Method for the selective production of ordered carbon nanotubes in a fluidised bed

Abstract

A method for the selective production of ordered carbon nanotubes includes decomposition of a carbon source in the gaseous state in contact with at least one solid catalyst, taking the form of metallic particles borne by carrier grains. The catalyst grains are adapted so as to be able to form a fluidised bed containing between 1% and 5% by weight of metallic particles having average dimensions of between 1 nm and 10 nm. The decomposition takes place in a fluidised bed of catalyst grains. The method can be used to obtain pure nanotubes with predetermined dimensions in a high yield.

Description

[0001] The present invention relates to the production of ordered carbon nanotubes.

[0002] Ordered carbon nanotubes within the meaning of the present invention have a tubular structure of diameter between 0.4 nm and 50 nm and a length greater than 100 times their diameter, in particular between 1000 and 100,000 times their diameter. They may exist either combined with particles of metallic catalyst, or separate from these particles. Carbon nanotubes have been described for a long time (S. Iijima “Helical nanotubules of graphitic carbon” Nature 354, 56 (1991)), but are still not exploited on an industrial scale. They could nevertheless be employed in numerous applications and in particular could be extremely useful and advantageous in the production of composite materials, flat screens, tips for nuclear power microscopes, storage of hydrogen or other gases, as catalyst supports, etc.

[0003] U.S. Pat. No. 4,663,230 and U.S. Pat. No. 5,500,200 describe a process for the catalytic preparation of carbon fibrils by high temperature decomposition of a source of gaseous carbon in contact with a solid catalyst in the form of metallic particles of size 3.5 nm to 70 nm, comprising at least one transition metal, carried by granules of solid support of size less than 400 &mgr;m. According to these documents the fibrils obtained should comprise an internal core of less ordered carbon surrounded by an external region of ordered carbon, and should have a diameter varying between 3.5 nm and 70 nm. U.S. Pat. No. 5,500,200 discloses that the process for obtaining these fibrils may be carried out in a fluidised bed, but does not provide any example of such a process. All the examples mentioned are carried out with a fixed bed, produce a moderate yield with respect to the carbon source (<20% by weight), and the actual characteristics of the products obtained are not given. These documents therefore do not provide any real information relating to the production of real nanotubes of ordered carbon and/or the use of a fluidised bed for the production of such nanotubes.

[0004] Other documents disclose the production of nanotubes of single-wall carbon by means of a catalytic composition formed from metallic particles that are either carried by support granules deposited in a crucible (WO-0017102) or introduced in the form of an aerosol (WO-9906618) to a reactor fed with a gaseous source of carbon such as carbon monoxide or ethylene. The yields obtained (nanotubes produced with respect to the source of carbon) with such processes are very low, and a certain amount of particles of pyrolitic or amorphous carbon is produced. However, it is important for the practical industrial exploitation of carbon nanotubes to be able to control precisely and simultaneously the dimensional characteristics, the production yields and the purity of the product obtained.

[0005] WO 01/94260, published on 13 Dec. 2001, describes a process and an apparatus for the production of carbon nanotubes in several stages, in which a preliminary treatment stage of the catalyst in order to extract air from the latter is followed by a stage involving the reduction of the catalyst. In such a process it is also necessary to eliminate the amorphous carbon formed in the reaction, which is thus not selective with regard to the nanotubes that are formed.

[0006] U.S. Pat. No. 4,650,657 and U.S. Pat. No. 4,767,737 describe a process for the production of a fibrous carbon-containing material containing a ferrous metallic component in a fluidised bed by decomposition of carbon monoxide in the presence of hydrogen and a neutral gas such as nitrogen, a powder of ferrous metallic catalyst and in the presence of an abrasive such as alumina that acts as a support. These documents mention that the effect of such a fluidised bed is to remove the carbon formed from the surface of the granules, to promote the fragmentation and to minimise the size of the reactive mass of the fluidised bed. These documents do not describe a process that can be applied to the production of carbon nanotubes. On the contrary, the products obtained are particles of carbon of average dimension 1&mgr; to 50&mgr; (Table 1 of U.S. Pat. No. 4,650,657).

