TWO-LAYER CATALYST, PROCESS FOR PREPARING SAME AND USE
FOR THE MANUFACTURE OF NANOTUBES
The present invention relates to novel two-layer
catalysts. It also relates to the process for preparing
these catalysts and their use for the manufacture of
nanotubes, especially carbon nanotubes.
Many studies have focused on catalysts of the supported
transition metal type, in particular for the
manufacture of carbon nanotube (CNT) powder.
CNTs have in recent years been the subject of intensive
research, with a view to replacing the volatile and
difficult to handle carbon black powder in all its
applications. CNTs also have the advantage of imparting
improved mechanical properties and electric and/or heat
conduction properties to any composite material
containing them, which are at least equal to those of
pulverulent carbon black, at lower contents. Their good
mechanical properties and especially the property of
resistance to elongation are partly linked to their
very high (length/diameter) aspect ratios.
They are composed of one or more graphite sheets
arranged concentrically about a longitudinal axis. For
nanotubes composed of a single sheet, reference is made
to SWNTs (single-walled nanotubes) and for nanotubes
composed of several concentric sheets, reference is
then made to MWNTs (multiwalled nanotubes). SWNTs are
in general more difficult to manufacture than MWNTs.
Carbon nanotubes may be manufactured according to
various processes such as electrical discharge, laser
ablation, chemical vapor deposition (CVD) or physical
vapor deposition (PVD).
2
According to the applicant, the process for
manufacturing CNTs that is the most promising in terms
of the quality of the CNTs, the reproducibility of the
characteristics of the CNTs and productivity is the CVD
process. This process consists in injecting a source of
carbon-rich gas into a reactor containing a metallic
catalyst brought to high temperature. In contact with
the metal, the gas source decomposes to graphite-plane
CNTs and hydrogen. In general, the catalyst consists of
a catalytic metal such as iron, cobalt or nickel,
supported by a solid substrate, in the form of grains,
and that is chemically inert, such as alumina, silica,
magnesia or else carbon.
The gaseous sources of carbon generally used are
methane, ethane, ethylene, acetylene or benzene.
By way of example of documents that describe this CVD
process, mention may be made of document WO 86/03455 by
Hyperion Catalysis International Inc. that can be
considered to be one of the base patents on the
synthesis of CNTs. This document describes carbon
fibrils (former name of CNTs) that are quasicylindrical,
the diameter of which is between 3.5 and
70 nm and the aspect ratio of which is greater than or
equal to 100, and also their preparation method.
The CNTs are synthesized by bringing a catalyst
containing iron (for example Fe304, Fe on a charcoal
support, Fe on an alumina support or Fe on a carboncontaining
fibril support) into contact with a carbonrich
gaseous compound, such as a hydrocarbon, in the
presence of another gas capable of reacting with the
carbon-rich gaseous compound. The synthesis is carried
out at a temperature chosen from the range extending
from 850 °C to 1200 °C. The catalyst is prepared by dry
impregnation, by precipitation or by wet impregnation.
3
Other documents describe improvements to this process,
such as the use of a continuous fluidized bed of
catalyst, which makes it possible to control the degree
of agglomeration of the catalyst and of the carbonbased
materials formed (see, for example WO 02/94713A1
in the name of the University of Tsinghua and
FR 2 826 646 INPT).
Many studies have also focused on improvement of the
catalyst, especially by combination of various
catalytic metals. Thus, US 2001/00036549 by Hyperion
Catalysis International Inc. described supported
bimetallic catalysts of Fe/Mo and Fe/Cr type and
demonstrated that molybdenum doping of the order of 1
to 2% by mass made it possible to double the
productivity relative to an iron monometallic catalyst,
in a temperature range from 500 °C to 1500 °C, but that
doping beyond 2.5% made the productivity drop. Mention
may also be made of patent application US 2008/0003169
which describes catalysts of Fe/Mo/alumina type
enabling good productivity. However, in this case, the
catalyst has a structure different from that of a
supported catalyst since it is obtained by
coprecipitation, on the one hand, of a solution of iron
salts and of molybdenum salts and, on the other hand,
of a solution of aluminum salts.
