U.S. patent application number 11/629028 was filed with the patent office on 2008-08-14 for method for selectively producing ordered carbon nanotubes.
This patent application is currently assigned to INSTITUT NATIONAL POLYTECHNIQUE DE TOULOUSE. Invention is credited to Massimiliano Corrias, Philippe Kalck, Philippe Serp.
Application Number | 20080193367 11/629028 |
Document ID | / |
Family ID | 34947376 |
Filed Date | 2008-08-14 |
United States Patent
Application |
20080193367 |
Kind Code |
A1 |
Kalck; Philippe ; et
al. |
August 14, 2008 |
Method for Selectively Producing Ordered Carbon Nanotubes
Abstract
The invention relates to a method for selectively producing
nanotubes made of carbon ordered by decomposing a gaseous carbon
source in contact with at least one solid catalyst in the form of
catalyst grains which are made of an alumina porous support
provided with a metallic ferrous non-oxidised deposit and whose
mean grain-size ranges from 25 .mu.m to 2.5 mm and on which said
metallic ferrous deposit covers more than 75% of the surface of the
microscopic alumina support and is embodied in the form of at least
one cluster formed by a plurality of metallic agglutinated
bulbs.
Inventors: |
Kalck; Philippe;
(Auzeville-Tolosane, FR) ; Serp; Philippe;
(Toulouse, FR) ; Corrias; Massimiliano; (Toulouse,
FR) |
Correspondence
Address: |
ARKEMA INC.;PATENT DEPARTMENT - 26TH FLOOR
2000 MARKET STREET
PHILADELPHIA
PA
19103-3222
US
|
Assignee: |
INSTITUT NATIONAL POLYTECHNIQUE DE
TOULOUSE
TOULOUSE FRANCE
FR
|
Family ID: |
34947376 |
Appl. No.: |
11/629028 |
Filed: |
June 21, 2005 |
PCT Filed: |
June 21, 2005 |
PCT NO: |
PCT/FR05/01542 |
371 Date: |
November 20, 2007 |
Current U.S.
Class: |
423/447.1 ;
977/742 |
Current CPC
Class: |
B82Y 40/00 20130101;
B01J 23/745 20130101; C01B 2202/36 20130101; B01J 35/006 20130101;
B82Y 30/00 20130101; C01B 2202/06 20130101; C01B 32/162
20170801 |
Class at
Publication: |
423/447.1 ;
977/742 |
International
Class: |
D01F 9/12 20060101
D01F009/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 23, 2004 |
FR |
0406804 |
Claims
1. A process for the selective manufacture of ordered carbon
nanotubes by decomposition of a carbon source in the gaseous state
brought into contact with catalyst particles comprising at least
one supported solid catalyst in the form of particles consisting of
a porous alumina support bearing an unoxidized ferrous metal
coating of at least one transition metal, including iron,
characterized in that said supported catalyst particles: have a
mean particle size of between 25 .mu.m and 2.5 mm; and said ferrous
metal coating covers more than 75% of the surface of the
macroscopic form of the porous alumina support.
2. The process as claimed in claim 1, characterized in that the
ferrous metal coating is in the form of at least one cluster formed
from a plurality of agglutinated metal bulbs.
3. The process as claimed in claim 1, characterized in that the
ferrous metal coating forms a homogeneous continuous ferrous metal
surface layer formed from metal bulbs.
4. The process as claimed in claim 1, characterized in that the
ferrous metal coating is designed to cover the alumina support in
such a way that its pores are made inaccessible.
5. The process as claimed in claim 1, characterized in that the
ferrous metal coating results from elemental metal deposition
carried out in a single step on the alumina support.
6. The process as claimed in claim 1, characterized in that the
bulbs have a mean dimension of between 10 nm and 1 .mu.m.
7. The process as claimed in claim 1, characterized in that the
unoxidized ferrous metal coating on each catalyst particle extends
superficially with a developed overall mean dimension of greater
than 35 .mu.m.
8. The process as claimed in claim 7, characterized in that the
unoxidized ferrous metal coating of each catalyst particle extends
superficially with a developed overall mean dimension of between
200 .mu.m and 400 .mu.m.
9. The process as claimed in claim 1, characterized in that the
ferrous metal coating of each catalyst particle extends
superficially with a mean apparent area of each catalyst particle
greater than 2.times.10.sup.3 .mu.m.sup.2.