[0007] The publication “Fe-catalyzed carbon nanotubes formation” by K. Hernadi et al., Carbon, 34, No. 10, (1996), 1249-1257 describes a process for the production of carbon nanotubes on various catalysts in a fixed bed or in a so-called “fluidised bed” reactor of 6.4 mm diameter. Such a diameter cannot produce a true fluidised bed. The catalysts are prepared by impregnation. This process limits the amorphous carbon produced to a laboratory scale exploitation and teaches that the use of such a “fluidised bed” would be less suitable than the use of a fixed bed.

[0008] In addition FR-2 707 526 describes a process for the preparation of a catalyst by chemical deposition in the vapour phase of metallic particles of size less than 2 nm in a fluidised bed of porous support granules at a temperature of less than 200° C. This document describes more particularly the preparation of a rhodium-containing catalyst and does not describe a catalyst suitable for the production of carbon nanotubes.

[0009] The object of the invention is thus to provide a process for the selective production of true nanotubes of ordered carbon of homogeneous average dimensions (varying only slightly around a mean value) under conditions compatible with an industrial-scale exploitation, particularly in terms of yield with respect to the carbon source, catalytic activity and production costs, and of purity in nanotubes of the product obtained.

[0010] The invention also provides such a process in which the characteristics of the nanotubes produced may be predetermined and adjusted by simple modification of the parameters involved in the implementation of the process.

[0011] The invention provides more particularly such a process in which the yield of produced nanotubes with respect to the carbon source is equal to or greater than 80% by weight.

[0012] The invention also provides a catalytic granular composition that may be used in a process for the production of ordered carbon nanotubes according to the invention, as well as a process for the preparation of such a catalytic granular composition.

[0013] (Throughout the text, all the terms and criteria relating to the characteristics of the fluidised bed are adopted within the meaning given in the reference work “Fluidization Engineering”, Kunii, D.; Levenspiel, O.; Butterworth-Heinemann Edition 1991.)

[0014] To this end, the invention relates to a process for the selective production of ordered carbon nanotubes by decomposition of a source of carbon in the gaseous state in contact with at least one solid catalyst in the form of metallic particles comprising at least one transition metal carried on granules of solid support, these support granules carrying the metallic particles, so-called catalyst granules, capable of being able to form a fluidised bed, the metallic particles having a mean dimension between 1 nm and 10 nm as measured after activation by heating to 750° C., in which a fluidised bed of the catalyst granules is produced in a reactor, the so-called growth reactor (30), and the carbon source is supplied continuously to the said growth reactor (30) in contact with the catalyst granules under conditions suitable for ensuring the fluidisation of the bed of catalyst granules, the decomposition reaction and the formation of nanotubes, characterised in that:

[0015] the catalyst granules of each catalyst are produced beforehand by deposition of metallic particles on support granules in a fluidised bed of support granules formed in a reactor, the so-called deposition reactor (20) supplied with at least one precursor capable of forming the metallic particles, and in such a way as to obtain catalyst granules comprising a proportion by weight of the metallic particles of between 1% and 5%,

[0016] the catalyst granules are then placed in the growth reactor (30) without coming into contact with the external atmosphere, which is followed by the formation of the fluidised bed of the catalyst granules and the formation of nanotubes in the growth reactor (30).

[0017] The inventors have surprisingly found that, contrary to the teaching of U.S. Pat. No. 4,650,657 and U.S. Pat. No. 4,767,737, the use of one fluidised bed to produce the catalyst(s) and of another fluidised bed to produce the nanotubes, without the catalyst(s) coming into contact with the atmosphere, under the conditions of the invention, not only does not result in the fragmentation of the carbon-containing products growing on the granules, but on the contrary enables ordered carbon nanotubes of very homogeneous dimensions (varying only slightly around the mean value) to be selectively produced, and in a yield of more than 80% by weight with respect to the carbon source.

[0018] The catalyst is not subjected to any atmospheric pollution, and in particular is not oxidised between its preparation and its use in the growth reactor.

[0019] Advantageously and according to the invention, the deposition reactor and the growth reactor are separate. Advantageously and according to the invention, the deposition reactor and the growth reactor are connected by at least one airtight line and the growth reactor is supplied with catalyst granules through this line. As a variant, the granules of the catalyst may be recovered and transferred from the deposition reactor under an inert atmosphere. Advantageously and according to the invention, the catalyst granules are produced by chemical deposition in the vapour phase.