The applicant has proposed in its patent application
WO 2006/082325 a novel type of supported catalyst which
may combine several types of metals. However, this
document concentrates solely on examples of an
Fe/alumina catalyst.
Finally, document EP 2 077 251 discloses a supported
catalyst for the production of single-walled carbon
nanotubes. This supported catalyst consists of a flat
substrate, made of quartz glass or of cordierite,
covered with a support based on non-porous alumina.
4
deposited on which are, according to a given process,
catalytic metals (molybdenum and iron). The latter form
a thin layer, which results in a low catalytic activity
of the catalysts from EP 2 077 251, which leads to the
formation of a film of carbon nanotubes, the thickness
of which does not exceed 10 pm.
Despite these various developments, there is still a
need for new catalysts, which make it possible to
further improve the productivity of the CNT synthesis
reactions in which they are used.
The present inventors have found that a supported
catalyst having a structure of "core-shell" type
enabled this improvement.
The invention thus aims to propose a catalyst material
for the preparation of nanotubes, especially carbon
nanotubes, said material being in the form of solid
particles, said particles comprising (and preferably
consisting of) a porous substrate supporting two
superposed catalytic layers, a first layer (referred to
as the "core"), positioned directly on the substrate,
comprising at least one transition metal, especially in
the reduced or metal state, from column VIB of the
Periodic Table, preferably molybdenum, and a second
layer (referred to as the "shell") positioned on the
first and comprising iron.
It is clearly understood that, in the present
description, the expression "at least one metal" means
one or more metals. Moreover, it is specified that
"iron" and "transition metal" refers to these metals in
the elemental state, that is to say in the 0 oxidation
state, or in the oxidized state. It is preferred
however that these metals are mainly in the elemental
state.
5
Such a catalyst material thus has a core-shell
structure positioned on a porous substrate.
The transition metal present in the first layer or core
is preferably chromium, molybdenum, tungsten or
mixtures thereof. Advantageously, molybdenum is used.
In the synthesis of carbon nanotubes, these catalytic
metals are known as having a reaction-initiating role,
and their presence is therefore useful at the start of
the carbon nanotube synthesis reaction. Iron, present
in the second layer or shell, is itself known as
playing a role during the elongation of the chain of
carbon nanotubes. The present inventors have observed
that the synthesis of CNTs takes place from the inside
of the catalyst to the outside and, without wishing to
be tied to any one theory, they are of the opinion that
by placing the initiating catalytic metal closer to the
part of the catalyst material where the initiation
takes place, that is to say toward the inside of the
catalyst material, and the chain-elongating catalytic
metal more toward the outside, the synthesis of CNTs is
favored.
The core may comprise, in addition to the transition
metal from column VIB of the Periodic Table, iron. In
this case, in the core, the amount, by mass, of iron
may be less than the amount, by mass, of transition
metal from column VIB of the Periodic Table. Likewise,
the shell may also comprise a transition metal from
column VIB of the Periodic Table, preferably
molybdenum, in addition to iron. In this case, in the
shell, the amount, by mass, of transition metal from
column VIB of the Periodic Table is generally less than
the amount, by mass, of iron.
According to one advantageous embodiment, the catalyst
according to the invention comprises (or even consists
of) a first catalytic layer comprising, as sole
6
catalytic metal, molybdenum, deposited on which is a
second catalytic layer comprising, as sole catalytic
metal, iron.
The iron content of the catalyst material according to
the invention is at least 25%, preferably from 30% to
40% by mass of the total mass of the catalyst material.
The content of transition metal from column VIB of the
Periodic Table, preferably molybdenum, is from 0.5% to
10%, in particular from 1.5% to 8%, preferably from 2%
to 4% by mass of the total mass of the catalyst
material.
The porous substrate advantageously has a BET specific
surface area of greater than 50 m^/g, preferably of
between 70 and 400 m^/g. The BET specific surface area
may be measured by the amount of nitrogen adsorbed by
the substrate, which method is well known to those
skilled in the art.
The substrate is preferably inert, namely chemically
inert with respect to the transition metal and iron and
the gaseous source of carbon, under the operating
conditions of the CVD synthesis process.
