WO2008065121A1 - Process for synthesizing nanotubes, especially carbon nanotubes, and their uses - Google Patents

Abstract

The subject of the present invention is a process for synthesizing nanotubes, especially carbon nanotubes, by decomposition of a gas source, at a temperature ranging from 400 to 1200 °C in a reactor, by bringing it into contact with at least one (one or more) multivalent transition metals, the transition metal(s) being supported on a support having a specific surface area determined by the BET method of greater than 50 m2/g, especially within the range from 70 m2/g to 400 m2/g. The support according to the invention is especially an inorganic support, for example an alumina having a multimodal porosity. The subject of the invention is also the nanotubes thus obtained and their use for improving the mechanical and/or electrical and/or thermal properties of materials, especially polymeric materials.

Description

PROCESS FOR SYNTHESIZING NANOTUBES, ESPECIALLY CARBON NANOTUBES, AND THEIR USES

(Field of the Invention)

The subject of the present invention is a process for synthesizing nanotubes, especially carbon nanotubes, by chemical vapour deposition employing a fluidized catalyst bed. The subject of the invention is also the nanotubes synthesized and their use for improving the mechanical and/or electrical and/or thermal properties of materials, especially polymeric materials.

(Prior art and the technical problem)

Inorganic or carbon nanotubes are recognized at the present time as being materials having great advantages because of their mechanical properties, their very high aspect ratio (length/diameter ratio) and their electrical and thermal conduction properties. In particular, these nanotubes are nanotubes consisting of carbon, boron, nitrogen, metal dichalocogenide MX₂ (M = Mo, Nb, Hf, W; X= S, Se), metal oxide MO_x, such as TiO₂, ZnO, ..., by themselves or combined. Nanotubes based on boron, nitrogen and/or carbon are composed of graphite sheets that are wound up and terminated by hemispheres consisting of pentagons and hexagons with a structure similar to fullerenes.

Nanotubes are known to be composed of either a single sheet - they are then referred to as single-walled nanotubes or SWNTs - or several concentric sheets - called multi-walled nanotubes or MWNTs.

Boron, nitrogen and/or carbon nanotubes may be produced by various processes, such as electrical discharge, laser ablation or chemical vapour deposition (CVD). In the case of metal-based nanotubes, sol-gel processes are used. Among these techniques, CVD seems to be the only one capable of manufacturing boron, nitrogen and/or carbon nanotubes in large quantity, an essential condition for achieving a cost price allowing them to be used industrially in bulk in materials based on polymers and/or resins, used in various industries, such as the automobile, electronics, optoelectronics, aeronautical and thermal or electrical protection industries.

In this CVD method, a source of nitrogen-containing, boron-containing and/or carbon-containing gas is injected at a relatively high temperature high temperature onto a catalyst, said catalyst possibly consisting of a metal supported on an inorganic solid. Among catalyst metals, mention may preferably be made of iron, cobalt, nickel, molybdenum, and among supports, alumina, silica and magnesia, or even carbon, are found. Carbon sources that may be envisaged are methane, ethane, ethylene, acetylene, benzene, ethanol, methanol, acetone or even CO/H₂ synthesis gas (the HIPCO process).

The gaseous source of boron is for example borane (B₂H₆), and the gaseous source of nitrogen is especially pyridine, ammonia or ethylenediamine. As prior art relating to the various types of nanotubes and their manufacture, the reader may refer to the doctoral thesis of Marie Castignolles: "Etudes de Ia synthese et de Ia structure par microscopie et spectroscopie electroniques de nanotubes de carbone purs et dopes a I'azote [Studies on the synthesis and structure, using electron microscopy and spectroscopy, of pure and nitrogen-doped carbon nanotubes]", University of Montpellier II, defended on 15 June 2006.

