesis methods, and the following two types of flocculants are disclosed. One is that the water-soluble organic solvent is a flocculant, and the flocculant to be added to the CNT dispersion is preferably 50 to 200% by volume, which requires a large amount of water-soluble organic solvent. Considering the amount of processing, it is not preferable. The other one uses a salt containing polyvalent cations as a flocculant and uses the complex formation of carboxyl groups introduced into CNTs and polyvalent cations. CNTs are separated and recovered from the CNT dispersion as aggregates. In some cases, CNT aggregates are necessarily recovered in the state where polyvalent cations are contained. As mentioned above, CNT is expected to be used as a conductive material. Therefore, in CNT aggregates containing polyvalent cations obtained in this way, polyvalent cations become impurities and affect the electrical properties, etc. of CNTs. Therefore, the range of use is limited. As described above, a method for recovering CNT is known only from a dispersion of CNT synthesized by a specific synthesis method. In addition, there is a great limitation on the cost for recovery and the use after recovery.

JP 2004-352605 A JP 2005-213108 A JP 2008-133178 A

J. Liu et al. Science 280, 1253-1256 (1998)

  An object of the present invention is to provide a method for efficiently aggregating CNTs from a dispersion of CNTs by a simple treatment regardless of the method for synthesizing CNTs.

As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that an aggregate can be obtained by adding an acid to the CNT dispersion, resulting in the present invention. That is, the present invention
(1) A method of obtaining an aggregate of carbon nanotubes, characterized by adding an acid to a dispersion in which carbon nanotubes are dispersed and setting the pH of the dispersion to 4.0 or less to aggregate the carbon nanotubes,
(2) A method of obtaining the carbon nanotube aggregate according to (1), wherein the pH of the dispersion is 3.0 or less,
(3) The method for obtaining the carbon nanotube aggregate according to (1), wherein the pH of the dispersion is 2.0 or less,
(4) The carbon nanotube aggregate according to any one of (1) to (3), wherein the acid contains at least one selected from nitric acid, sulfuric acid, hydrochloric acid, hydrobromic acid, trifluoroacetic acid, and phosphoric acid. Method,
(5) A method of obtaining the carbon nanotube aggregate according to any one of (1) to (4), wherein the carbon nanotube has a functional group,
(6) The method for obtaining the carbon nanotube aggregate according to (5), wherein the functional group includes a C—O bond or a C═O bond.

  The present invention also provides a CNT aggregate obtained using such a method for obtaining a CNT aggregate.

  According to the present invention, CNT dispersed in a liquid can be recovered by an easy method of adding an acid. Therefore, a complicated device for separation is not required, and the cost for the device can be saved. In addition, CNTs can be aggregated by adding acid to lower the pH without the need for a special flocculant, and can be easily recovered in combination with general-purpose separation operations. Further, since the CNT flocculant disclosed in the present invention is an acid, unlike the system using a polyvalent cation as the flocculant, the removal of the flocculant from the CNT aggregate is facilitated.

  Hereinafter, the present invention will be described in detail. The shape of the CNT contained in the CNT dispersion liquid used in the present invention is not particularly limited as long as it is dispersed in a solvent. In particular, a relatively short CNT having a relatively small diameter of a single layer to five layers and a length of 200 nm or less. It is particularly effective against. Moreover, there is no restriction | limiting about the manufacturing method of CNT, It can apply to the CNT manufactured by the chemical vapor deposition method, the laser evaporation method, the arc discharge method, or other manufacturing methods.