Advantageously, this substrate is made from an
inorganic material. It represents, in particular, from
50% to 85%, for example from 52% to 83.5%, by mass of
the catalyst material.
The substrate may be chosen from alumina, activated
charcoal, silica, a silicate, magnesia, titanium oxide,
zirconia, a zeolite or else carbon fibers. According to
one advantageous embodiment, the substrate is alumina,
for example of gamma or theta type.
The macroscopic form of the substrate particles, and of
the catalyst material particles, may be overall
7
substantially spherical or not. The invention also
applies to grains having a macroscopic shape that is
more or less flattened (flakes, discs, etc.) and/or
elongated (cylinders, rods, ribbons, etc.). In any
case, the substrate is in pulverulent form and not in
an agglomerated, especially planar form.
According to the invention, the shape and the dimension
of the particles are suitable for allowing the
formation of a fluidized bed of the catalyst material.
In practice, in order to ensure a reasonable
productivity, it is preferred that the substrate
particles have a larger dimension between 20 and
500 microns, preferably between 75 and 150 microns.
This particle size may be measured by dry or wet laser
particle size analysis.
Furthermore, according to one embodiment of the
invention, the catalyst material is in the form of
spherical particles having a unimodal particle size
distribution, the equivalent diameter of the particles
being between 80% and 120% of the average diameter of
the particles of the catalyst material. As a variant,
the particles may have a bimodal particle size
distribution with an equivalent diameter ranging from
30% to 350%.
Advantageously, the catalyst material according to the
invention comprises alumina particles supporting a
molybdenum core on which an iron shell is positioned,
the mass percentages of the various constituents being
32 for iron, 2 for molybdenum and 66 for alumina,
relative to the total mass of the catalyst material.
The invention extends to a process for preparing the
catalyst material described previously, which comprises
a first step of impregnation of the substrate with an
impregnation solution comprising a salt of a transition
8
metal from column VIB of the Periodic Table, preferably
molybdenum, and a second step of impregnation with an
impregnation solution comprising an iron salt. Each of
the impregnation solutions may be an alcoholic or
aqueous solution. The iron salt may be an iron nitrate,
and in particular iron nitrate nonahydrate. The
molybdenum salt may be ammonium molybdate, and
especially ammonium molybdate tetrahydrate.
Advantageously, the first impregnation solution is an
aqueous solution of ammonium molybdate and the second
solution is an aqueous solution of iron nitrate
nonahydrate.
Each impregnation step is preferably carried out under
a stream of dry gas, preferably under a stream of air.
It is carried out at a temperature measured in situ
ranging from 100 to 150°C, preferably of around 120°C.
The amount of impregnation solution, at any moment, in
contact with the substrate or the subjacent layer is
generally just sufficient to ensure the formation of a
film at the surface of the particles of substrate or of
the subjacent layer.
The process for preparing the catalytic material
according to the invention also comprises, after the
impregnation steps, a step of drying at a temperature
ranging for example from 150 to 250°C, measured in
situ, advantageously followed by a step of
denitrification, preferably under an inert atmosphere
at a temperature ranging from 350 to 450°C, measured in
situ.
The invention also extends to a catalyst material
obtained by a process according to the invention as
defined above.
The invention also extends to a process for
manufacturing nanoparticles of a material chosen from
9
silicon, carbon or boron and a mixture of these
elements, optionally combined with nitrogen or doped
with nitrogen, characterized in that at least one
catalyst material according to the invention is used.
Advantageously and according to the invention, it is a
reaction for the selective manufacture of carbon
nanotubes by thermal decomposition of a gaseous source
of carbon. Thus, the invention relates more
particularly to a process for manufacturing carbon
nanotubes by decomposition of a source of carbon in the
gas state, comprising the following steps:
a) the introduction, especially the placement in a
fluidized bed, in a reactor, of a catalyst material as
defined previously;
b) the heating of said catalyst material at a
temperature ranging from 620 to 680°C, preferably of
around 650°C;
c) the bringing into contact of a source of carbon
(alkane or alkene), preferably of ethylene, with the
catalyst material from step b) , in order to form, at
the surface of said catalyst material, carbon nanotubes
and hydrogen by catalytic decomposition of said carbon
source;
d) the recovery of the carbon nanotubes produced in
c) .