10. The process as claimed in claim 9, characterized in that the
ferrous metal coating of each catalyst particle extends
superficially with a mean apparent area of between 10.sup.4
.mu.m.sup.2 and 1.5.times.10.sup.5 .mu.m.sup.2.
11. The process as claimed in claim 1, characterized in that a
supported catalyst is used in the form of particles whose shapes
and dimensions are adapted so as to allow the formation of a
fluidized bed of these catalyst particles, in that a fluidized bed
of the catalyst particles is formed in a reactor and in that the
carbon source is continuously delivered into the reactor,
contacting the catalyst particles under conditions suitable for
fluidizing the bed of catalyst particles and for ensuring that the
decomposition reaction and the formation of nanotubes take
place.
12. The process as claimed in claim 1, characterized in that a
supported catalyst having a mean particle size of between 100 .mu.m
and 200 .mu.m is used.
13. The process as claimed in claim 1, characterized in that the
ferrous metal coating covers 90% to 100% of the surface of the
particles.
14. The process as claimed in claim 1, characterized in that the
ferrous metal coating forms a metal shell covering the entire
surface of the porous alumina support and making its pores
inaccessible.
15. The process as claimed in claim 1, characterized in that the
ferrous metal coating extends over a thickness of greater than 0.5
.mu.m.
16. The process as claimed in claim 1, characterized in that the
alumina core has a specific surface area of greater than 100
m.sup.2/g and in that the supported catalyst has a specific surface
area of less than 25 m.sup.2/g.
17. The process as claimed in claim 1, characterized in that a
supported catalyst comprising more than 20% by weight of unoxidized
ferrous metal coating is used.
18. The process as claimed in claim 1, characterized in that the
ferrous metal coating consists mainly of iron.
19. The process as claimed in claim 1, characterized in that the
ferrous metal coating consists exclusively of iron.
20. The process as claimed in claim 1, characterized in that the
ferrous metal coating is formed from iron and from at least one
metal chosen from nickel and cobalt.
21. The process as claimed in claim 1, characterized in that a
quantity of carbon source such that the ratio of the mass of carbon
of the initial carbon source introduced per hour to the mass of
metal of the supported catalyst is greater than 100 is used.
22. The process as claimed in claim 1, characterized in that the
carbon source is ethylene.
23. The process as claimed in claim 1, characterized in that the
bulbs have a mean dimension of between 30 nm and 100 nm.
24. The process as claimed in claim 1, characterized in that the
ferrous metal coating extends over a thickness of around 2 to 20
.mu.m.
Description
[0001] The invention relates to the manufacture of ordered carbon
nanotubes.
[0002] For the purpose of the present invention, the ordered carbon
nanotubes have a tubular structure with a diameter between 0.4 nm
and 30 nm and a length of greater than 100 times their diameter,
especially between 1000 and 100 000 times their diameter. They may
either be associated with metal catalyst particles or be free of
such particles (after purification). Carbon nanotubes were
described a long time ago (S. Iijima "Helical nanotubes of
graphitic carbon", Nature, 354, 56 (1991)), but they have still not
been exploited on an industrial scale. However, they could be used
for many applications, and especially be very useful and
advantageous in the manufacture of composites, flat screens, tips
for atomic force microscopes, the storage of hydrogen or other
gases, as catalyst supports, etc.
[0003] WO-03/002456 describes a process for the selective
manufacture of ordered carbon nanotubes in a fluidized bed in the
presence of a supported catalyst formed from iron on alumina,
comprising from 1 to 5% by weight of highly dispersed atomic iron
by fluidized-bed CVD on alumina grains about 120 .mu.m or 150 .mu.m
in size. The iron particles deposited are dispersed and have a
dimension of around 3 to 6 nm. This process makes it possible to
obtain a good selectivity and a good yield (greater than 90%)
relative to the carbon source.
[0004] In particular in the case of unoxidized metals used to
catalyze the formation of carbon nanotubes by thermal decomposition
in a gas phase of a carbon source, it is considered necessary to
provide a multiplicity of discontinuous metal catalyst sites
dispersed to the maximum on grains of a support, the size of the
dispersed metal sites corresponding to the diameter of the
nanotubes to be formed. A very considerable amount of research has
been carried out in this regard. Another solution would be to use
isolated catalyst particles of a size equivalent to the diameter of
the nanotubes to be formed. This is because a metal particle is
entrained to the end of each nanotube.