As an example of the CVD method, the process described in document WO 86/03455A1 from Hyperion Catalysis International Inc. may be mentioned. The synthesis of carbon nanotubes (CNTs) is carried out by bringing a catalyst containing iron (for example Fe₃O₄, iron on a carbon support, iron on an alumina support or iron on a carbon-containing fibril support) into contact with a carbon-containing gaseous compound (preferably CO or one or more hydrocarbons), advantageously in the presence of a compound capable of reacting with carbon in order to produce gaseous products (for example CO, H₂ or H₂O). The catalysts are prepared by dry impregnation, by precipitation or by wet impregnation of a support.

The desire to increase the productivity by weight (quantity of nanotubes produced relative to the quantity of gas and catalyst used) or to achieve better control of the quality of the nanotubes formed has led several authors to consider Co/Fe catalyst mixtures.

Thus, the article "Metal mixtures catalyzed carbon nanotubes", by Z. Konya, N. Nagaraju, A. Fonseca, J. B. Nagy, A. Tamasi and K.M. Mukhopadhyay, AIP Conference Proceedings, (1999), 486, 249-253, may be mentioned. This document explains that the Fe/Co catalyst mixtures are more effective for synthesizing MWNTs than Co or Fe by themselves on the aluminas used. These aluminas were prepared from hydrolysed aluminium isopropoxide or commercial alumina having a low specific surface area determined by the BET method.

Z. Fonseca et al. in "Synthesis of SWNT by catalytic decomposition of hydrocarbons", Chem. Commun. (1999), 1344-1344, teach that a Co/Fe catalyst mixture on silica or alumina results in better CNT yields than Fe by itself and that alumina is a better catalyst support than silica. Controlling the diameter of nanotubes is mentioned in "XPS characterization of catalysts during production of multiwall carbon nanotubes" by Z. Konya, J. Kiss, A. Oszko, A. Siska and I. Kiricsi, Physical Chemistry, Chemical Physics (2001 ), βϋL 155-158. Thus, this article mentions that the CNTs synthesized using a Co/AI₂O₃ or Fe/AI₂O₃ catalyst in the presence of acetylene have a diameter ranging from 20 to 40 nm, whereas they are finer (8 to 12 nm diameter) if an Fe-Co/ AI₂O₃ catalyst is used.

The article "Control of the outer diameter of thin carbon nanotubes synthesized by catalytic decomposition of hydrocarbons", by J. Willems, Z. Konya, JF. Colomer, G. van Tenderloo, N. Nagaraju, A. Fonseca and J. B. Nagy, CP 544, Electronic Properties of Novel Materials-Molecular Nanostructures, published by Kuzmany et al., (2000), 242-245, shows that the outside diameter of the CNTs is controlled by the metal.

The object of the present invention is to provide a novel process that is effective for manufacturing nanotubes, especially carbon nanotubes, having good weight productivity and good reproducibility. This process also makes it easier to purify the nanotubes, should this step be necessary for their application.

(Detailed description of the Invention)

The subject of the present invention is a process for synthesizing nanotubes, especially carbon nanotubes, by decomposition of a gas source, at a temperature ranging from 400 to 1200⁰C in a reactor, by bringing it into contact with at least one (one or more) multivalent transition metals, the transition metal or metals being supported on a support having a specific surface area determined by the BET method of greater than 50 m²/g.

The BET method is based on the molecular multilayer adsorption of gas at low temperature, well known to those skilled in the art.

In particular, the catalyst is brought into contact with the gases in a fluidized bed.

According to one embodiment of the invention, the specific surface area of the support is chosen to be in the range from 70 m²/g to 400 m²/g.

Among supports according to the invention, it is particularly useful to use inorganic supports, for example a support consisting of at least one alumina, the intraparticle porosity of which is multimodal, as determined by the mercury porosimetry method.

According to one particular embodiment of the invention, the support is a multimodal alumina (having 2 or more than 2 porosity peaks), the total mercury pore volume of which is greater than 0.9 cm³/g, said alumina having at least one porosity peak in the range from 50 to 3000 nm.

According to one particular embodiment, the supports can be impregnated with an amount of transition metal(s) ranging up to 50% by weight of the final catalyst and especially in a range from 10 to 50% by weight of the final catalyst.

