WO2020148670A2

WO2020148670A2 - Carbon nanotubes and method of producing carbon nanotubes

Info

Publication number: WO2020148670A2
Application number: PCT/IB2020/050301
Authority: WO
Inventors: Sabelo Dalton MHLANGA; Edward Ndumiso NXUMALO
Original assignee: Sabinano (Pty) Ltd.
Priority date: 2019-01-15
Filing date: 2020-01-15
Publication date: 2020-07-23
Also published as: WO2020148670A3; ZA202105786B

Abstract

The invention relates to carbon nanotubes and, more particularly, the production thereof. The invention provides a method of preparing a catalyst for producing carbon nanotubes. The method includes, irradiating, with microwave radiation, a catalyst support supporting a metal compound, to form the catalyst. The invention also extends to a catalyst prepared according to a conventional method of preparing a catalyst for producing carbon nanotubes. The invention also provides a method of producing carbon nanotubes. The invention further provides use of the catalyst of the invention in producing carbon nanotubes.

Description

CARBON NANOTUBES AND METHOD OF PRODUCING CARBON NANOTUBES

FIELD OF INVENTION

THIS INVENTION is in the field of carbon nanotubes and, more particularly, the production thereof. The invention provides a method of preparing a catalyst for producing carbon nanotubes. The invention extends to a catalyst prepared according to a conventional method of preparing a catalyst for producing carbon nanotubes. The invention also provides a method of producing carbon nanotubes. The invention further provides use of the catalyst of the invention in producing carbon nanotubes. The invention also extends to carbon nanotubes produced according to the method of producing carbon nanotubes.

BACKGROUND OF INVENTION

CARBON NANOTUBES (CNTs) are a form of synthetic carbon with unique physical and chemical properties such as, size, electronic conductivity, strength, and thermal tolerance and behaviour. These properties can be tailored during manufacture. The abovementioned properties make CNTs ideal for use in many industries such as electronics, polymers, plastics, strong lightweight materials, energy, biotechnology, separations, health, water treatment and membrane technology.

Generally, CNTs are produced by causing growth thereof on a solid catalyst. Such a solid catalyst is conventionally prepared by mixing a calculated amount of a metal compound in the form of a metal precursor salt, such as iron nitrate, cobalt nitrate, and/or nickel nitrate, with a liquid such as deionised water to form a solution, and then to disperse a catalyst support or substrate, such as calcium carbonate, in the solution.

This mixture, or slurry, is stirred for at least 30 minutes, and then filtered. The insoluble solid product obtained from the filter, which is the catalyst support supporting the metal compound, is dried in an air oven at 120°C for 12 hours, cooled to room temperature, ground, and finally screened through a 150pm sieve to form a fine powder, as the catalyst.

The dried catalyst support supporting the metal compound, in powder form, is then calcined at 400°C for at least 16 hours in a static air oven, thus providing a solid catalyst.

The resultant weight percentage of metals, i.e. the iron and/or cobalt and/or nickel, derived from the dissolved precursor metal salt, in the solid catalyst is typically between 5 to 10 wt% of the total weight of the solid catalyst. Chemical vapour deposition (CVD) is then used to cause the growth of CNTs onto the solid catalyst.

In as much as CVD is one of the most widely used or preferred processes used in making CNTs, this process often results in the manufacture of impure CNTs. High production costs incurred in manufacturing the solid catalyst and poor quality or low purity of the manufactured CNTs generally limit the application of CNTs. The present invention seeks to address this, with reference to the catalyst. SUMMARY OF INVENTION

ACCORDING TO A FIRST ASPECT OF THE INVENTION IS PROVIDED a method of preparing a solid catalyst for producing carbon nanotubes, the method including irradiating, with microwave radiation, a catalyst support supporting a metal compound.

The catalyst support may be selected from calcium carbonate, limestone, calcium oxide, and any two or more thereof. In this sense, the method may include irradiating a mixture of different catalyst supports supporting a metal compound.