[0005] Highly dispersed catalysts with a low metal content make it
possible to achieve a good metal catalyst activity A* (grams of
nanotubes formed per gram of metal per hour) and a rather moderate
catalytic activity A (grams of nanotubes formed per gram of
catalytic composition per hour). However, this good activity is
obtained to the detriment of a low productivity (grams of nanotubes
formed per gram of catalytic composition). For example, the process
described in WO-03/002456 makes it possible to achieve at best an
activity A* 13.1 and an activity A of 0.46 for a productivity of
0.46.
[0006] Now, from the economic and industrial standpoints, it is
desirable not only for the reaction to be selective in terms of
nanotubes (as opposed to other forms of carbon that may be
produced, namely soot, fibers, etc.) and for the activity to be
high so that the reaction is rapid, but also for its productivity
to be high in order to avoid the need for purification steps, in
order to separate the catalyst from the nanotubes, and the costs
incurred.
[0007] Certain authors (Lyudmila B. Avdeeva et al in
"Iron-containing catalysts of methane decomposition: accumulation
of filamentous carbon", Applied Catalysis A: General 228, 53-63
(2002)) have recently proposed a use of alumina catalysts with a
high iron or iron/cobalt content produced by precipitation or
coprecipitation or impregnation. The best results announced with an
Fe/Co/Al.sub.2O.sub.3 catalyst containing 50% iron by weight and 6%
cobalt by weight make it possible to obtain, after 40 hours, a
productivity of 52.4 for an activity A of 1.31 and an activity A*
of 2.34, and with a material produced that contains both carbon
nanotubes and other fibrous structures (poor selectivity).
[0008] Thus, it may be thought that a high proportion of metal on a
catalyst produced by impregnation or precipitation makes it
possible to increase the productivity, but to the detriment of the
activity and/or the nanotube production selectivity.
[0009] It remains the case that the mechanisms involved in the
catalysis of carbon nanotube formation are still largely
unexplained and poorly controlled, and the processes and catalysts
envisaged are defined in an essentially empirical manner.
[0010] The object of the invention is therefore to alleviate these
drawbacks by proposing a process using a catalyst of astonishingly
high performance. More particularly, the aim of the invention is to
propose a process for obtaining, simultaneously, a high
productivity, especially of about 25 or higher, a high activity,
especially of about 10 or higher, and a very high selectivity,
especially greater than 90%, or even close to 100%, in terms of
carbon nanotubes produced, especially multiwalled nanotubes.
[0011] The object of the invention is more particularly to propose
a process for manufacturing ordered carbon nanotubes, especially
multiwalled nanotubes, having a production rate and a yield that
are compatible with the constraints of exploitation on an
industrial scale.
[0012] To do this, the invention relates to a process for the
selective manufacture of ordered carbon nanotubes by decomposition
of a carbon source in the gaseous state brought into contact with
at least one supported solid catalyst in the form of particles,
called catalyst particles, consisting of a porous alumina support
bearing an unoxidized metal coating of at least one transition
metal, including iron, referred to as ferrous metal coating,
characterized in that a supported catalyst is used that is mainly
formed from catalyst particles: [0013] having a mean particle size
of between 25 .mu.m and 2.5 mm; [0014] on which the ferrous metal
coating covers more than 75% of the surface of the macroscopic form
of the alumina support (without taking the porosity into
account).
[0015] Advantageously, and according to the invention, the ferrous
metal coating is in the form of at least one cluster formed from a
plurality of agglutinated metal bulbs.
[0016] Advantageously, and according to the invention, the ferrous
metal coating forms a homogeneous continuous ferrous metal surface
layer formed from metal bulbs. Each cluster, especially the ferrous
metal layer, is formed from bulbs, that is to say mutually
agglutinated rounded globules.
[0017] Inexplicably and in complete contradiction with the teaching
of the prior art, the inventors have in fact found that the
specific catalyst formed by an unoxidized ferrous metal coating,
especially produced in the form of clusters, or of a continuous
layer, of bulbs covering more than 75% of the alumina support has a
very greatly superior performance than the known catalysts, in
particular making it possible simultaneously to obtain a high
activity and a high productivity with a carbon nanotube selectivity
close to 100%.
[0018] Advantageously, and according to the invention, the ferrous
metal coating is designed to cover the alumina support in such a
way that its pores are made inaccessible. It should be noted that
the fact that these pores (mesopores in the case of a mesoporous
alumina) are made inaccessible by the metal coating may be easily
verified by simply measuring the change in specific surface area
due to the presence of the ferrous metal coating and/or by
calculating the volume of residual mesopores and/or micropores
and/or by XPS analysis, making it possible to demonstrate that the
constituent chemical elements of the alumina support are no longer
accessible on the surface. Thus, in particular, the composition
according to the invention has a specific surface area
corresponding to that of grains whose pores are inaccessible.