Advantageously, the size of the support particles is chosen so as to allow good fluidization of the catalyst during the CNT synthesis reaction. In practice, to ensure correct productivity, the support particles preferably have a mean diameter D₅₀ ranging from 20 to 500 μm. According to one particular embodiment of the process of the invention, the catalyst is prepared by impregnating the support particles, especially in a stream of dry gas, with an impregnation solution containing at least one transition metal salt, especially an iron and/or cobalt and/or molybdenum salt, at a temperature lying within the range from room temperature to the boiling point of the solution. The amount of impregnation solution is chosen so that the support particles are, at all times, in contact with a sufficient amount of solution to ensure formation of a film of the impregnation solution on the surface of the support particles. In particular, when the transition metal is iron, the iron impregnation solution may be an aqueous iron nitrate solution.

According to the invention, before the nanotubes are synthesized, the catalyst is calcined in a furnace, especially at a temperature between 300 and 750⁰C, for the purpose of purifying them and, for example, denitrifying them.

The fact of working "dry", that is to say having at all times just the quantity of liquid needed to create a liquid film on the surface of the catalyst support particles, prevents aqueous discharges (for example aqueous nitrate discharges when the impregnation solution contains iron nitrate; after impregnation, the product obtained is heated to between 300⁰C and 400⁰C in a gas, whether inert or not, in order to remove the nitrates). According to one particular embodiment of the invention, the catalyst is reduced in situ in the synthesis reactor and the catalyst does not see air again before the synthesis of the nanotubes. The iron thus remains in metallic form.

According to the invention, the carbon source may be chosen from any type of carbon-containing material, such as methane, ethane, propane, butane or any other aliphatic alkane containing more than 4 carbon atoms, cyclohexane, ethylene, propylene, butane, isobutene or any other aliphatic alkane containing more than 4 carbon atoms, benzene, toluene, xylene, cymene, ethyl benzene, naphthalene, phenanthrene, anthracene, acetylene or any other alkyne containing more than 4 carbon atoms, formaldehyde, acetaldehyde, acetone, methanol, ethanol, carbon monoxide, by themselves or as a mixture.

According to the invention, the boron source is for example borane (B₂H₆). According to the invention, the nitrogen source is for example pyridine, ammonia or ethylenediamine. The gas source and its composition fixes the composition of the nanotubes. Thus, a carbon source allows carbon nanotubes to be manufactured.

The subject of the present invention is also nanotubes, especially carbon nanotubes, obtained by the above process. The nanotubes thus obtained are multi-walled nanotubes having an external diameter lying within the range from 10 to 30 nm.

These nanotubes may be used as agents for improving the mechanical and/or electrical and/or thermal conductivity properties, especially in compositions based on polymers and/or resins. These nanotubes may be used in many fields, especially in electronics (depending on the use temperature and their structure, they may be conductors, semiconductors or insulators); in the mechanical field, for example for the reinforcement of composites, for example in the automotive field, aeronautical field (CNTs are one hundred times stronger and six times lighter than steel) and in the electromechanical field (they can elongate or contract by charge injection). For example, mention may be made of the use of CNTs in macromolecular compositions intended for example for the packaging of electronic components, for the manufacture of fuel (petrol or diesel) lines, antistatic coatings, in thermistors, in electrodes for the energy sector, especially for supercapacitors, as agents dispersed in aqueous media, such as electromagnetic screening, etc.

Because the catalyst support has multimodal porosity, the method of purifying the nanotubes, in order to remove the catalyst residues, for example using an acid solution, is made easier owing to greater accessibility to the support.

The present invention will now be illustrated by particular examples of its implementation described below. It should be pointed out that the purpose of these examples is not in any way to limit the scope of the present invention.

EXAMPLES:

The instrument used for carrying out the BET specific surface area measurements was a Micromeritics ASAP^® 2000 machine.