The catalyst support may be in particulate form, e.g. in powder form. The metal of the metal compound may be selected from iron, cobalt, nickel, and any two or more thereof. In this sense “two or more thereof” refers to the metal compound, and not to the metals as such. Thus, the catalyst support supporting a metal compound may comprise one or more catalyst supports (e.g. selected from those described above) each supporting one or more metal compounds (e.g. selected from those described below).

The metal compound may be a metal salt, specifically a water-soluble metal salt. Typically, the metal salt would be a metal nitrate. The metal salt may therefore typically be selected from iron nitrate, cobalt nitrate, nickel nitrate, and any two or more thereof. The method may include a prior step of preparing the catalyst support supporting the metal compound.

The prior step of preparing the catalyst support supporting a metal compound may include dissolving the metal compound in a solvent, to obtain a solution of the metal compound, and dispersing the catalyst support in the solution, thus obtaining a suspension of the catalyst support supporting a metal compound in the solvent, e.g. as a slurry.

Thus, the catalyst support supporting a metal compound may be suspended in the solvent when being irradiated.

The solvent may be deionized water or tap water. The invention does not require the water to be deionised water, and it is in fact preferred that the water is not deionised water.

In an embodiment, the ratio of the catalyst support (by mass) to the metal compound in the solvent (by volume) is at least 1 g: 25 ml.

In an embodiment, the weight percentage loading of the metal compound on the catalyst support is between 5 - 10 wt%. The molar concentration of the metal compound in solution is in the range 0.1 - 0.3 M. For example, for a 5 wt% Fe- Co/CaCC>3 catalyst (i.e. with 10 g of CaCC>3), the mass of metal salts used is 1.81 g (or 0.13M) for Fe and 1.23 g (or 0.08M) for Co. This is equivalent to 1 :1 percentage loading of Fe:Co. This can be varied to a 2:1 ratio of Fe:Co.

Irradiating the catalyst support supporting a metal compound with microwave radiation may provide the solid catalyst.

In an embodiment, the typical microwave reaction parameters that results to formation of the solid catalyst are irradiation power of between about 1000 - 1700W, irradiation time of between about 5 - 20 min, and irradiation temperature of between about 25 - 90 °C.

Thus, when the catalyst support supporting a metal compound is in suspension in the solvent when it is irradiated, the solid catalyst would also be in suspension post irradiation.

In such a case, the method may include separating, for example by filtering, the solid catalyst from the solvent.

The method may then also include drying the solid catalyst. For example, in drying the solid catalyst, the method may include subjecting the solid catalyst to centrifuging. Additionally, or alternatively, the solid catalyst may be subjected to air drying followed by heating, e.g. in an oven.

Alternatively, the solid catalyst may be subjected to freeze drying.

It is preferred that both irradiation of the catalyst support supporting a metal compound and drying of the solid catalyst take place at non-calcining temperatures. Thus, it is preferred that the method does not include, i.e. omits, a calcination step, in producing the solid catalyst.

The method may comprise subjecting the solid catalyst to size classification, preferably by subjecting it to screening using a screen/sieve having an aperture size of about 150 pm. THE INVENTION EXTENDS, AS A SECOND ASPECT THEREOF, to a solid catalyst produced in accordance with the first aspect of the invention.

ACCORDING TO A THIRD ASPECT OF THE INVENTION IS PROVIDED a method of producing carbon nanotubes, the method including contacting a solid catalyst according to the second aspect of the invention with a carbon containing gas such that carbon nanotubes form on the surface of the solid catalyst.

The method may include a prior step of producing the solid catalyst according to the method of the first aspect of the invention. The carbon nanotubes formed are mainly multi-walled however the method can be extended to produce single-walled CNTs.

The method may be carried out in a chemical vapour deposition reactor, and thus using a chemical vapour deposition technique.