[0019] Advantageously, and according to the invention, each
catalyst particle has an unoxidized ferrous metal coating forming a
homogeneous continuous surface layer extending over at least one
portion of a closed surface around a porous alumina core.
[0020] The term "continuous" layer denotes the fact that it is
possible to pass continuously over the entire surface of this layer
without having to pass through a portion of another nature
(especially a portion containing no unoxidized ferrous metal
coating). Thus, the ferrous metal coating is not dispersed on the
surface of each alumina grain but on the contrary forms a
continuous layer with an apparent area corresponding substantially
to that of the grains. This layer is also "homogeneous" in the
sense that it is formed from iron or from a plurality of metals
including iron, and has an identical solid composition throughout
its volume.
[0021] The expression "closed surface" is used in the topological
sense of the term, that is to say it denotes a surface that
delimits and surrounds a finite internal space, which is the core
of the grain, and can adopt various shapes (sphere, polyhedron,
prism, torus, cylinder, cone, etc.)
[0022] The ferrous metal coating forms the outer layer of the
catalyst particles, immediately after its manufacture and if the
catalyst composition is not brought into the presence of an
oxidizing medium. If the catalyst composition is in contact with
the atmospheric air, an oxide layer may form on the periphery. This
oxide layer may if necessary be removed by a reduction step, before
the catalyst particles are used.
[0023] Advantageously, and according to the invention, the ferrous
metal coating results from elemental metal deposition (i.e. in
which one (or more) metal(s) is (are) deposited in the elemental
state, that is to say in atomic or ionic form) carried out in a
single step on the alumina support.
[0024] Thus, the ferrous metal layer forms part of an elemental
ferrous metal coating deposited in a single step on the solid
alumina support. Such an elemental metal coating deposited in a
single step may result in particular from a vacuum evaporation
deposition (PVD) operation or a chemical vapor deposition (CVD)
operation or an electroplating operation.
[0025] However, this coating cannot result from a process carried
out in several steps in liquid phase, especially by precipitation
or impregnation, or by deposition in the molten state and
solidification, or by deposition of one or more metal oxides
followed by a reduction step. The catalyst composition used in a
process according to the invention is distinguished in particular
from a composition obtained by milling pieces of pure metal
manufactured metallurgically.
[0026] An elemental metal coating deposited in a single step is
formed from crystalline microdomaines of the metal(s). Such an
elemental metal coating is formed from mutually agglutinated metal
bulbs (rounded globules).
[0027] Furthermore, advantageously and according to the invention,
the bulbs have a mean dimension of between 10 nm and 1 .mu.m,
especially between 30 nm and 100 nm.
[0028] Advantageously, and according to the invention, the ferrous
metal coating covers 90% to 100% of the surface of the macroscopic
form (the envelope surface considered without taking the porosity
into account) of the particles which is itself a closed surface.
This coverage of the surface of the alumina support by the ferrous
metal coating may be determined by XPS analysis. The ferrous metal
coating thus extends over 90% to 100% of a closed surface.
[0029] Advantageously, and according to the invention, the ferrous
metal coating extends over a thickness of greater than 0.5 .mu.m,
especially around 2 to 20 .mu.m. Furthermore, advantageously and
according to the invention, the ferrous metal coating of each
catalyst particle extends superficially with a mean apparent area
(on the external surface of the particle) of greater than
2.times.10.sup.3 .mu.m.sup.2. More particularly, advantageously and
according to the invention, the ferrous metal coating of each
catalyst particle extends superficially with a mean apparent area
of between 10.sup.4 .mu.m.sup.2 and 1.5.times.10.sup.5
.mu.m.sup.2.
[0030] Furthermore, advantageously and according to the invention,
the unoxidized ferrous metal coating of each catalyst particle
extends superficially with a developed overall mean dimension of
greater than 35 .mu.m. The developed overall mean dimension is the
equivalent radius of the disk circumscribing the ferrous metal
coating after it has been virtually developed in a plane.
Advantageously, and according to the invention, the unoxidized
ferrous metal coating of each catalyst particle extends
superficially with a developed overall mean dimension of between
200 .mu.m and 400 .mu.m.
