Method for purifying carbon nanotubes

Abstract

The present invention relates to a method for purifying carbon nanotubes, capable of obtaining high-purity purified carbon nanotubes in a high yield by heating crude carbon nanotubes containing a high content of metal oxides as impurities under a gas atmosphere and simultaneously supplying a halogen material and an oxygen removing material to the heated crude carbon nanotubes to induce a reaction.

Description

Method for purifying carbon nanotubes

[0001] Cross-citation with related applications

[0002] This application claims the benefit of priority from Korean Patent Application No. 10-2024-0087294, dated July 3, 2024, the entire contents of which are incorporated herein by reference.

[0003] Technology field

[0004] The present application relates to a method for purifying carbon nanotubes, and more specifically, to a method for purifying carbon nanotubes for obtaining high-purity carbon nanotubes with a high yield by purifying unpurified carbon nanotubes containing a high content of metal oxides as impurities.

[0005] When manufacturing a specific grade of carbon nanotube (CNT), metal oxides such as aluminum oxide (Al2O3) and magnesium oxide (MgO) are used as catalyst supports. However, the boiling point of the metal oxide is about 200℃ or higher than the reaction temperature during carbon nanotube manufacturing, so a large amount of metal oxide remains as impurities in the manufactured carbon nanotubes. Metal impurities such as metal oxides lower the conductivity of carbon nanotubes or accelerate the decomposition of organic solvents, so they need to be removed.

[0006] In this way, in order to remove metal component impurities contained in carbon nanotubes synthesized with low purity at more than 50 wt%, a method has been used in the past to vaporize the impurities by applying a high vacuum at an ultra-high temperature higher than the boiling point of the metal component impurities, to dissolve the metal component impurities in a liquid phase using a heated strong acid such as nitric acid or hydrochloric acid, or to react a halogen-containing gas with a metal oxide to convert it into a halogenated metal having a low boiling point, thereby vaporizing the halogenated metal component impurities under relatively mild conditions.

[0007] Among these, the method of vaporizing after conversion to a halogenated metal has the advantage of being advantageous for commercialization in that it can lower the purification temperature and does not require separate washing or drying. However, when purifying CNTs containing a large amount of metal impurities such as aluminum oxide or magnesium oxide used as a catalyst support, the oxygen generated by the substitution of the metal oxide and halogen reacts with the CNTs and causes combustion, which rapidly reduces the CNT yield. In particular, if the metal oxide is present in an excess amount compared to the CNTs in the sample before purification, a large amount of oxygen may be generated during the purification process, which may cause the CNTs to completely burn. Therefore, when amorphous carbon is additionally added to prevent CNT combustion, there is still the problem of reducing the CNT yield due to the low oxygen selectivity of the amorphous carbon compared to the CNTs.

[0008] Meanwhile, methods have been proposed to prevent CNT combustion during the refining process, using carbon-containing halogenated hydrocarbons or halogenated carbons, or mixing halogen gases with other hydrocarbons. However, these methods pose the problem of forming solid, incompletely combusted compounds, such as coke, which can clog exhaust pipes and generate air pollutants.

[0009] Accordingly, there is a need to develop a carbon nanotube purification process that can purify low-purity, unrefined carbon nanotubes containing a large amount of metal impurities into high-purity and high-yield carbon nanotubes without generating incomplete combustion compounds or air pollutants.

[0010] The problem to be solved in the present disclosure is to improve the yield and purity of purified carbon nanotubes by minimizing the consumption of carbon nanotubes during purification of crude carbon nanotubes containing a high content of metal oxide as an impurity, in order to solve the problem mentioned in the background technology of the above invention.

[0011] In addition, the purpose is to provide a carbon nanotube purification method that prevents the formation of solid carbides in exhaust gas generated during the purification process, thereby preventing the problem of exhaust port clogging.

[0012] However, the problems that the present invention seeks to solve are not limited to the problems mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the description below.

[0013] According to one embodiment of the present disclosure for solving the above problem, a method for purifying carbon nanotubes is provided, comprising the steps of heating an unpurified carbon nanotube containing impurities under an inert gas atmosphere; and simultaneously supplying a halogen substance and an oxygen removing substance to the heated unpurified carbon nanotube to react, thereby obtaining a purified carbon nanotube.