The carbon-containing gas may be selected from acetylene and methane.

Acetylene may be used, typically as a mixture in conjunction with nitrogen and, optionally, carbon dioxide, to produce multi-walled carbon nanotubes.

Methane may be used, typically as a mixture in conjunction with nitrogen and carbon dioxide, to produce single-walled carbon nanotubes. Carbon dioxide may be required when the catalyst support of the solid catalyst is calcium oxide. In the case of multi-walled carbon nanotubes, carbon dioxide may be omitted when the catalyst support of the solid catalyst is calcium carbonate and/or limestone. When producing multi-walled carbon nanotubes, the solid catalyst may be heated, typically using the carbon containing gas or, more specifically, the mixture of which it forms part, to a temperature ranging from 600°C to 700°C. When producing single- walled carbon nanotubes, the temperature may be higher typically between 950 -

1050 °C. The method may include recovering carbon nanotubes. This may include separating carbon nanotubes from the catalyst. Separating carbon nanotubes from the catalyst may include dissolving the catalyst in a suitable acid that does not dissolve the carbon nanotubes.

IN ACCORDANCE WITH A FOURTH ASPECT OF THE INVENITON IS PROVIDED

use of a solid catalyst in accordance with the second aspect of the invention, in producing carbon nanotubes. The use of the fourth aspect of the invention may be in accordance with the method of the third aspect of the invention.

THE INVENTION EXTENDS, AS A FIFTH ASPECT THEREOF, to carbon nanotubes produced in accordance with the method of the third aspect of the invention.

BRIEF DESCRIPTION OF DRAWINGS

THE INVENTION WILL NOW BE DESCRIBED IN MORE DETAIL by way of non- limiting example, with reference to the accompanying diagrammatic drawings, in which:

Figure 1 shows a high-level block-diagram showing a process of preparing a solid catalyst in accordance with the corresponding method of invention; Figure 2a shows a transmission electron microscope (TEM) image of a Fe- Co/CaC03 catalyst;

Figure 2b shows an energy-dispersive X-ray spectrum of the catalyst indicating the presence of Ca, Fe and Co elements; and

Figure 3 shows a high-level block-diagram showing a process of producing carbon nanotubes in accordance with the corresponding method of the invention, using the solid catalyst produced by the process depicted in Figure 1.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

REFERRING TO THE DRAWINGS, and in particular Figure 1 , reference numeral 10 generally indicates a process for preparing a solid catalyst for use in producing carbon nanotubes.

The process 10 includes the following stages: an impregnation solution preparation stage 12;

- a catalyst support impregnation stage 14;

a microwave irradiation stage 16;

a solid catalyst recovery stage 18; and

a drying stage 20. In the impregnation solution preparation stage 12, a metal compound selected from iron nitrate, cobalt nitrate, nickel nitrate, and any two or more thereof, is/are dissolved in a solvent, which is deionized water or tap water, to form an impregnation solution.

In the catalyst support impregnation stage 14, a catalyst support is dispersed in the impregnation solution. The catalyst support is in particulate form, in particular in powder form, and is selected from calcium carbonate, limestone, calcium oxide, and/or any two or more thereof.

Thus, the catalyst support is impregnated with the metal compound in the impregnation solution, thereby providing a suspension of the catalyst support supporting the metal compound in suspension in the solvent. In the microwave irradiation stage 16, the catalyst support supporting the metal compound, in suspension in the solvent, is subjected to microwave irradiation. Typically, this is achieved using an industrial microwave vessel, equipped with an agitator and a magnetic stirrer. More specifically, in the microwave irradiation stage 16, while continuously stirring the catalyst support supporting the metal compound in suspension in the solvent, the catalyst support supporting the metal compound is irradiated with microwaves, up to a predetermined temperature, typically up to 90 °C, at an irradiation power of about 1700W, for about 5 min. In the microwave irradiation stage, the solid catalyst is thus obtained, in suspension in the solvent.