[0031] Advantageously, a process according to the invention is
characterized in that a supported catalyst is used in the form of
particles whose shapes and dimensions are adapted so as to allow
the formation of a fluidized bed of these catalyst particles, in
that a fluidized bed of the catalyst particles is formed in a
reactor and in that the carbon source is continuously delivered
into the reactor, contacting the catalyst particles under
conditions suitable for fluidizing the bed of catalyst particles
and for ensuring that the decomposition reaction and the formation
of nanotubes take place.
[0032] More particularly, advantageously and according to the
invention, a supported catalyst having a mean particle size
(D.sub.50) of between 100 .mu.m and 200 .mu.m is used. The shape of
the catalyst particles may or may not be substantially spherical
overall. The invention also applies to a process in which catalyst
particles of relatively flat shape (flakes, disks, etc.) and/or of
elongate shape (cylinders, rods, ribbons, etc.) are used.
[0033] Advantageously, and according to the invention, each
particle comprises an alumina core covered with a shell formed from
said ferrous metal coating. Thus, advantageously and according to
the invention, the ferrous metal coating forms a metal shell
covering the entire surface of the porous alumina support and
making its pores inaccessible.
[0034] The shape of each particle depends on that of the alumina
core and on the conditions under which the ferrous metal coating is
formed on this core.
[0035] Advantageously, and according to the invention, the alumina
has a specific surface area of greater than 100 m.sup.2/g, but the
supported catalyst has a specific surface area of less than 25
m.sup.2/g.
[0036] It should be noted that the thickness of the ferrous metal
coating may extend, at least partly, into the thickness of the
porous alumina core and/or, at least partly, as an overthickness
relative to the porous core. However, it is not always easy to
precisely and clearly determine the interface between the porous
alumina core impregnated with the ferrous metal coating and the
pure ferrous metal layer extending away from the alumina core and
their relative disposition.
[0037] Furthermore, advantageously and according to the invention,
a supported catalyst comprising more than 20% by weight, especially
around 40% by weight, of ferrous metal coating is used.
[0038] Advantageously, and according to the invention, the ferrous
metal coating consists exclusively of iron.
[0039] As a variant, advantageously and according to the invention,
the ferrous metal coating is formed from iron and from at least one
metal chosen from nickel and cobalt. This is because it is known in
particular that an Fe/Ni or Fe/Co bimetallic catalyst can be used
with similar results to a pure iron catalyst, all other things
being equal. Preferably, the ferrous metal coating consists mainly
of iron.
[0040] The supported catalyst composition used in a process
according to the invention is advantageously formed mainly from
such particles, that is to say it contains more than 50% of such
particles, preferably more than 90% of such particles.
[0041] The invention also relates to a process for the selective
manufacture of ordered carbon nanotubes, in which a supported
catalyst composition formed exclusively, apart from impurities,
from such particles is used, that is to say the particles of which
catalyst composition are all in accordance with all or some of the
features defined above or below.
[0042] The use of such a high-performance supported catalyst makes
it possible in particular for the quantity of initial carbon source
to be considerably increased.
[0043] Thus, in a process according to the invention, a quantity of
carbon source such that the ratio of the mass of the initial carbon
source, especially the mass of carbon introduced into the reactor
per hour, to the mass of metal of the supported catalyst,
especially when present in the reactor, is greater than 100 is
used. Advantageously, and according to the invention, the carbon
source is ethylene. Other carbon-containing gases may be used.
[0044] Other objects, features and advantages of the invention will
become apparent on reading the following description of its
embodiments, with reference to the appended figures in which:
[0045] FIG. 1 is a diagram of an embodiment of an installation for
manufacturing a catalyst composition that can be used in a process
according to the invention;
[0046] FIG. 2 is a diagram of an embodiment of an installation for
producing carbon nanotubes with a process according to the
invention;
[0047] FIG. 3 is a micrograph of the surface of a particle of a
catalytic composition that can be used in a process according to
the invention, obtained in example 1;
[0048] FIGS. 4 and 5 are micrographs of the surface of the
particles of a catalytic composition obtained in example 2 that can
be used in a process according to the invention;
[0049] FIG. 6 is a graph showing the diameter distribution of the
nanotubes obtained in example 4; and
[0050] FIGS. 7a and 7b are micrographs on two different scales
showing nanotubes obtained in example 4.
[0051] FIG. 1 is a diagram of an installation for the
implementation of a process for manufacturing a divided solid
catalytic composition used in a process according to the invention.
This installation comprises a reactor, called a deposition reactor
20, for synthesizing the catalytic composition by chemical vapor
deposition (CVD), which includes a glass sublimator 1 into which
the organometallic precursor is introduced. This sublimator
comprises a sintered plate and can be heated to the desired
temperature by a heated bath 2.