[0014] Additionally, the unrefined carbon nanotube may contain 50 to 95 wt% of the metal oxide as an impurity based on the total weight of the unrefined carbon nanotube.

[0015] According to the method for purifying carbon nanotubes of the present disclosure, the purity of CNTs can be improved, thereby improving product quality.

[0016] In addition, by using halogen substances and oxygen removing substances during the purification of low-purity synthesized crude carbon nanotubes, CNT loss can be minimized, thereby increasing product productivity and reducing costs.

[0017] In addition, by suppressing the generation of carbon compounds during the CNT purification process, clogging can be prevented, ensuring process stability and preventing the emission of pollutants.

[0018] The effects that can be obtained from this invention are not limited to the effects mentioned above, and other effects not mentioned can be clearly understood by a person having ordinary skill in the art to which the present disclosure pertains from the description below.

[0019] Figure 1 is a graph comparing the impurity removal rate and CNT residue rate during purification by a method according to an example and comparative example of the present disclosure.

[0020] Figure 2 is a graph comparing the CNT residual ratio in cases where the impurity removal rate is 95% or higher during purification by a method according to an embodiment and comparative example of the present disclosure.

[0021] The terms or words used in the description and claims of this disclosure should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted as meanings and concepts that conform to the technical idea of ​​this disclosure, based on the principle that the inventor can appropriately define the concept of the term to best explain his or her own invention.

[0022] The singular form of a noun corresponding to an item may include one or more of said items, unless the relevant context clearly indicates otherwise.

[0023] In this disclosure, each of the phrases "A or B", "at least one of A and B", "at least one of A or B", "A, B, or C", "at least one of A, B, and C", and "at least one of A, B, or C" may include any one of the items listed together in the corresponding phrase, or all possible combinations thereof.

[0024] The term "and / or" includes any combination of a plurality of related described elements or any one of a plurality of related described elements.

[0025] Terms such as "first," "second," or "first" or "second" may be used simply to distinguish one component from another and do not qualify the components in any other respect (e.g., importance or order).

[0026] Terms such as "include" or "have" are intended to specify the presence of a feature, number, step, operation, component, part or combination thereof described in the present disclosure, but do not preclude the presence or addition of one or more other features, numbers, steps, operations, components, parts or combinations thereof.

[0027] When we say that a component is "on" another component, this includes not only cases where the component is in contact with the other component, but also cases where there is another component between the two components.

[0028] In this disclosure, when a part is said to “include” a certain component, this does not mean that other components are excluded, but rather that other components may be included, unless specifically stated otherwise.

[0029] In the present disclosure, the term “combination thereof” included in the expression in the Makushi format means one or more mixtures or combinations selected from the group consisting of the components described in the expression in the Makushi format, and means including one or more selected from the group consisting of the components.

[0030] In addition, the terms "about", "substantially", etc. used in this disclosure are used in the sense of or close to the numerical value when manufacturing and material tolerances inherent to the meanings mentioned are presented, and are used to prevent unscrupulous infringers from unfairly using the disclosure contents in which exact or absolute numerical values ​​are mentioned to aid understanding of this disclosure.

[0031] The term “fluid” as used in this disclosure may include one or more components of gas, liquid, and solid.

[0032] Additionally, “pressure” as referred to in the present disclosure means gauge pressure measured based on atmospheric pressure.

[0033] A method for purifying carbon nanotubes according to the present disclosure includes a heating process of unpurified carbon nanotubes and a purification process using a halogen substance and an oxygen removing substance.

[0034] According to one embodiment of the present disclosure, crude carbon nanotubes containing impurities are heated under an inert gas atmosphere. Specifically, the crude carbon nanotubes may be filled into a purification furnace filled with an inert gas and then heated.

[0035] The crude carbon nanotubes according to the present disclosure may be manufactured by growing carbon nanotubes through decomposition of a carbon source using a catalyst support, and the metal catalyst supported on the catalyst support may be a material that promotes the growth of carbon nanotubes. The supported catalyst may primarily be alumina or magnesium oxide supported with Fe, Co, Ni, Mo, or the like.