[0020] According to another possible variant of the invention, one and the same reactor may be used both as deposition reactor and as growth reactor. In other words, the two stages of preparation of the catalyst granules (deposition) followed by production of the carbon nanotubes (growth) may be carried out successively in one and the same reactor by modifying the gases and reactants at the inlet of the reactor as well as the operating parameters between the two stages.

[0021] Advantageously and according to the invention, the fluidised bed of the catalyst granules is formed in a cylindrical growth reactor of diameter greater than 2 cm and having a wall height capable of containing 10 to 20 times the volume of the initial non-fluidised bed of the catalyst granules as determined in the absence of any gaseous feed. Such a reactor enables a true fluidised bed to be formed.

[0022] Advantageously and according to the invention, a fluidised bed of the catalyst granules is formed under a bubbling regime at least substantially free of leakage.

[0023] Furthermore, advantageously and according to the invention, in order to form the fluidised bed of catalyst granules:

[0024] a bed of catalyst granules is formed at the bottom of the growth reactor,

[0025] the growth reactor is fed underneath the bed of catalyst granules with at least one gas whose velocity is greater than the minimal velocity of fluidisation of the bed of catalyst granules and less than the minimal velocity of occurrence of a plunger-type regime.

[0026] Advantageously and according to the invention, in order to form the fluidised bed of the catalyst granules, the growth reactor is fed underneath the catalyst granules with the carbon source in the gaseous state, and with at least one neutral carrier gas.

[0027] More particularly, advantageously and according to the invention, the growth reactor is fed with at least one carbon-containing precursor forming the carbon source, at least one reactive gas, and at least one neutral gas, which are mixed before being introduced into the growth reactor. The term “reactive gas” is understood to denote a gas such as hydrogen that is capable of participating in and promoting the production of nanotubes.

[0028] Advantageously and according to the invention, the source of carbon comprises at least one carbon-containing precursor selected from hydrocarbons. Among the hydrocarbons that may advantageously be used, there may be mentioned ethylene and methane. As a variant or in combination, there may however also be used an oxide of carbon, in particular carbon monoxide.

[0029] Advantageously and according to the invention, the molar ratio of the reactive gas(es) to the carbon-containing precursor(s) is greater than 0.5 and less than 10, and in particular is of the order of 3.

[0030] Advantageously and according to the invention, the growth reactor (30) is fed at a flow rate of carbon-containing precursor(s) of between 5% and 80%, in particular of the order of 25%, of the total gas flow rate.

[0031] Advantageously and according to the invention the fluidised bed is heated to a temperature between 600° C. and 800° C.

[0032] The invention also covers a catalytic granular composition suitable for the implementation of a production process according to the invention.

[0033] The invention thus relates to a catalytic granular composition comprising metallic particles containing at least one transition metal carried by granules of solid support, so-called catalyst granules, characterised in that:

[0034] the catalyst granules are capable of being able to form a fluidised bed,

[0035] the proportion by weight of metallic particles is between 1% and 5%,

[0036] the metallic particles have a mean particle dimension of between 1 nm and 10 nm as measured after heating at 750° C.

[0037] Throughout the text the expression “mean dimension” of the particles or granules denotes the mean value (maximum of the distribution curve of the dimensions of the particles or granules) of the dimensions of all the particle or granules as determined by conventional granulometry, in particular by the sedimentation rate, before use. The term “dimension” used in isolation denotes, for a given particle or a given granule, its largest real dimension as determined for example by static measurements obtained by observations with a scanning or transmission electron microscope, also before use.