The machine used to carry out the mercury porosimetry measurement was a Micromeritics AUTOPORE^® machine operating from 3 to 4000 bar. > Preparation of the catalysts:

Counter-example

A catalyst containing 35% iron by weight was prepared by impregnation of

Puralox^® SCCA 5-150 alumina from Sasol using the following protocol: 300 g of alumina were introduced into a jacketed 3-litre reactor heated to

100⁰C, a stream of air being passed therethrough. By means of a pump, 1600 ml of an iron solution containing 545 g/l of iron nitrate nonahydrate was then continuously injected. Since the intended (mass of metal/mass of final catalyst) ratio was 35% by weight of iron in metallic form, the iron solution was added over a period of 23 h and the rate of addition of this solution was equal to the rate of evaporation of the water. The catalyst was then heated at 100⁰C in an oven for

16 h.

Initially, the particles of this alumina had a median diameter of about 85 μm and the surface area and porosity characteristics indicated below: BET surface area (m²/g) 148

Hg total pore volume (cm³/g) 0.87

Example 1 (Ref: 2017 C27) (according to the invention)

An alumina was prepared by spray drying, without prior micronization, a suspension consisting of water, a calcined alumina (Sasol Puralox^® UF 5/230) and a pseudoboehmite (Sasol Dispersal^® 40). After calcination to convert the pseudoboehmite into γ-alumina, the catalyst was prepared as explained in the counter-example.

Example 2 (Ref: 2017 COD (according to the invention)

An alumina was prepared by milling a bimodal alumina from Norton, supplied in the form of extrudates 5 mm in length having a BET surface area of 252 m²/g.

Example 3 (Ref: 2017 C54) (according to the invention)

An alumina was prepared by spray drying, with prior micronization, a suspension consisting of water, a calcined alumina (Sasol Puralox^® UF 5/230) and a pseudoboehmite (Eurosupport Versal^® 250). The solids content was 21.3% by weight. After calcination to convert the pseudoboehmite into γ-alumina, the catalyst was prepared as explained in the counter-example.

Example 4 (Ref: 2017 C70) (according to the invention)

An alumina was prepared by spray drying, with prior micronization, a suspension consisting of water, a calcined alumina (Sasol Puralox^® UF 5/230) and a pseudoboehmite (Sasol Pural^® 400). The solids content was 42.5% by weight. After calcination to convert the pseudoboehmite into γ-alumina, the catalyst was prepared as explained in the counter-example.

Example 5 (Ref: 2017 C94) (according to the invention)

An alumina was prepared by spray drying, without prior micronization, a suspension consisting of water and a pseudoboehmite (Sasol Versal^® 250). The solids content was 26% by weight. After calcination to convert the pseudoboehmite into γ-alumina, the catalyst was prepared as explained in the counter-example.

Example 6 (Ref: 2017 C93) (according to the invention) An alumina was prepared by spray drying, without prior micronization, a suspension consisting of water and a pseudoboehmite (Sasol Versal^® 250). The solids content was 15% by weight. After calcination to convert the pseudoboehmite into γ-alumina, the catalyst was prepared as explained in the counter-example.

Example 7 (Ref: 1870 C161 ) (according to the invention)

An alumina was prepared by milling a bimodal alumina in the form of extrudates 1.2 mm in length from Norton.

The main data regarding these aluminas are given in Table 1 below. Table 1

D₅₀: Apparent mean diameter of 50% of the particle population.

Example 8 (according to the invention)

An alumina was prepared by spray drying, without prior micronization, a suspension consisting of water and a pseudoboehmite (Sasol Versal^® 250). The solids content was 15% by weight. After calcination to convert the pseudoboehmite into γ-alumina, the catalyst was prepared by adding a solution composed of cobalt acetate dihydrate and iron nitrate, so as to have a total metal content of 35 wt% with a Co/Fe ratio = 1.