[0036] The above-mentioned crude carbon nanotubes may be carbon nanotubes synthesized with low purity, containing a catalyst support such as a metal oxide used in the synthesis of CNTs as an impurity. The low-purity carbon nanotubes may mean that the content of the metal oxide as an impurity is greater than that of the CNTs. For example, the crude carbon nanotubes may contain 50 wt% or more of the metal oxide as an impurity based on the total weight of the crude carbon nanotubes. For example, the content of the metal oxide based on the total weight of the crude carbon nanotubes may be 50 wt% to 95 wt%, 60 wt% to 95 wt%, 70 wt% to 95 wt%, or 80 wt% to 95 wt%.

[0037] The method for purifying carbon nanotubes according to the present disclosure may be particularly effective for low-purity, unpurified carbon nanotubes containing 50 wt% or more of metal oxides as impurities. In the present disclosure, low-purity carbon nanotubes and / or unpurified carbon nanotubes may refer to carbon nanotubes having a CNT purity of less than 50%, preferably less than 30%, and most preferably less than 15%.

[0038] According to one embodiment, the metal oxide may include a metal selected from the group consisting of Al2O3, MgO, Fe2O3, Fe3O4, Co3O4, and combinations thereof, and preferably includes Al2O3 and / or MgO.

[0039] According to one embodiment, the pressure of the inert gas may be supplied at a range of 500 torr to 1,000 torr, for example, at a pressure of 600 torr to 900 torr, or 700 torr to 800 torr.

[0040] According to one embodiment, the inert gas may include, for example, a gas containing nitrogen, helium, neon, argon, krypton, xenon, radon, or a mixture thereof, and nitrogen gas may be specifically used. Such inert gases are chemically very stable and have the property of not giving, receiving, or sharing electrons, and thus can play a role in allowing CNTs to flow and move due to the introduction of the gas without reacting with the CNTs.

[0041] According to one embodiment, the step of heating the crude carbon nanotubes may include heating to a temperature of 600°C to 1,500°C, preferably 700°C to 1,500°C, and more preferably 800°C to 1,500°C. When the above temperature range is satisfied, energy suitable for performing a reaction for removing impurities in the purification step described below can be supplied. For example, when the temperature range is exceeded, damage to the CNTs due to a rapid halogenation reaction may rather be accelerated, and when the temperature range is lower than the above temperature range, the reaction rate may be slow or the reaction may not occur.

[0042] In addition, the heating step may be performed by heating at a rate of 10°C / min to 50°C / min, preferably 10°C / min to 40°C / min, and most preferably 10°C / min to 30°C / min under an inert gas atmosphere. If the heating rate in the heating step is too slow, there may be a problem that the purification time becomes long. On the other hand, if the heating rate is too fast, the oxidation temperature may be reached before moisture or oxygen in the CNT sample is completely removed, which may damage the CNT, and furthermore, it may cause a thermal shock to the purification furnace (graphite, quartz, etc.) in which the purification process is performed, which may cause problems in terms of equipment maintenance.

[0043] Next, purified carbon nanotubes can be obtained by simultaneously supplying a halogen substance and an oxygen removing substance to the heated, unpurified carbon nanotubes and causing a reaction. Specifically, the halogen substance and the oxygen removing substance can be simultaneously supplied into the interior of a purification furnace in which the heated, unpurified carbon nanotubes exist. By simultaneously supplying the halogen substance and the oxygen removing substance, the metal oxide corresponding to the impurity can be modified into a halide metal, and at the same time, the oxygen that causes CNT loss can be removed.

[0044] Modifying metal oxides with metal halides has the advantage of lowering their boiling points, allowing for the vaporization and removal of halogenated metal impurities under relatively mild conditions. Therefore, conventionally, impurities in crude carbon nanotubes are typically removed by substituting metal oxides with halogens, followed by vaporization, as illustrated in Scheme 1 below.

[0045] Reaction Scheme 1: Unpurified CNT (CNT + metal oxide) + halogen substance → CNT + halogenated metal (g) + O2 (g)

[0046] At this time, as in the above reaction scheme 1, oxygen is generated when the metal oxide is replaced with a halogen substance. However, when crude CNTs with a low metal oxide content are purified using only a halogen substance, the effect of oxygen is minimal. However, when crude carbon nanotubes containing 50 wt% or more of metal oxide are purified, a large amount of oxygen is generated, and under the high-temperature conditions during the purification process, the oxygen reacts with the CNTs and causes combustion. In particular, when the metal oxide content is 85% or more, the CNTs may be completely consumed using conventional purification methods.