[0052] The inert carrier gas 3, for example helium, which entrains
the vapor of the organometallic precursor used, is stored in a
bottle and admitted into the sublimator 1 via a flow regulator (not
shown).
[0053] The sublimator 1 is connected to a lower gas compartment 4,
which comprises a sintered plate, into which compartment water
vapor is introduced, which serves to activate the decomposition of
the organometallic precursor. The presence of water makes it
possible to obtain an unoxidized metal coating (thanks to the
gas-to-water displacement reaction) containing no impurities, and
thus a highly active catalyst. The compartment 4 has a jacket
thermostatted to a temperature that can be adjusted by means of a
temperature regulator (not shown). The water vapor is entrained by
and with an inert carrier gas 5, for example nitrogen, stored in a
bottle and admitted into the compartment 4 via a flow regulator
(not shown). A supply of inert carrier gas 6, for example nitrogen,
is intended to adjust the flow rates so as to obtain the
fluidization conditions. This carrier gas 6 is stored in a bottle
and admitted into the compartment 4 via a flow regulator (not
shown).
[0054] The top of the compartment 4 is connected in a sealed manner
to a glass fluidization column 7, for example 5 cm in diameter,
which is provided at its base with a gas distributor. This jacketed
column 7 is thermostatted at a temperature which may be adjusted by
means of a temperature regulator 8.
[0055] The top of the column 7 is connected to a vacuum pump 9 via
a trap, in order to retain the decomposition gases released.
[0056] The operating protocol for the embodiments relating to the
production of the catalysts according to the invention by CVD is
the following.
[0057] A mass M.sub.p of precursor is introduced into the
sublimator 1.
[0058] A mass M.sub.g of alumina support grains is poured into the
column 7 and a quantity (for example around 20 g) of water is
introduced into the compartment 4 using a syringe. A vacuum is
created in the assembly formed by the compartment 4 and the column
7. The temperature of the bed is brought to T.sub.1.
[0059] The sublimator 1 is brought to the temperature T.sub.s and
the pressure is set to the value P.sub.a throughout the apparatus
by introducing the carrier gases 3, 5 and 6 (total flow rate Q).
The deposition then starts and lasts for a time t.sub.d.
[0060] At the end of deposition, the temperature is brought back
down to room temperature by slow cooling, and the vacuum pump 9 is
stopped. Once the system has returned to room temperature and
atmospheric pressure, the catalytic granular composition is removed
from the column 7 under an inert gas atmosphere (for example a
nitrogen atmosphere). The composition is ready to be used for
manufacturing nanotubes in a growth reactor 30.
[0061] The growth reactor 30 consists of a quartz fluidization
column 10 (for example 2.6 cm in diameter) provided in the middle
of it with a distributing plate 11 (made of quartz frit) on which
the powder of catalytic granular composition is placed. The column
10 may be brought to the desired temperature using, an external
oven 12, which may slide vertically along the fluidization column
10. In the protocol used, this oven 12 has either a high position,
where it does not heat the fluidized bed, or a low position where
it heats the bed. The gases 13 (inert gas such as helium, carbon
source and hydrogen) are stored in bottles and admitted into the
fluidization column via flow regulators 14.
[0062] At the top, the fluidization column 10 is connected in a
sealed manner to a trap 15 designed to collect any fines of the
catalytic granular composition or a catalytic granular
composition/nanotube mixture.
[0063] The height of the column 10 is adapted so as to contain, in
operation, the fluidized bed of catalyst particles. In particular,
it is at least equal to 10 to 20 times the gas height, and must
correspond to the heated zone. In the embodiments, a column 10
having a total height of 70 cm, heated over a height of 60 cm by
the oven 12, is chosen.
[0064] The operating protocol for the embodiments relating to the
manufacture of nanotubes according to the invention is the
following:
[0065] A mass M.sub.c of granular supported catalyst is introduced
into the fluidization column 10 with an atmosphere of inert
gas.
[0066] When the oven 12 is in the low position relative to the
catalyst bed, its temperature is brought to the desired temperature
T.sub.n for synthesizing the nanotubes, either in an inert gas
atmosphere or in an inert gas/hydrogen (reactive gas) mixture.
[0067] When this temperature is reached, the carbon source, the
hydrogen and an addition of inert gas are introduced into the
column 10. The total flow rate Q.sub.T ensures that the bed is in a
bubbling regime at the temperature T.sub.n, without expulsion.