In the solid catalyst recovery stage 18, the formed solid catalyst is recovered from the solvent, typically by filtering and/or centrifuging.

In the drying stage 20, the recovered solid catalyst is subjected to air drying to avoid agglomeration thereof. Also in the drying stage, the solid catalyst is subjected to oven drying in an oven at a temperature of 1 10°C for at least 3 hours.

It should be noted that the oven drying temperature is lower than a typical calcination temperature of about 400°C, which indicates that the method of preparing the solid catalyst in accordance with the invention obviates the conventional calcination step, which is generally regarded as essential in preparing solid catalysts for producing carbon nanotubes.

The drying stage may also include drying the solid catalyst in the absence of heat i.e. by freeze drying. This process also eliminates agglomeration of the metal particles which typical occurs when a solution containing metals in heated to temperatures beyond the boiling point of water.

Although not illustrated, the process 10 also includes, a size classification step, in which the solid catalysts (which are typically in particulate form, preferably in powder form) are subjected to size classification, i.e. screening, through a screen/sieve of an aperture size of about 150pm. The undersized product from the screen is characterized to ascertain the purity of the solid catalyst, elemental composition and dispersion of the metals on the support.

As shown in Figure 2a, which shows a TEM image of the solid catalyst comprising of a CaC03 catalyst support supporting Fe and Co, the arrows on Figure 2a show a Fe- Co nanoparticle supported on CaCC>3 catalyst support. Figure 2b shows an energy- dispersive X-ray spectrum of the catalyst indicating the presence of Ca, Fe and Co elements. The peaks of Cu are derived from a copper grid (not shown) used to hold the sample during analysis, and therefore the Cu does not form part of the formed solid catalyst.

The solid catalyst is then used in a chemical vapour deposition process, to provide nucleation sites for the growth of multi-walled or single-walled carbon nanotubes on the surface of the solid catalyst.

In respect of the production of carbon nanotubes, reference is made to Figure 3, which shows a process 100 of producing carbon nanotubes in accordance with the corresponding method of the invention, by growing carbon nanotubes on a solid catalyst prepared in the process 10 illustrated in Figure 1 by chemical vapour deposition.

As shown in Figure 3, the process 100 includes a loading stage 102, in which the solid catalyst is loaded into a chemical vapour deposition reactor 102. The process 100 also includes a chemical vapour deposition reaction stage 104, in which the solid catalyst is heated, for producing multi-walled carbon nanotubes, to a temperature of about 650 to 700 °C, using a carbon-containing gas selected from methane and acetylene.

In one embodiment, in which the solid catalyst is based on a calcium oxide catalyst support, the carbon-containing gas comprises of nitrogen, carbon dioxide and acetylene.

In another embodiment, in which the solid catalyst is based on a calcium carbonate catalyst support, the carbon-containing gas comprises of nitrogen and acetylene.

In a further embodiment, in which the solid catalyst is based on either calcium carbonate or calcium oxide, the carbon bearing gas comprises of nitrogen, carbon dioxide and methane, and the solid catalyst is heated to a higher temperature, typically between about 950 - 1050 °C, with the resultant carbon nanotubes being single-walled. The method further includes the step of recovering the formed carbon nanotubes from the surface of the solid catalyst. This may include separating the carbon nanotubes from the solid catalyst. Separating carbon nanotubes from the catalyst may include dissolving the catalyst in a suitable acid that does not dissolve the carbon nanotubes. DISCUSSION

When the metal compounds (i.e. metal salts) are dissolved in water, ions of the metal are formed. For example, if salts of Fe and Co are used, Fe³⁺ and Co²⁺ are formed. The concentration of the ions are determined by the amount of metal salt used. These metal ions are not active as catalysts and must be converted to metal oxides such and Fe2C>3 and CoO. Typically, this is achieved by heating the metal ions in air at temperatures between 400 - 500°C in a process that oxidizes them to form the metal oxides. This process is called calcination. This process also removes other unwanted substances such as nitrate and water from the catalyst.