CNTs produced by any production method can be made into a CNT dispersion in order to increase the affinity for the solvent. In the production process, especially during the purification process, or in the functionalization process, a C—O bond such as a carboxyl group is used. Or a functional group containing a C═O bond is preferably introduced. The presence or absence of a functional group can be confirmed by X-ray photoelectron spectroscopy (XPS). For example, the peak of O1s is in the vicinity of 532 to 533 (eV) under the conditions of excitation X-ray: Monochromatic Al K _{1 and 2} , X-ray diameter: 1000 μm, photoemission angle: 90 ° (detector inclination with respect to the sample surface) This can be confirmed by detecting peaks of C—O groups and C═O groups. There is no restriction | limiting in the method to introduce | transduce a functional group, For example, it can select from a gaseous-phase oxidation process, a liquid phase oxidation process, or those combinations. The gas phase oxidation treatment is performed by contacting with oxygen under a temperature condition of 200 to 800 ° C., for example. The liquid phase oxidation treatment is performed by heating and refluxing in a liquid containing an oxidizing substance such as hydrofluoric acid or nitric acid. Although a difference appears in the dispersibility in the solvent and the agglomeration property after dispersion depending on the reflux time, the time required for heating and reflux is preferably 5 hours to 30 hours, and more preferably 10 hours to 15 hours. When combining a gas phase oxidation process and a liquid phase oxidation process, it is preferable to perform a liquid phase oxidation process after a gas phase oxidation process. In particular, from the viewpoint of performing uniform oxidation treatment and improving dispersibility in a solvent, it is preferable to include liquid phase oxidation treatment, and it is more preferable that the liquid phase oxidation treatment is performed last.

  The solid component is recovered from the liquid containing CNTs obtained by these operations by solid-liquid separation. The solid-liquid separation method is not particularly limited as long as it is a method capable of obtaining a composition containing carbon nanotubes as a solid part with high purity, and a known method such as filtration or centrifugation is used. Can do. Further, after adding water to the obtained solid portion, the same solid-liquid separation operation is performed to wash away the remaining oxidizing substance. Purified CNTs are obtained by repeated washing. There is no restriction on the number of washings, and it is sufficient that the pH of the filtrate obtained for each washing operation is constant. In this process, some CNTs are dispersed in the liquid, pass through the filtration membrane, and move to the filtrate side. The pH is measured at room temperature (20 ° C.) using a pH meter calibrated with a standard solution at pH 4 and pH 7, and judged by two significant digits.

  If the CNT concentration of the CNT dispersion is 0.01% by mass or more, aggregation can be sufficiently observed by acid addition. The CNT aggregate of the present invention refers to a black solid that can be separated by a membrane filter (1.0 μm, manufactured by Millipore).

  A CNT aggregate can be obtained by adding an acid to the filtrate in which the CNTs are dispersed and allowing to stand. Although there is no restriction | limiting about the kind of acid, It is preferable that it is a strong acid, For example, nitric acid, a sulfuric acid, hydrochloric acid, hydrobromic acid, a trifluoroacetic acid, phosphoric acid etc. are mentioned, By these acids, or those combinations It may be a mixed acid. By adding these acids, the pH of the filtrate in which the CNTs are dispersed is preferably 4 or less, more preferably 3 or less, and most preferably 2 or less. Moreover, it is preferable to stand after the acid addition, and the standing time is more preferably 6 hours or more, and most preferably 24 hours or more.