[0038] As regards the metallic particles, the values of the dimension or of the mean dimension that are given throughout the text are those measured before use for the production of the nanotubes, but after heating the catalytic composition to 750° C. The inventors have in fact found that the dimensions of the particles before heating are not, in general, capable of analysis, the particles being invisible under a microscope. This operation is effected by contact with a neutral atmosphere, for example helium and/or nitrogen, at 750° C., for a sufficient time in order to obtain stable values of dimensions. This time is in practice very low (of the order of a minute or a few minutes). The activation may be effected in a fluidised bed (in the fluidised bed of the catalyst granules before feeding the carbon source) or in any other way, for example in a fixed bed. Furthermore the temperature of 750° C. should be regarded solely as a value for the measurement of the size of the particles and does not correspond to a temperature value that should necessarily be used in a process according to the invention or in order to obtain a catalytic composition according to the invention (even if this value may advantageously be that used in certain embodiments of the invention). In other words, it enables the invention to be characterised uniquely by dimensional criteria, although a catalytic composition not subjected to this specific temperature may also be in accordance with the invention.

[0039] Advantageously the catalytic granular composition according to the invention is characterised in that the mean dimension of the metallic particles is between 2 nm and 8 nm, in particular of the order of 4 to 5 nm, and in that for at least 97% by number of the metallic particles, the difference between their dimension and the mean dimension of the metallic particles is less than or equal to 5 nm, and in particular is of the order of 3 nm.

[0040] The catalytic granular composition may comprise a small amount of metallic particles of dimension very much greater than the mean dimension (typically more than 200% of the mean dimension). Nevertheless, advantageously and according to the invention the dimension of the metallic particles is less than 50 nm as measured before use and installation in the fluidised bed, and after activation at 750° C.

[0041] Advantageously and according to the invention, the metallic particles consist in an amount of at least 98 wt. % of at least one transition metal and are substantially free of non-metallic elements other than traces of carbon and/or oxygen and/or hydrogen and/or nitrogen. Several different transition metal may be used in order to be deposited on the support granules. Likewise, several different catalytic compositions according to the invention (whose support granules and/or metallic particles have distinct characteristics) may be used as a mixture. The traces of impurity may derive from the preparation process of the metallic particles. Apart from these traces, the 2% maximum remaining amount may comprise one or more metallic elements other than a transition metal. Preferably, advantageously and according to the invention, the metallic particles consist of a pure metallic deposit of at least one transition metal, with the exception of traces of impurity. Advantageously and according to the invention, the proportion by weight of metallic particles, in particular of iron, is between 1.5% and 4%.

[0042] Advantageously and according to the invention, the catalyst granules have a mean dimension between 10&mgr; and 1000&mgr;. Advantageously and according to the invention, the difference between the dimension of the catalyst granules and the mean dimension of the catalyst granules is less than 50% of the value of the said mean dimension.

[0043] It has been found in fact that these dimensional distributions of the metallic particles and of the granules enable excellent results to be obtained within the context of a fluidised bed.

[0044] Furthermore, advantageously and according to the invention, the support has a specific surface greater than 10 m2/g.

[0045] Advantageously and according to the invention, the support is a porous material having a mean pore size greater than the mean dimension of the metallic particles. Advantageously and according to the invention, the support is a mesoporous material, the pores having a mean size of less than 50 nm. Advantageously and according to the invention, the support is chosen from alumina (Al2O3), an activated carbon, silica, a silicate, magnesia (MgO), titanium dioxide (TiO2), zirconia (ZrO2), a zeolite or a mixture of granules of several of these materials.

[0046] In particular, in the case where the carbon source is ethylene, advantageously and according to the invention the metallic particles consist of pure iron deposited in the dispersed state on granules of alumina.

[0047] Advantageously, in a process for the production of nanotubes according to the invention, the catalyst granules are produced beforehand by chemical deposition in the vapour phase of the metallic particles on the support granules in a fluidised bed of the support granules fed with at least one precursor capable of forming the metallic particles.

[0048] The invention also covers a process for the preparation of a catalytic granular composition according to the invention.

[0049] The invention thus relates to a process for the preparation of a catalytic granular composition comprising metallic particles containing at least one transition metal carried on solid support granules, so-called catalyst granules, in which a chemical deposition in the vapour phase of the metallic particles on the support granules is performed in the vapour phase, characterised in that the deposition, particularly in the form of a chemical deposition, of the metallic particles on the support granules is carried out in a fluidised bed of the support granules fed with at least one precursor capable of forming the said metallic particles, and in that the support granules are chosen and the parameters of the deposition are adjusted so that:

[0050] the catalyst granules are capable of being able to form a fluidised bed,

[0051] the proportion by weight of the metallic particles is between 1% and 5%,

[0052] the metallic particles have a mean particle dimension between 1 nm and 10 nm as measured after heating to 750° C.