When microwaves are used, there is no need to calcine the catalyst. The microwaves assist the oxidation of the metal salts onto the support. Studies are underway to understand the actual chemistry that takes places. Flowever, in the meantime, it has been observed that the effect of the use of microwaves is evident from the results obtained when the catalyst is used to make CNTs. When a catalyst support which is impregnated with the metal compound is not calcined, no CNTs can be formed on the catalyst under all possible reaction conditions. Flowever, when the catalyst support supporting the metal compound is microwave irradiated and not calcined, CNTs produced by the method 100 described above are formed in high yields.

Microwave irradiation provides a number of advantages compared to the conventional calcination method, namely: (i) Microwave irradiation energy is effective since it directly heats the combination of the solvent containing the metal compound as well as the catalyst support, as opposed to the conventional process where the vessel containing same is heated first then the heat is transferred to the contents of the vessel via convection.

(ii) Microwave irradiation provides rapid heating rates and temperature homogeneity while the conventional methods are characterized by slow heating rates and increased thermal gradients.

(iii) Microwave irradiation can also achieve volumetric heating throughout the volume of the solvent loaded with the metal compound as well as the catalyst support since microwave radiation can penetrate the core of the solvent and supply energy.

(iv) Microwave irradiation also leads to high product yield due to the elimination of secondary reactions because of the selective heating provided by the microwaves. (v) The formation of metal nanoparticles whose growth is highly sensitive to reaction conditions benefits from the homogenous and efficient heating provided by microwave irradiation.

IT IS REGARDED AS ADVANTAGEOUS that the method of the invention does not include, and in fact obviates, calcination, which was surprisingly found by the applicant to be a result of employing microwave irradiation. Avoiding calcination, and thus avoiding operating at the high temperatures that it requires, is regarded as beneficial from a cost perspective. The utilisation of microwave irradiation was also surprisingly found by the applicant to result in the formation of smaller, more uniform reaction sites on the catalyst support, thus providing a higher percentage of nucleation sites than those that are reported in the state of the art. The applicant has also surprisingly found that the use of microwave irradiation in producing the catalyst results in high yields of high purity CNTs, i.e. yields which are up to 10 times higher when compared to the conventional methods of making the solid catalyst. The applicant further found it surprising that the method of the invention of producing the catalyst can be carried out using tap water, and thus avoiding the commonly accepted need to use deionised water.

The method of the invention is also environmentally friendly with less energy consumption. The method further does not result in the production of greenhouse gases. This is also a consequence of avoiding calcination. The only gas that is produced is hydrogen, which is a clean and valuable gas, potentially adding economic value to the invention. The invention also advantageously involves the use of small amounts of carbon dioxide to produce the CNTs, which gas can be captured from other carbon dioxide producing processes and used in the method of producing CNTs, thus reducing the negative impact of carbon dioxide in the atmosphere.

The method also advantageously uses iron metal which is a relatively less expensive metal and easily found in the market. Cobalt and nickel are highly active and promote the catalytic reaction for the growth of carbon nanotubes onto the surface of the solid catalyst. The catalyst supports (i.e. calcium carbonate and calcium oxide) are readily available and can dissolve in dilute acids such as nitric acids, making them easy to be removed from the final product i.e. the CNTs.

Claims

1. A method of preparing a solid catalyst for producing carbon nanotubes, the method including irradiating, with microwave radiation, a catalyst support supporting a metal compound.

2. The method of claim 1 , wherein the catalyst support is selected from calcium carbonate, limestone, calcium oxide, and any two or more thereof.

3. The method of claim 1 , wherein the catalyst support is in particulate form.

4. The method of claim 1 , wherein the metal of the metal compound is selected from iron, cobalt, nickel, and any two or more thereof.