[0047] Accordingly, in a CNT purification method according to one embodiment of the present disclosure, in order to remove oxygen generated by the reaction between a metal oxide and a halogen substance as in the above reaction formula 1 when purifying a crude CNT containing a large amount of metal oxide, an oxygen removing substance is supplied to the crude CNT at the same time as the halogen substance is supplied during the purification process, thereby preventing CNTs from being consumed and thereby improving the yield of CNTs.

[0048] The halogen substance may be a gas or liquid containing fluorine, chlorine, bromine, iodine, or a mixture thereof. For example, the halogen substance may include F2, Cl2, Br2, I2, HF, HCl, Hbr, HI, CHCl3, and CCl4, and chlorine gas (Cl2) is preferred.

[0049] When the halogen substance is a gas at room temperature, the flow rate of the gaseous halogen substance supplied to the crude carbon nanotubes may be 0.1 to 10 times, preferably 0.5 to 5 times, the internal volume of the purification per minute (min). If the halogen substance supply rate is too slow, the time required for impurities to be removed from the CNTs becomes longer, and impurities may accumulate in the exhaust line, causing pipe blockage. If the halogen substance supply rate is too fast, the consumption of the halogen substance increases without improving the purification efficiency, which is not desirable.

[0050] Meanwhile, when the halogen substance is a liquid at room temperature, the flow rate of the liquid halogen substance supplied to the unrefined carbon nanotubes may be 0.0001 to 0.01 times, preferably 0.0005 to 0.005 times, the internal volume of the purifier per minute. When the flow rate is satisfied, there is an advantage in that there is no safety issue due to temperature drop or explosion within the purifier while supplying an appropriate amount of the halogen substance.

[0051] The above oxygen-removing material is a material that does not generate incomplete combustion compounds or air pollutants when reacted with oxygen, and may be a liquid or gas at room temperature. However, when the oxygen-removing material is supplied to the crude carbon nanotubes, it is preferably in a gaseous state.

[0052] The oxygen-removing substance that is liquid at room temperature is a substance having a boiling point of 400°C or lower, preferably 50°C to 400°C, and examples thereof include acetaldehyde and formic acid. The oxygen-removing substance that is liquid at room temperature can be supplied in a vaporized state to a purification process performed at a temperature of 600°C or higher to react with oxygen.

[0053] Examples of oxygen scavenging substances that are gases at room temperature include carbon monoxide, formaldehyde, hydrogen sulfide, and sulfur dioxide, with carbon monoxide being most preferred. For example, when the oxygen scavenging substance is carbon monoxide (CO), if carbon monoxide is supplied simultaneously with the supply of a halogen substance, oxygen (O2) generated by the metal oxide reforming reaction represented by Reaction Scheme 1 reacts with carbon monoxide (CO), as shown in Reaction Scheme 2 below, thereby preventing damage to CNTs.

[0054] Reaction equation 2: O2(g) + 2CO (g) → 2CO2(g)

[0055] According to one embodiment, the process may further include a step of generating the oxygen removal agent. For example, among the oxygen removal agents, carbon monoxide may be generated by mixing sulfuric acid and formic acid in a 1:1 ratio, heating the mixture, and then introducing the resulting carbon monoxide into the purification process.

[0056] In one embodiment, when both the halogen material and the oxygen removing material are supplied in a gaseous state, the ratio of the supply flow rates of the halogen material and the oxygen removing material may be 9:1 to 1:9 based on volume, for example, 9:1 to 3:7, 7:3 to 4:6, or 6:4 to 4:6. Here, when the above ratio range is satisfied, there is an advantage in that the amount of the halogen material can be maximized while the oxygen removing material participates in the reaction as much as possible.

[0057] The reaction of the above-described crude carbon nanotubes, halogenated substance, and oxygen-removing substance may be performed for 10 minutes, 15 minutes, 30 minutes, 45 minutes, or 60 minutes or more and 120 minutes, 150 minutes, or 200 minutes or less. If the reaction time (i.e., purification time) is too short, impurities may not be sufficiently removed, thereby lowering the purity of the CNT, and thus deteriorating the physical properties of the CNT. In addition, if the reaction (purification) time is too long, the CNT residue rate may be lowered, making it impossible to secure the minimum CNT yield. In addition, the CNT purification method according to the present disclosure can secure an excellent impurity removal rate and CNT residue rate without performing a separate reduction reaction to increase the CNT yield before the halogenation reaction for the metal oxide contained in the crude CNT, while relatively shortening the total time required for the entire purification process.