[0068] The growth of the nanotubes then starts, and lasts for a
time t.sub.n.
[0069] After the growth, the oven 12 is placed in the high position
relative to the catalyst bed, the flows of gases corresponding to
the carbon source and hydrogen are stopped, and the temperature is
brought back down to room temperature by slow cooling.
[0070] The carbon nanotubes associated with the metal particles and
attached to the support grains are extracted from the growth
reactor 30 and stored without taking any particular precaution.
[0071] The quantity of carbon deposited is measured by weighing and
by thermogravimetric analysis.
[0072] The nanotubes thus manufactured are analyzed by transmission
electron microscopy (TEM) and scanning electron microscopy (SEM)
for the size and dispersion measurements and by X-ray
crystallography and Raman spectroscopy for evaluating the
crystallinity of the nanotubes.
EXAMPLES
Example 1
[0073] A catalyst composition containing 24 wt % Fe/Al.sub.2O.sub.3
was prepared by the fluidized-bed CVD technique described above.
The carrier gas was nitrogen. The organometallic precursor was iron
pentacarbonyl and the support was mesoporous .gamma.-alumina (pore
volume: 0.54 cm.sup.3/g) that had been screened between 120 .mu.m
and 150 .mu.m and had a specific surface area of 160 m.sup.2/g.
[0074] The operating conditions were the following: [0075]
M.sub.g=50 g; [0076] M.sub.p=15.8 g; [0077] T.sub.1=220.degree. C.;
[0078] P.sub.a=40 torr; [0079] T.sub.s=35.degree. C.; [0080] Q=250
cm.sup.3/min; [0081] t.sub.d=95 min.
[0082] The composition obtained was formed from alumina grains
covered with clusters of iron bulbs (the mean size of the bulbs is
around 20 nm), covering the surface of the alumina with a surface
composition having 22% aluminum as measured by XPS analysis (FIG.
3).
Example 2
[0083] The purpose of this example is to prepare a supported
catalyst composition consisting of 40 wt % iron on alumina
(Al.sub.2O.sub.3) as indicated in example 1, but with the following
operating conditions: [0084] M.sub.g=25 g; [0085] M.sub.p=58.5 g;
[0086] T.sub.1=220.degree. C.; [0087] P.sub.a=40 torr; [0088]
T.sub.s=35.degree. C.; [0089] Q=250 cm.sup.3/min; [0090]
t.sub.d=200 min.
[0091] The composition obtained was formed from alumina grains
completely covered with an iron shell consisting of clusters of
iron bulbs 30 nm to 300 nm in size (FIGS. 4 and 5). The specific
surface area of the final material was 8 m.sup.2/g and the XPS
analyses showed that aluminum was no longer present on the
surface.
Example 3
[0092] Multiwalled carbon nanotubes were manufactured from the 24%
Fe/Al.sub.2O.sub.3 catalyst of example 1 in an installation
according to FIG. 2, using gaseous ethylene as carbon source.
[0093] The operating conditions were the following: [0094]
M.sub.c=0.100 g; [0095] T.sub.n=650.degree. C.; [0096]
Q(H.sub.2)=100 cm.sup.3/min; [0097] Q(C.sub.2H.sub.4)=200
cm.sup.3/min; [0098] Z=500 (ratio of the mass of carbon introduced
per hour to the mass of iron present in the reactor); [0099] for
t.sub.n=120 min: [0100] A=13.4 (activity expressed in grams of
nanotubes produced per gram of catalytic composition per hour);
[0101] P=26.8 (productivity expressed in grams of nanotubes
produced per gram of catalytic composition).
[0102] The selectivity was close to 100% in terms of multiwalled
nanotubes.
Example 4
[0103] Multiwalled carbon nanotubes were manufactured from the 40%
Fe/Al.sub.2O.sub.3 catalyst of example 2 in an installation
according to FIG. 2, using gaseous ethylene as carbon source.
[0104] The operating conditions were the following: [0105]
M.sub.c=0.100 g; [0106] T.sub.n=650.degree. C.; [0107]
Q(H.sub.2)=100 cm.sup.3/min; [0108] Q(C.sub.2H.sub.4)=200
cm.sup.3/min; [0109] Z=300; [0110] for t.sub.n=120 min: A=15.6 and
P=30.3; [0111] for t.sub.n=240 min: A=9.9 and P=39.6.
[0112] In all cases, the multiwalled-nanotube selectivity was close
to 100%.