The source of carbon may be an alkane such as methane
or ethane or preferably an alkene which may be chosen
from the group comprising ethylene, isopropylene,
propylene, butene, butadiene, and mixtures thereof.
This source of carbon may be of renewable origin as
described in patent application EP 1 980 530. The
alkene preferably used is ethylene.
Advantageously and according to the invention, the
source of carbon, and preferably ethylene, is mixed in
step c) with a stream of hydrogen.
10
The source of carbon/hydrogen ratio may in this case be
between 90/10 and 60/40, preferably between 70/30 and
80/20. Advantageously, step c) is carried out with an
ethylene/hydrogen mixture in a ratio of 75/25.
The various steps are preferably carried out
simultaneously or continuously in one and the same
reactor.
Moreover, this process may comprise other (preliminary,
intermediary or subsequent) steps, as long as they do
not adversely affect the production of carbon
nanotubes.
Thus, advantageously, the catalyst material is reduced
in situ in the CNT synthesis reactor. Thus, the
catalytic layers are in the reduced state at the moment
when the catalyst is used.
If necessary, a step of milling the nanotubes in situ
or ex situ relative to the reactor may be envisaged,
before or after step d). It is also possible to provide
a step of chemical and/or thermal purification of the
nanotubes before or after step d) .
The productivity obtained with the process of the
invention is particularly high, since it is always
greater than 20, even greater than 25, said
productivity being calculated as the ratio of the mass
of carbon formed to the mass of catalyst used.
Furthermore, the carbon nanotubes formed have less
tendency to agglomerate than in the prior art
processes.
The invention also extends to the carbon nanotubes
capable of being obtained according to the process
described previously. These are advantageously multi11
walled nanotubes, comprising for example from 5 to 15,
and preferably from 7 to 10, concentrically rolled
graphene sheets. The nanotubes obtained according to
the invention customarily have an average diameter
ranging from 0.1 to 200 nm, preferably from 0.4 to
100 nm, more preferably from 0.4 to 50 nm and better
still from 1 to 30 nm and advantageously a length of
more than 0.1 ^m and advantageously from 0.1 to 20 pm,
for example around 6 pm. Their length/diameter ratio is
advantageously greater than 10 and usually greater than
100. Their specific surface area is for example between
100 and 600 m^/g and their bulk density may especially
be between 0.01 and 0.5 q/cTxc' and more preferably
between 0.07 and 0.2 g/cm^.
The invention also relates to the use of nanotubes,
capable of being obtained as described previously, in
composite materials, in order to impart thereto
improved electric and/or heat conduction properties
and/or improved mechanical properties, especially
resistance to elongation. In particular, the CNTs may
be used in macromolecular compositions intended for
wrapping electronic components or for the manufacture
of fuel lines or antistatic coatings or paints, or in
thermistors or electrodes for supercapacitors or else
for the manufacture of structural parts in the
aeronautic, nautical or automotive fields.
The invention will be described in greater detail with
reference to the following examples which are given for
purely illustrative purposes and are in no way
limiting, taken in combination with the appended figure
which illustrates a catalyst grain according to the
invention covered with a film of carbon nanotubes.
12
EXAMPLES
Example 1:
A 25Fe3Mo7Fe/Al203 catalyst is prepared from Puralox®
SCCa-5/150 alumina having a median diameter equal to
around 85 lam and a specific surface area of 160 m^/g. In
a IL reactor equipped with a jacket and heated at
120 °C, 100 g of alumina are introduced and the- reactor
is purged with air. Using a pump, 150 ml of a solution
of iron nitrate and ammonium molybdate containing
535 g/1 of iron nitrate nonahydrate and 60 g/1 of
ammonium molybdate tetrahydrate then 520 ml of a
solution of iron nitrate containing 535 g/1 of iron
nitrate nonahydrate are then continuously injected.
Since the targeted ratio (mass of metal/mass of
catalyst) is 32% for the iron and 3% for the
molybdenum, the addition duration is set at 25 h. The
catalyst is then heated in situ at 220 °C under a purge
of dry air for 8 hours than placed in a muffle furnace
at 400°C for 8 hours.