  The method of obtaining this CNT aggregate is not limited to the dispersion liquid containing CNT that has passed through the filtration membrane obtained in the above process and moved to the filtrate side, and is prepared by dispersing CNT in a dispersion medium. It can also be applied to CNT dispersions. Usually, this dispersion is used by adhering CNTs by contacting with a film or a glass substrate. However, the CNT dispersion after being used in such a manner has a reduced CNT content. It is difficult to use. However, by applying the present invention, it becomes possible to recover CNT from such a CNT dispersion. In addition, the dispersion liquid referred to here contains a dispersant for enhancing the dispersibility of the dispersion medium, and in some cases CNT. The dispersion medium of the dispersion is not limited, and can be selected from water, alcohol, ester, ketone, ether, hydrocarbon, aromatic compound, and aprotic polar solvent. Suitable alcohols include methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, t-butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol, amyl alcohol, benzyl alcohol, and the like. Methanol, ethanol and n-propanol are preferred. Esters include methyl formate, ethyl formate, propyl formate, butyl formate, isobutyl formate, pentyl formate, methyl acetate, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, isobutyl acetate, sec-butyl acetate, pentyl acetate, isopentyl acetate , 3-methoxybutyl acetate, sec-hexyl acetate, 2-ethylbutyl acetate, 2-ethylhexyl acetate, cyclohexyl acetate, benzyl acetate, methyl propionate, ethyl propionate, butyl propionate, isopentyl propionate, ethylene glycol monoacetate, diethylene glycol Monoacetate, monoacetin, diacetin, triacetin, monobutyrin, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, dibutyl carbonate, etc. are suitable Cage, in particular ethyl acetate, propyl acetate, butyl acetate is preferred. As the ketone, acetone, 2-butanone, 2-pentanone, 3-pentanone, 2-hexanone, cyclohexane, 2-methylcyclohexanone, acetophenone, 2-methylacetophenone, etc. are suitable, especially acetone, 2-butanone, cyclohexanone. Is preferred. Examples of ethers include dimethyl ether, ethyl methyl ether, diethyl ether, dipropyl ether, butyl methyl ether, t-butyl methyl ether, dibutyl ether, ethyl phenyl ether, diphenyl ether, tetrahydrofuran, 1,4-dioxane, diglyme, triglyme and tetraglyme. Etc., and dimethyl ether, ethyl methyl ether, diethyl ether, and tetrahydrofuran are particularly preferable. As the hydrocarbon, pentane, hexane, heptane, octane, nonane, decane, undecane, dodecane, cyclohexane, methylcyclohexane, methylcyclohexene, isoprene, etc. are suitable, especially heptane, octane, nonane, decane, undecane, dodecane, cyclohexane. Is preferred. As the aromatic compound, benzene, toluene, ethylbenzene, propylbenzene, o-xylene, m-xylene, p-xylene, o-ethyltoluene, m-ethyltoluene, p-ethyltoluene and the like are suitable, and benzene, toluene, o-Xylene, m-xylene, p-xylene and ethylbenzene are preferred. Examples of the aprotic polar solvent include dimethyl sulfoxide, N-methylpyrrolidone, N, N-dimethylformamide and the like. In particular, from the viewpoint of safety, hygiene, and price, water, ethanol, propanol, 2-butanone, N-methylpyrrolidone, N, N-dimethylformamide are preferable, and water is most preferable. These may use only 1 type and may mix 2 or more types.

  There is no particular limitation on the dispersant. For example, when water is used as the solvent, sodium dodecylbenzenesulfonate (NaDDBS), sodium dodecylsulfonate, Triton X-100 (GE Healthcare Biosciences), polyvinylpyrrolidone, and the like are preferable. These may be used alone or in combination of two or more. With this CNT dispersion, CNT aggregates can be obtained by the above-described method. The recovery method is not particularly limited, and a known method such as filtration or centrifugation can be used.

  EXAMPLES Hereinafter, although an Example demonstrates this invention, this invention is not limited to these.

[Reference Example 1]
(Support of metal salt on TS-1 type zeolite powder)
Add 0.01 g of ferrous acetate (Aldrich) and 0.21 g of cobalt acetate tetrahydrate (Nacalai Tesque) to 40 ml of ethanol (Nacalai Tesque) and suspend in an ultrasonic cleaner for 10 minutes. did. To this suspension, 2.0 g of TS-1 zeolite (manufactured by NE Chemcat, silicon / titanium ratio 50) was added, treated with an ultrasonic cleaner for 10 minutes, ethanol was removed at a constant temperature of 60 ° C., and TS A solid catalyst having a metal salt supported on -1 type zeolite powder was obtained.

(Synthesis of CNT-containing composition)
On a quartz wool at the center of a quartz tube having an inner diameter of 32 mm, 1.0 g of a solid catalyst prepared by loading a metal salt on the TS-1 type zeolite powder was taken, and argon gas was supplied at 600 cc / min. The quartz tube was installed in an electric furnace, and the center temperature was heated to 800 ° C. (temperature rising time 30 minutes). After reaching 800 ° C., after supplying ultra-high purity acetylene gas (manufactured by High Pressure Gas Industry) at 5 cc / min for 30 minutes, the supply of acetylene gas was stopped, the temperature was cooled to room temperature, and a composition containing CNTs was I took it out.