[0058] According to one embodiment of the present disclosure, a step of heat-treating the purified carbon nanotubes to remove residual impurities may be additionally performed, wherein the residual impurities may include amorphous carbon. For example, the heat treatment to remove the amorphous carbon may be performed at 400°C to 500°C. If the heat treatment temperature is too low, the amorphous carbon may not be oxidized, and if the heat treatment temperature is too high, the CNTs may be oxidized and damaged.

[0059] Meanwhile, the impurity removal rate of the purified carbon nanotube obtained in the method for purifying carbon nanotubes according to the present disclosure may be 80 wt% or more, preferably 90 wt% or more, and more preferably 95 wt% or more, based on the total weight of impurities in the unpurified CNT. The higher the impurity removal rate, the higher the purity of the purified CNT. Since the higher the purity of CNT, the better the physical properties such as conductivity, it is necessary to secure the impurity removal rate so that the desired physical properties can be achieved.

[0060] In addition, the residue ratio of the purified carbon nanotubes may be 10 wt% or more, preferably 15 wt% or more, and more preferably 20 wt% or more, based on the total weight of CNTs in the crude CNTs. A higher CNT residue ratio means a higher yield of purified CNTs, and a lower CNT residue ratio means a lower yield of the final product obtained.

[0061] Above, the method for purifying carbon nanotubes according to the present disclosure has been described and illustrated in the drawings, but the description and illustration in the drawings describe and illustrate only the core components for understanding the present disclosure, and in addition to the processes and devices described and illustrated in the drawings, processes and devices not described and illustrated separately can be appropriately applied and utilized to carry out the method for purifying carbon nanotubes according to the present disclosure.

[0062] Hereinafter, the present disclosure will be described in more detail through examples. However, the following examples are intended to further illustrate the present disclosure, and the scope of the present disclosure is not limited by the following examples.

[0063] [Example]

[0064] Examples 1-1 to 1-3

[0065] About 10 g of synthetic CNT (purity approximately 10%) containing impurity Al2O3 was introduced into a purification furnace, purged with N2 as an inert gas for 5 minutes, and then heated to 900°C at 10°C / min.

[0066] Next, 100 mL of concentrated sulfuric acid (95%) and 100 mL of formic acid were mixed and heated to 80°C, and the generated carbon monoxide (CO) gas was continuously supplied to the purification furnace. In addition, while supplying the carbon monoxide gas, chlorine (Cl2) gas was supplied to the purification furnace simultaneously, and CNT purification was performed for 15 minutes, 30 minutes, and 60 minutes, respectively. At this time, chlorine gas was supplied to the purification furnace at 90% by volume, and carbon monoxide gas was supplied at 10% by volume.

[0067] Examples 2-1 to 2-3

[0068] The same process as in Examples 1-1 to 1-3 was performed, but chlorine gas was supplied to the purifier at 70% by volume, and carbon monoxide gas was supplied at 30% by volume.

[0069] Examples 3-1 to 3-3

[0070] The same process as in Examples 1-1 to 1-3 was performed, but chlorine gas was supplied to the purifier at 50% by volume, and carbon monoxide gas was supplied at 50% by volume.

[0071] Comparative Examples 1-1 to 1-3

[0072] About 10 g of synthetic CNT (purity approximately 10%) containing impurity Al2O3 was introduced into a purification furnace, purged with N2 as an inert gas for 5 minutes, and then heated to 900°C at 10°C / min.

[0073] Next, CNT purification was performed for 15 minutes, 30 minutes, and 60 minutes, respectively, while supplying only chlorine gas to the purification furnace.

[0074] Comparative Examples 2-1 to 2-3

[0075] About 10 g of synthetic CNT (purity approximately 10%) containing impurity Al2O3 was introduced into a purification furnace, purged with N2 as an inert gas for 5 minutes, and then heated to 900°C at 10°C / min.