Example 2 (comparative):
A 3Mo7Fe25Fe/Al203 catalyst containing 32% of iron and
3% of molybdenum is prepared under the conditions of
example 1 by firstly injecting 520 ml of the iron
nitrate solution then 150 ml of the solution of iron
nitrate and ammonium molybdate.
Example 3;
A 32Fe2Mo/Al203 catalyst containing 32% of iron and 2%
of molybdenum is prepared under the conditions of
example 1, by injecting firstly 90 ml of a solution of
ammonium molybdate containing 60 g/1 of Mo then 650 ml
of a 535 g/1 iron nitrate solution.
Example 4 (comparative);
A 32Fe/Al203 catalyst is prepared from Puralox®
SCCa-5/150 alumina having a median diameter equal to
13
around 85 ^m and a specific surface area of 160 m^/g. In
a IL reactor equipped with a jacket and heated at
120 °C, 100 g of alumina are introduced and the reactor
is purged with air. Using a pump, 630 ml of an iron
nitrate solution containing 535 g/1 of iron nitrate
nonahydrate are then continuously injected. Since the
targeted ratio (mass of iron/mass of catalyst) is 32%,
the addition duration is set at 25 h. The catalyst is
then heated in situ at 220 °C under a purge of dry air
for 8 hours then placed in a muffle furnace at 400 °C
for 8 hours.
Example 5:
A catalytic test is carried out by placing a mass of
around 2.3 g of catalyst as a layer in a reactor having
a diameter of 5 cm and an effective height of 1 meter.
It is heated at 650°C under 2.66 1/min of nitrogen for
30 minutes then a reduction hold is maintained for 30
minutes under 2 1/min of nitrogen and 0.66 1/min of
hydrogen. Once this hold has ended, a flow rate of
2 1/min of ethylene and of 0.66 1/min of hydrogen is
introduced. After 60 minutes, the heating is stopped
and the reactor is cooled under a 2.66 1/min stream of
nitrogen. The amount of product formed is evaluated by
calculating the mass remaining after a calcination of
around 2 g of composite at 800°C for 6 hours.
Catalyst of Reference Productivity Activity
e x a m p l e ( g c / g c a t a l y s t ) ( g c / g m e t a l /h)
1 25Fe3Mo7Fe/Al203 29 82.9
2 3Mo7Fe25Fe/Al203 17 48.6
3 32Fe2Mo/Al203 26 76.5
4 I 32Fe/Al203 | 15 | 46.9
The catalysts in accordance with the invention make it
possible to obtain a carbon nanotube productivity and
an activity that are greater than those obtained with
the catalysts from the comparative examples.
14
The appended figure furthermore illustrates a catalyst
grain according to the invention, covered with a film
of carbon nanotubes which is formed according to a
process similar to that described above. As shown in
this figure, the film of nanotubes has a thickness of
greater than 100 ^m. In order to obtain a film
thickness value more representative of the whole of the
sample tested, a particle size analysis of the catalyst
grains was carried out at the end of the reaction.
After subtracting the average diameter (D50) of the
catalyst grains before reaction, it was deduced
therefrom that the average thickness of the film of
nanotubes was, for this sample, approximately 200 ]im..
The nanotubes obtained according to the invention may
be introduced into a polymer matrix in order to produce
composite materials having improved mechanical and/or
thermal and/or conductive properties.

CLAIMS
1. A catalyst material for the preparation of
nanotubes, especially carbon nanotubes, said material
being in the form of solid particles, said particles
comprising a porous substrate supporting two superposed
catalytic layers, a first layer, positioned directly on
the substrate, comprising at least one transition metal
from column VIB of the Periodic Table, preferably
molybdenum, and a second layer positioned on the first
and comprising iron.
2. The catalyst material as claimed in claim 1,
characterized in that the first layer also comprises
iron, and/or the second layer also comprises a
transition metal from column VIB of the Periodic Table,
preferably molybdenum.
3. The catalyst material as claimed in claim 1,
characterized in that it comprises a first catalytic
layer comprising, as sole catalytic metal, molybdenum,
deposited on which is a second catalytic layer
comprising, as sole catalytic metal, iron.
