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 MOx,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 la synthese et de la 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 a/, 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), 3(11, 155-158. Thus, this article mentions that the CNTs
synthesized using a CO/AI2O3 or Fe/AI2C>3 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 a/., (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 1200 °C 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 urn. 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 750 °C, 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 300°C and 400°C 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 30nm.
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
100°C, 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 100 °C in an oven for
16 h.
Initially, the particles of this alumina had a median diameter of about 85 um
and the surface area and porosity characteristics indicated below:

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 y-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 Y-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 y-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 Y-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 y-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.


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 y-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 350°C 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 650°C 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

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

AMENDED CLAIMS
1. Process for synthesizing nanotubes, especially carbon nanotubes,
comprising a decomposition of a gas source, at a temperature of between 400
and 1200 °C, by bringing it into contact with a catalyst comprising at least one or
more multivalent transition metals, said transition metal or metals being
supported on a support having a BET specific surface area of greater than 50
m2/g and consisting of at least one alumina whose intraparticle porosity is
multimodal.
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. 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.
4. 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.
5. 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.
6. 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 urn.
7. 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.
8. 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.
9. Process according to one of the preceding claims, characterized in
that said catalyst is prepared by impregnating the support particles with an
impregnation solution containing at least one transition metal salt especially an
iron and/or cobalt and/or molybdenum salt.
10. 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.
11. Process according to one of the preceding claims, characterized in
that the catalyst is calcined in a furnace before the nanotubes are synthesized.
12. 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.
13. Process according to any one of the preceding claims,
characterized in that the gas source is a carbon source.
14. Nanotubes that can be obtained by the process as defined in any
one of the preceding claims.
15. Use of the nanotubes, especially carbon nanotubes, obtained
according to any one of Claims 1 to 13 as agents for improving the mechanical
and/or electrical and/or thermal conductivity properties, especially in
compositions based on polymers and/or resins.
16. Use according to the preceding claim of the compositions based on
polymers for packaging electronic components, for manufacture of fuel lines,
antistatic coatings, thermistors, electrodes for the energy sector, as agents
dispersed in aqueous media or as electromagnetic screening.

The subject of the present invention is aprocess 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.