> Preparation of the carbon nanotubes:

Example 9 (according to the invention) The denitrification operations, corresponding to the step of purifying the catalysts obtained according to the counter-example and examples 1 to 8, were carried out at 350⁰C in an oven under a stream of air for 2 h. About 2.5 g of catalyst thus denitrified was introduced, as a layer, into a reactor having a diameter of 5 cm and an effective height of 1 m, fitted with a separator intended to prevent fine particles from being entrained towards the top of the reactor. The reactor was heated for about 30 minutes up to 650⁰C and then the catalysts were reduced under 25 vol% H₂/75 vol% N₂ for 30 minutes. The nitrogen was then replaced with ethylene, the reaction was left to continue for 1 hour and then the nanotubes formed were collected. In all cases, the total N₂, H₂/N₂ or C₂H₂/H₂ flow rates were constant at 160 Sl/min.

After the nanotubes formed were discharged and collected, the productivity was determined by loss of ignition of the CNTs and the quality of the CNTs determined by electron microscopy.

The results are given in Table 2 below:

Table 2

MWNT: multi-walled nanotubes; 0: diameter of the nanotubes.

Except for the counter-example, all the other catalysts were γ-alumina/iron catalysts exhibiting two peaks in the region of pores smaller in size than 5 μm.

Table 2 shows that the best productivity is obtained with catalysts having a multimodal porosity.

Table 2 also shows that the combination of iron and cobalt results in better CNT productivity and smaller CNTs. It may also be seen that the amount of catalyst has no influence on the productivity nor on the reproducibility of the CNTs in terms of diameter and structure.

Claims

1. Process for synthesizing nanotubes, especially carbon nanotubes, by decomposition of a gas source, at a temperature of between 400 and 1200⁰C, by bringing it into contact with at least one or more multivalent transition metals, characterized in that the transition metal or metals are supported on a support having a BET specific surface area of greater than 50 m²/g.

2. Nanotube synthesis process according to Claim 1 , characterized in that the transition metal or metals are supported on a support having a BET specific surface area lying within the range from 70 m²/g to 400 m²/g.

3. Nanotube synthesis process according to Claim 1 or 2, characterized in that the transition metal or metals are supported on an inorganic support.

4. Process according to one of the preceding claims, characterized in that the support consists of at least one alumina whose intraparticle porosity is multimodal.

5. Process according to one of the preceding claims, characterized in that the support is an alumina, the total mercury pore volume of which is greater than 0.9 cm³/g, said alumina having at least one porosity peak in the range from 50 to 3000 nm.

6. Process according to one of the preceding claims, characterized in that the amount of transition metal(s) represents up to 50% by weight of the final catalyst.

7. Process according to one of the preceding claims, characterized in that the amount of transition metal(s) lies within the range from 10 to 50% by weight of the final catalyst.

8. Process according to one of the preceding claims, characterized in that the support particles have a mean diameter lying within the range from 20 to 500 μm.

9. Process according to one of the preceding claims, characterized in that the catalyst is prepared by impregnating the support particles with an impregnation solution containing at least one transition metal salt.

10. Process according to one of the preceding claims, characterized in that said catalyst is prepared by impregnating the support particles at a temperature lying within the range from room temperature to the boiling point of the solution, the support particles being at all times in contact with a sufficient amount of impregnation solution to ensure formation of a film of the impregnation solution on the surface of the support particles.

11. Process according to one of the preceding claims, characterized in that the catalyst is prepared by impregnating the support particles with an iron impregnation solution.

12. Process according to one of the preceding claims, characterized in that the catalyst is calcined in a furnace before the nanotubes are synthesized.

13. Process according to one of the preceding claims, in which the catalyst is reduced in situ and does not see air again before the synthesis of the nanotubes.

14. Process according to any one of the preceding claims, characterized in that the gas source is a carbon source.

15. Nanotubes that can be obtained by the process as defined in any one of the preceding claims.

16. Use of the nanotubes, especially carbon nanotubes, obtained according to any one of Claims 1 to 14 as agents for improving the mechanical and/or electrical and/or thermal conductivity properties, especially in compositions based on polymers and/or resins.

17. Use according to the preceding claim of the compositions based on polymers in fuel lines, antistatic coatings or in electrodes for the energy sector.