4. The catalyst material as claimed in any one of
claims 1 to 3, characterized in that the iron content
is at least 25%, preferably from 30% to 40% by mass of
the total mass of the catalyst material.
5. The catalyst material as claimed in any one of
claims 1 to 4, characterized in that the content of
transition metal from column VIB of the Periodic Table
is from 0.5% to 10%, in particular from 1.5% to 8%,
preferably from 2% to 4% by mass of the total mass of
the catalyst material.
6. The catalyst material as claimed in any one of
claims 1 to 5, characterized in that the porous
16
substrate has a BET specific surface area of greater
than 50 m^/g, preferably of between 70 and 400 m^/g.
7. The catalyst material as claimed in any one of
claims 1 to 6, wherein the substrate is chosen from
alumina, activated charcoal, silica, a silicate,
magnesia, titanium oxide, zirconia, a zeolite and
carbon fibers, preferably the substrate is alumina.
8. The catalyst material as claimed in any one of
claims 1 to 7, characterized in that the substrate
particles have one larger dimension between 20 and
500 microns, preferably between 75 and 150 microns.
9. The catalyst material as claimed in any one of
claims 3 to 8, characterized in that the substrate is
made of alumina and in that it supports a first layer
of molybdenum on which a second layer of iron is
placed, and in that the mass percentages of the various
constituents are 32 for iron, 2 for molybdenum and 66
for alumina, relative to the total mass of catalyst
material.
10. A process for preparing the catalyst material as
claimed in any one of claims 1 to 9, by impregnation of
the substrate with a first impregnation solution
comprising a salt of a transition metal from column VIB
of the Periodic Table, preferably a molybdenum salt,
then with a second impregnation solution of iron salt,
preferably of iron nitrate, each of said impregnations
preferably being carried out under a stream of dry gas.
11. The process as claimed in claim 10, in which each
impregnation is carried out at a temperature ranging
from 100 to 150°C, measured in situ.
12. The process as claimed in any one of the preceding
claims, wherein the amount of impregnation solution, at
17
any moment, in contact with the substrate or the
subjacent layer is just sufficient to ensure the
formation of a film at the surface of the particles of
substrate or of subjacent layer.
13. The process as claimed in any one of claims 10 to
12, which comprises, after the impregnation steps, a
step of drying at a temperature ranging from 150 to
250°C, measured in situ, optionally followed by a step
of denitrification, preferably under an inert
atmosphere at a temperature ranging from 350 to 450°C,
measured in situ.
14. A process for manufacturing nanotubes, especially
carbon nanotubes, comprising the following steps:
a) the introduction, especially the placement in a
fluidized bed, in a reactor, of a catalyst material as
defined in one of claims 1 to 9 or prepared as claimed
in any one of claims 10 to 13;
b) the heating of said catalyst material at a
temperature ranging from 620 to 680°C, preferably of
around 650°C;
c) the bringing into contact of a source of carbon,
preferably of ethylene, with the catalyst material from
step b) , in order to form, at the surface of said
catalyst, carbon nanotubes and hydrogen by catalytic
decomposition of said carbon source;
d) the recovery of the carbon nanotubes produced in
c) .
15. The process as claimed in claim 14, characterized
in that the source of carbon is mixed in step c) with a
stream of hydrogen.
16. The process as claimed in claim 15, wherein the
source of carbon/hydrogen ratio is between 90/10 and
60/40, preferably between 70/30 and 80/20.
17. The process as claimed in claim 16, wherein ethylene is
used as the source of carbon and the ethylene/hydrogen ratio
is 75/25.
18. The carbon nanotubes capable of being obtained according
to the process as claimed in any one of claims 14 to 17.
19. The process as claimed in claim 14, wherein the carbon
nanotubes are used in composite materials in order to impart
thereto improved electric and/or heat conduction properties
and/or improved mechanical properties, especially resistance
to elongation.
20. The process as claimed in claim 11^, wherein the carbon
nanotubes are used in macromolecular compositions intended
for wrapping electronic components or for the manufacture of
fuel lines or antistatic coatings or paints, or in
thermistors or electrodes for supercapacitors or else for
the manufacture of structural parts in the aeronautic,
nautical or automotive fields.