[0076] Next, 100% by volume of carbon monoxide gas was first supplied as a reducing agent to the purification furnace, and a reduction reaction was performed for 15 minutes, 30 minutes, and 60 minutes, respectively. Then, 100% by volume of chlorine gas was sequentially supplied, and CNT purification was performed for 15 minutes, 30 minutes, and 60 minutes, respectively. At this time, the purification process was performed for a total of 30 minutes, 60 minutes, and 120 minutes.

[0077] Experimental example

[0078] Table 1 below describes the purification process conditions and the impurity removal rate (%) and CNT residual rate (%) of the purified CNTs measured according to the above examples and comparative examples, and the results are shown in Fig. 1.

[0079] Here, the impurity removal rate and CNT residue rate were calculated using the following equations (1) and (2), respectively.

[0080] Equation (1): Impurity removal rate (%) = (mass of impurities removed after purification / mass of impurities before purification) * 100

[0081] Equation (2): CNT residue (%) = (CNT mass after purification / CNT mass before purification) * 100

[0082] Specifically, the upper limit of the impurity removal rate according to the above formula (1) and the CNT residual rate according to the above formula (2) are each 100%. For example, a higher impurity removal rate means that more impurities are removed, which means a higher CNT purity, and a higher CNT residual rate means that a lower amount of CNT is consumed during the purification process, which means a higher CNT yield.

[0083] Purification process conditionsImpurity removal rate(%)CNT residual rate(%)Feed material compositionProcess time (min)Example 1-1Cl2 90% : CO 10%1544.439.2Example 1-2Cl2 90% : CO 10%3089.022.6Example 1-3Cl2 90% : CO 10%6098.011.9Example 2-1Cl2 70% : CO 30%1534.048.0Example 2-2Cl2 70% : CO 30%3068.639.0Example 2-3Cl2 70% : CO 30%6010023.8Example 3-1Cl2 50% : CO 50%1523.351.4Example 3-2Cl2 50% : CO 50% 3048.149.6 Example 3-3 Cl2 50% : CO 50% 6098.236.9 Comparative Example 1-1 Cl2 100% 1548.87.2 Comparative Example 1-2 Cl2 100% 3099.60 Comparative Example 1-3 Cl2 100% 6099.50 Comparative Example 2-1 CO 100% → Cl2 100% 15 → 15 (30) 48.117.6 Comparative Example 2-2 CO 100% → Cl2 100% 30 → 30 (60) 99.92.9 Comparative Example 2-3 CO 100% → Cl2 100% 60 → 60 (120) 99.31.1

[0084] Referring to Table 1 and Fig. 1 above, it was confirmed that Examples 1-1 to 3-3, in which the purification process was performed by simultaneously injecting CO gas and Cl2 gas when the purification time was the same, had significantly superior CNT retention rates compared to each comparative example with the same purification time. In addition, it was confirmed that the CNT retention rate tended to improve as the fraction of CO gas increased, but it was confirmed that when the fraction of CO gas increased, a longer purification time was required to improve the impurity removal rate.

[0085] In addition, in Comparative Examples 1-1 to 1-3, where the purification process was performed using only Cl2, the impurity removal rate tended to be excellent, but the CNT residual rate was less than 7.2%, resulting in excessive CNT loss. In particular, in Comparative Examples 1-2 and 1-3, it was confirmed that all CNTs were lost. In other words, it is judged that injecting CO after CNT purification with chlorine gas is meaningless due to the problem of CNT loss.

[0086] Meanwhile, Fig. 2 compares the CNT residue rates of experimental groups with impurity removal rates of 95% or higher, i.e., Examples 1-3, 2-3, 3-3 and Comparative Examples 1-2, 1-3, 2-2, 2-3.

[0087] Referring to FIG. 2, in the case of Examples 1-3, 2-3 and 3-3 according to the present disclosure, a high CNT purity was secured with an impurity removal rate of 95% or higher, while CNT residue rates of 11.9%, 23.8% and 36.9% were shown, respectively.

[0088] On the other hand, in Comparative Examples 2-2 and 1-3, which did not use CO gas, the CNT residue rate was 0%, indicating that all CNTs were consumed during the purification process. This is because a large amount of oxygen was generated during the purification process, causing all CNTs to be burned.