(Purification of composition containing CNT)
0.4 g of the composition containing CNT obtained by the synthesis of the composition containing CNT described above was heated to 400 ° C. (temperature raising time: 40 minutes) in the air atmosphere. After holding at 400 ° C. for 60 minutes, it was cooled to room temperature (temperature drop time 60 minutes). Furthermore, 0.4 g of this CNT-containing composition was put into 200 ml of hydrofluoric acid having a concentration of 5.0 mol / liter, and then stirred for 5 hours while maintaining at room temperature. Thereafter, the mixture was filtered using a filter paper (Toyo Filter Paper Co., Ltd., Filter Paper No. 125 mm) and separated into solid and liquid. The solid on the filter paper was washed with 500 ml of purified water, and the solid was further poured into 200 ml of hydrochloric acid having a concentration of 3.0 mol / liter, followed by stirring for 1 hour while maintaining at room temperature. Then, it filtered using the filter paper (Toyo Filter Paper Co., Ltd., Filter Paper No. 125mm), and solid-liquid-separated, and obtained CNT cake. The CNT cake was washed several times with 500 ml of purified water to obtain a filtrate in which some CNTs were dispersed. When the pH of the filtrate in which the CNTs were dispersed was measured with a pH meter (20 ° C., HM-30G, manufactured by Toa Denpa Kogyo Co., Ltd.), the pH was 5.0. Further, when a black solid obtained by concentrating and drying this CNT dispersion was analyzed by X-ray photoelectron spectroscopy (XPS), peaks of C—O groups and C═O groups were detected in the vicinity of 532 to 533 (eV).

[Reference Example 2]
(Support of metal salt on light magnesia)
5 g of ammonium iron citrate (manufactured by Wako Pure Chemical Industries, Ltd.) was dissolved in 250 mL of methanol (manufactured by Kanto Chemical Co., Inc.). To this solution, 50 g of light magnesia (manufactured by Wako Pure Chemical Industries, Ltd., bulk density was 0.16 g / mL) was added, treated with an ultrasonic cleaner for 60 minutes, and stirred at 40 ° C. to 60 ° C. Methanol was removed by drying to obtain a solid catalyst in which a metal salt was supported on light magnesia powder.

(Synthesis of CNT)
The reactor was synthesized with a cylindrical quartz tube having an inner diameter of 32 mm and a length of 120 mm. A quartz sintered plate is provided in the central part, an inert gas and source gas supply line is provided in the lower part of the quartz tube, and an exhaust gas line and a catalyst charging line are provided in the upper part. Furthermore, a heater surrounding the circumference of the reactor is provided so that the reactor can be maintained at an arbitrary temperature. The heater is provided with an inspection port so that the flow state in the apparatus can be confirmed. A catalyst was set on a quartz sintered plate by passing 12 g of the catalyst from a closed type catalyst feeder through a catalyst charging line. Subsequently, supply of argon gas from the gas supply line was started at 1000 mL / min. After the inside of the reactor was placed in an argon gas atmosphere, the temperature was heated to 850 ° C. (temperature rising time 30 minutes). After reaching 850 ° C., the temperature was maintained, the argon flow rate in the gas supply line was increased to 2000 mL / min, and fluidization of the solid catalyst on the quartz sintered plate was started. After confirming fluidization from the heating furnace inspection port, methane was further supplied to the reactor at 95 mL / min (methane concentration 4.5 vol%. After supplying the mixed gas for 30 minutes, switching to the flow of only argon gas was performed. The synthesis was terminated.

  The heating was stopped and the mixture was allowed to stand at room temperature. After reaching room temperature, the composition containing the catalyst and CNTs was taken out from the reactor.

(Calcination and purification treatment of CNT-containing composition)
The CNT composition was calcined in air at 400 ° C. for 1 hour, then added to 6N hydrochloric acid, and stirred for 2 hours in an 80 ° C. water bath. The recovered product obtained by filtration was further added to 6N hydrochloric acid and stirred in a water bath at 80 ° C. for 1 hour. This was filtered, washed with purified water several times, and then the filtrate was dried in an oven at 120 ° C. overnight to remove magnesia and metals, and CNT was purified. When this CNT was analyzed by X-ray photoelectron spectroscopy (XPS), peaks of C—O groups and C═O groups were detected in the vicinity of 532 to 533 (eV).