[0113] What was thus obtained was both a high catalytic activity A
(expressed in grams of nanotubes produced per gram of catalytic
composition and per hour), of the order of or greater than 10, and,
simultaneously, also a high productivity P (expressed in grams of
nanotubes produced per gram of catalytic composition), of the order
of or greater than 25, and to do so with a nanotube selectivity
close to 100%.
[0114] The result is extremely surprising in so far as, with all
known catalysts, either a good activity A* is obtained to the
detriment of a low productivity (the case for catalysts having a
low proportion of metal on the support) or, on the contrary, a high
productivity to the detriment of a low activity (the case for
catalysts having a high proportion of metal). Now, these parameters
are both important in the context of an industrial production line.
The productivity associated with the selectivity makes it possible
to dispense with the subsequent purification steps. A high activity
allows the reaction time to be minimized.
[0115] FIG. 6 also shows that the diameter of the nanotubes
obtained in example 4 is predominantly around 10 nm to 25 nm,
whereas the particles of the composition had a diameter of around
150 .mu.m and the iron bulbs had sizes from 30 to 300 nm. Here
again, this result is surprising and inexplicable, going counter to
all the prior teaching.
[0116] FIGS. 7a and 7b show the high selectivity in the nanotubes
produced in example 4, which can thus be used directly, in
particular taking into account the low proportion of residual
porous support in the nanotubes that it was necessary to remove in
the previously known processes.
Comparative Example 5
[0117] Multiwalled carbon nanotubes were manufactured from a 5%
Fe/Al.sub.2O.sub.3 catalyst obtained as indicated in example 1 with
the following operating conditions: [0118] M.sub.g=100 g; [0119]
M.sub.a=18.45 g; [0120] t.sub.d=21 min.
[0121] The carbon nanotubes were prepared in an installation as
shown in FIG. 2 using gaseous ethylene as carbon source.
[0122] The operating conditions for manufacturing the nanotubes
were the following: [0123] M.sub.c=0.100 g; [0124]
T.sub.n=650.degree. C.; [0125] Q(H.sub.2)=100 cm.sup.3/min; [0126]
Q(C.sub.2H.sub.4)=200 cm.sup.3/min; [0127] Z=2400; [0128] for
t.sub.n=30 min: A=1.6 and P=0.8.
[0129] As may be noticed, using a catalyst less charged, covering
75% of the surface of the particles, while keeping a nanotube
selectivity close to 100%, it is impossible to obtain high values
of A and P.
Comparative Example 6
[0130] A 20 wt % Fe/Al.sub.2O.sub.3 catalyst composition was
prepared by the fluidized-bed CVD technique described above. The
carrier gas was nitrogen. The organometallic precursor was iron
pentacarbonyl and the support was nonporous .alpha.-alumina
(specific surface area (BET method): 2 m.sup.2/g).
[0131] The operating conditions were the following: [0132]
M.sub.g=50 g; [0133] M.sub.a=14 g; [0134] T.sub.1=220.degree. C.;
[0135] P.sub.a=40 torr; [0136] T.sub.s=35.degree. C. [0137] Q=250
cm.sup.3/min; [0138] t.sub.d=15 min.
[0139] The composition obtained was formed from alumina particles
covered with a shell formed by a cluster of iron bulbs entirely
covering the surface of the alumina with a surface composition in
which aluminum was absent, as measured by XPS analysis.
[0140] Multiwalled carbon nanotubes were manufactured from this
iron/nonporous alumina catalyst in an installation as shown in FIG.
2, using gaseous ethylene as carbon source.
[0141] The operating conditions were the following: [0142]
M.sub.c=0.100 g; [0143] T.sub.n=650.degree. C.; [0144]
Q(H.sub.2)=100 cm.sup.3/min; [0145] Q(C.sub.2H.sub.4)=200
cm.sup.3/min; [0146] Z=500; [0147] for t.sub.n=60 min: A=0.9 and
P=0.2.
[0148] These results are 30 times inferior to those obtained in
accordance with the invention for a catalyst according to the
invention (example 1) and under the same operating conditions. In
addition, the selectivity obtained, as evaluated by transmission
electron microscopy and thermogravimetric analysis, was poor.
[0149] These results cannot be explained in so far as the sole
difference between the two catalytic compositions lies in the
porous or nonporous nature of the core, which is not accessible on
the surface on account of the metal shell.
[0150] The invention may be the subject of many alternative
embodiments and applications other than those of the examples
mentioned above.
* * * * *