[0089] Meanwhile, in the case of Comparative Examples 2-2 and 2-3, where CO gas was first introduced as a reducing agent and then Cl2 gas was introduced, it was confirmed that the CNT yield was significantly low, with the CNT residue rate being less than 2.9%.

[0090] As in the examples according to the present disclosure, the reaction in which CO and Cl2 simultaneously convert Al2O3 to AlCl3 [Al2O3(s) + 3CO(g) + 3Cl2→ 2AlCl3(s) + 3CO2(g)] is a spontaneous reaction (△G <0), whereas as in Comparative Example 2, when CO gas is introduced first, the reaction in which CO reduces Al2O3 to Al [Al2O3(s) + 3CO(g) → 2Al(s) + 3CO2(g)] is a non-spontaneous reaction (△G>0), and since reduction of metal oxides is difficult, it is believed that the metal oxide reforming reaction is performed only when Cl2 gas is introduced, resulting in CNT loss due to oxygen. However, it was confirmed that in Comparative Example 2, the CNT residue rate was higher than in Comparative Example 1 due to the influence of CO present in the purification furnace at the initial stage of Cl2 gas introduction.

[0091] Although exemplary embodiments of the present disclosure have been described above, the present disclosure is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and spirit of the claims set forth below.

Claims

1. A step of heating an unrefined carbon nanotube containing impurities under an inert gas atmosphere; and A step of obtaining purified carbon nanotubes by simultaneously supplying a halogen substance and an oxygen removing substance to the heated unrefined carbon nanotubes and causing a reaction; A method for purifying carbon nanotubes, comprising 50 to 95 wt% of metal oxide as impurities based on the total weight of the unrefined carbon nanotubes.

2. In paragraph 1, A method for purifying carbon nanotubes, wherein the oxygen removing material comprises a substance selected from the group consisting of acetaldehyde, formic acid, carbon monoxide, formaldehyde, hydrogen sulfide, sulfur dioxide, and combinations thereof.

3. In paragraph 1, A method for purifying carbon nanotubes, wherein the halogen material is selected from the group consisting of F2, Cl2, Br2, I2, HF, HCl, Hbr, HI, CHCl3CCl4, and combinations thereof.

4. In paragraph 1, A method for purifying carbon nanotubes, wherein the metal oxide is selected from the group consisting of Al2O3, MgO, Fe2O3, Fe3O4, Co3O4, and combinations thereof.

5. In paragraph 1, The above method for purifying carbon nanotubes is performed in a purification furnace, A method for purifying carbon nanotubes, wherein, when the halogen substance is a gas at room temperature, the supply flow rate of the halogen substance is 0.1 to 10 times the internal volume of the purifier per minute.

6. In paragraph 1, The above method for purifying carbon nanotubes is performed in a purification furnace, A method for purifying carbon nanotubes, wherein when the halogen substance is a liquid at room temperature, the supply flow rate of the halogen substance is 0.0001 to 0.01 times the internal volume of the purifier per minute.

7. In paragraph 1, A method for purifying carbon nanotubes, wherein the step of heating the above-mentioned unrefined carbon nanotubes includes heating to a temperature of 600°C to 1,500°C.

8. In paragraph 1, A method for purifying carbon nanotubes, wherein the step of heating the above-mentioned unrefined carbon nanotubes includes heating at a rate of 10°C / min to 50°C / min.

9. In paragraph 1, A method for purifying carbon nanotubes, wherein the reaction time is 10 to 200 minutes.

10. In paragraph 1, Simultaneously supplying a halogen substance and an oxygen removing substance to the above heated unrefined carbon nanotubes to cause a reaction, A method for purifying carbon nanotubes, comprising modifying the metal oxide by reacting it with the halogen substance, and removing oxygen generated by the reaction between the metal oxide and the halogen substance by reacting it with the oxygen removing substance.

11. In paragraph 1, A method for purifying carbon nanotubes, further comprising a step of heat-treating the purified carbon nanotubes at 400°C to 500°C to remove residual impurities.

12. In paragraph 1, A method for purifying carbon nanotubes, wherein the supply flow rate ratio of the halogen substance and the oxygen removing substance is 9:1 to 3:

7.

13. In paragraph 1, A method for purifying carbon nanotubes, wherein the oxygen removing material is supplied in a gaseous state to the heated, unrefined carbon nanotubes.

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