5. The method of claim 1 , wherein the metal compound is a metal salt.

6. The method of claim 5, wherein the metal salt is a water-soluble metal salt.

7. The method of claim 6, wherein the metal salt is a metal nitrate selected from iron nitrate, cobalt nitrate, nickel nitrate, and any two or more thereof.

8. The method of claim 1 , including a prior step of preparing the catalyst support supporting the metal compound.

9. The method of claim 8, wherein the prior step of preparing the catalyst support supporting a metal compound includes dissolving the metal compound in a solvent, to obtain a solution of the metal compound, and dispersing the catalyst support in the solution, thus obtaining a suspension of the catalyst support supporting a metal compound in the solvent.

10. The method of claim 9, wherein the solvent is deionized water or tap water.

1 1 . The method of claim 9, wherein the ratio of the catalyst support (by mass) to the metal compound in the solvent (by volume) is at least 1 g: 25 ml.

1 2. The method of claim 1 1 , wherein the weight percentage loading of the metal compound on the catalyst support is between 5 - 10 wt%, and the molar concentration of the metal compound in solution is in the range 0.1 - 0.3 M.

13. The method of claim 12, wherein the typical microwave reaction parameters that results to formation of the solid catalyst are irradiation power of between about 1000 - 1700W, irradiation time of between about 5 - 20 min, and irradiation temperature of between about 25 - 90 °C.

14. The method of claim 13, wherein irradiating the catalyst support supporting a metal compound with microwave radiation produces the solid catalyst, thus, when the catalyst support supporting a metal compound is in suspension in the solvent when it is irradiated, the solid catalyst would also be in suspension post-irradiation, and therefore the method including separating the produced solid catalyst from the solvent.

15. The method of claim 14, including drying the solid catalyst.

16. The method of claim 15, wherein the drying of the solid catalyst includes subjecting the solid catalyst to centrifuging, air drying followed by heating, and/or freeze drying.

17. The method of claim 16, including subjecting the solid catalyst to size classification.

18. A solid catalyst produced in accordance with the method of claim 1.

19. A method of producing carbon nanotubes, the method including contacting a solid catalyst with a carbon containing gas such that carbon nanotubes form on the surface of the solid catalyst, wherein the solid catalyst is prepared by irradiating, with microwave radiation, a catalyst support supporting a metal compound.

20. The method of claim 19, wherein the carbon nanotubes formed are mainly multi-walled.

21 . The method of claim 19, wherein the carbon nanotubes formed are mainly single-walled.

22. The method of claim 19, wherein the production of the carbon nanotubes is carried out using a chemical vapour deposition technique.

23. The method of claim 19, wherein the carbon-containing gas is selected from acetylene and methane.

24. The method of claim 23, wherein acetylene is used, typically as a mixture in conjunction with nitrogen and, optionally, carbon dioxide, to produce multi- walled carbon nanotubes.

25. The method of claim 23, wherein methane is used, typically as a mixture in conjunction with nitrogen and carbon dioxide, to produce single-walled carbon nanotubes.

26. The method of claim 20, wherein when producing multi-walled carbon nanotubes, the solid catalyst is heated using the carbon containing gas or, more specifically, the mixture of which it forms part, to a temperature ranging from 600°C to 700°C.

27. The method of claim 21 , wherein when producing single-walled carbon nanotubes, the solid catalyst is heated using the carbon containing gas or, more specifically, the mixture of which it forms part, to a temperature that is between 950 - 1050 °C.

28. The method of claim 19, including recovering carbon nanotubes.

29. The method of claim 28, wherein the recovering of the carbon nanotubes includes separating carbon nanotubes from the solid catalyst.

30. The method of claim 29, wherein separating carbon nanotubes from the catalyst includes dissolving the solid catalyst in a suitable acid that does not dissolve the carbon nanotubes.

31. Use of a solid catalyst of claim 18 in producing carbon nanotubes.

32. Carbon nanotubes produced in accordance with the method of claim 19.