(CNT dispersion preparation)
Weigh 10 mg of the above CNT, 30 mg of poly (3,4-ethylenedioxythiophene) poly (styrenesulfonic acid) (PEDOT / PSS) (Starck) into a 50 mL container, add 10 mL of distilled water, and output with an ultrasonic homogenizer. Dispersion treatment was carried out under ice cooling at 25 W for 20 minutes to prepare a CNT composition liquid. Aggregates were not visually confirmed in the prepared liquid, and CNTs were well dispersed. The obtained liquid was centrifuged at 20000 G for 15 minutes using a high-speed centrifuge, and the supernatant was stored in a 50 ml sample tube. The precipitated CNTs were dried and then calcined at 400 ° C. for 1 hour and measured for weight. As a result, the amount of precipitated CNTs was 12% by mass of the CNTs contained in the entire liquid. The above dispersion was impregnated with a polyethylene terephthalate (PET) film (Toray Co., Ltd. (Lumirror U36) and supported with CNTs. The CNT content of the remaining CNT dispersion was calculated by the same operation as described above. 1% by mass, resulting in a dilute dispersion.

<Example 1>
Nitric acid (manufactured by Wako Pure Chemical Industries, Ltd.) was added to 25 mL of the filtrate in which the CNTs shown in Reference Example 1 were dispersed. The pH of the filtrate in which the CNTs were dispersed was measured with a pH meter (20 ° C., HM-30G, manufactured by Toa Denpa Kogyo Co., Ltd.) and found to be 1.6. When this was allowed to stand at room temperature for one day in the dark, precipitation of black material due to CNT aggregates was observed. The black material obtained as described above was filtered off with a membrane filter (1.0 μm, manufactured by Millipore) and dried in a ventilated dryer at 120 ° C. for 2 hours to obtain a black powder. When this black powder was observed with a scanning electron microscope (acceleration voltage 5.0 kV, 15000 times, JSM-6700F, manufactured by JEOL Ltd.), CNTs were observed.

<Comparative Example 1>
Nitric acid (manufactured by Wako Pure Chemical Industries, Ltd.) was added to 25 mL of the filtrate in which the CNTs shown in Reference Example 1 were dispersed to adjust the pH to 4.2. Even when this was allowed to stand at room temperature in the dark for one day, no CNT aggregates were observed.

<Examples 2 to 6>
In the same manner as in Example 1, sulfuric acid, hydrochloric acid, hydrobromic acid, trifluoroacetic acid (TFA), and phosphoric acid were used instead of nitric acid. The results are shown in Table 1.

<Example 3>
An appropriate amount of nitric acid (manufactured by Wako Pure Chemical Industries, Ltd.) was added to 25 mL of the CNT dispersion liquid shown in Reference Example 2 to adjust the pH to 1.6. When this was allowed to stand at room temperature for one day in the dark, precipitation of black material due to CNT aggregates was observed. This black substance was filtered off with a membrane filter (1.0 μm, manufactured by Millipore) and dried in a ventilated dryer at 120 ° C. for 2 hours to obtain a black powder. When this black powder was observed with a scanning electron microscope (acceleration voltage 5.0 kV, 15000 times, JSM-6700F, manufactured by JEOL Ltd.), CNTs were observed.
<Comparative example 2>
An appropriate amount of nitric acid was added to 25 mL of the CNT dispersion to adjust the pH to 4.2. Even when this was allowed to stand at room temperature in the dark for one day, no CNT aggregates were observed.

  According to the present invention, expensive CNTs can be recovered and reused from CNTs dispersed on the filtrate side obtained in the filtration and washing process in the CNT manufacturing process or from diluted CNT dispersions. In addition, it is preferable to collect CNT from a solution containing CNT from the viewpoint of reducing industrial waste.

Claims (6)

A method of obtaining an aggregate of carbon nanotubes, characterized by adding an acid to a dispersion in which carbon nanotubes are dispersed, and adjusting the pH of the dispersion to 4.0 or less to aggregate the carbon nanotubes. The method of obtaining the carbon nanotube aggregate of Claim 1 which makes pH of a dispersion liquid 3.0 or less. The method of obtaining the carbon nanotube aggregate of Claim 1 which makes pH of a dispersion liquid 2.0 or less. The method for obtaining the carbon nanotube aggregate according to any one of claims 1 to 3, wherein the acid contains at least one selected from nitric acid, sulfuric acid, hydrochloric acid, hydrobromic acid, trifluoroacetic acid, and phosphoric acid. The method of obtaining the carbon nanotube aggregate in any one of Claim 1 to 4 in which a carbon nanotube has a functional group. The method for obtaining the carbon nanotube aggregate according to claim 5, wherein the functional group contains a C—O bond or a C═O bond.