[0053] Advantageously and according to the invention the deposition is carried out at a temperature between 200° C. and 300° C.

[0054] Advantageously and according to the invention the fluidised bed of the support granules is fed with at least one organometallic precursor, in particular Fe(CO)5.

[0055] Advantageously and according to the invention, the precursor(s) in the vapour state is/are continuously diluted in a gaseous mixture that is supplied continuously to a deposition reactor under conditions capable of ensuring the fluidisation of the support granules. Thus, advantageously and according to the invention, the fluidised bed is fed continuously with precursor(s). Advantageously and according to the invention, the gaseous mixture comprises a neutral gas and at least one reactive gas. Advantageously and according to the invention, steam (water vapour) is used as reactive gas. Between 200° C. and 300° C. the steam in fact enables the precursor Fe(CO)5 to be decomposed, releasing atoms of Fe. In addition all manifestations of fritting and agglomeration of the metallic catalyst into excessively large metallic particles is avoided.

[0056] The invention also relates to a process for the production of nanotubes, a catalytic granular composition and a process for the preparation of a catalytic granular composition, characterised by a combination of all or some of the characteristics mentioned hereinbelow or hereinafter.

[0057] Other objects, advantages and characteristics of the invention are disclosed in the following description and examples, which refer to the accompanying drawings in which:

[0058] FIG. 1 is a diagram of a first variant of an installation for implementing a process for producing nanotubes according to the invention,

[0059] FIG. 2 is a diagram of a second variant of an installation of a process for producing nanotubes according to the invention,

[0060] FIG. 3 is a histogram of the dimensions of the metallic particles of a catalytic composition according to the invention obtained in Example 5,

[0061] FIGS. 4 and 5 are micrographs of the nanotubes obtained according to the invention as described in Example 9.

[0062] FIG. 1 is a diagram of an installation enabling a process for producing nanotubes according to the invention to be implemented. This installation comprises two reactors: a reactor, so-called deposition reactor 20, for the synthesis of the catalyst, and a reactor, so-called growth reactor 30, for the preparation of the nanotubes.

[0063] The deposition reactor 20 for the synthesis of the catalyst by chemical deposition in the vapour phase (CVD) comprises a glass sublimator 1 to which is added the organometallic precursor. This sublimator comprises a fritted plate and may be heated to the desired temperature by a heating bath 2.

[0064] The neutral carrier gas 3, for example helium, which entrains the vapours of the organometallic precursor that is used is stored in a cylinder and introduced into the sublimator 1 with the aid of a flow regulator (not shown).

[0065] The sublimator 1 is connected to a lower glass compartment 4 that comprises a fritted plate into which is introduced steam that serves to activate the decomposition of the organometallic precursor. The presence of steam enables a very active catalyst to be obtained. This compartment 4 comprises a double jacket that is thermostatically controlled at a temperature that may be adjusted by means of a temperature regulator (not shown). The steam is entrained by a neutral carrier gas 5, for example nitrogen, stored in a cylinder and added to the compartment 4 with the aid of a flow regulator (not shown). A feed of neutral carrier gas 6, for example nitrogen, is intended to adjust the flow rates to those prevailing under the fluidisation conditions. This carrier gas 6 is stored in a cylinder and added to the compartment 4 by means of a flow regulator (not shown).

[0066] The upper part of the compartment 4 is connected in a gas-tight manner to a glass fluidisation column 7 of 5 cm diameter, which is equipped at its base with a gas distributor. This double jacket column 7 is thermostatically controlled to a temperature that may be adjusted by means of a temperature regulator 8.

[0067] The upper part of the column 7 is connected to a vacuum pump 9 via a trap in order to retain the released decomposition gases.

[0068] The procedure for implementing the examples relating to the preparation of the catalysts by CVD is as follows: A mass Ma of precursor is added to the sublimator 1.

[0069] A mass Ms of support granules Ms is added to the column 7 and a mass Me of water is added to the compartment 4 by means of a syringe. A vacuum is applied to the arrangement consisting of the compartment 4 and the column 7. The temperature of the bed is raised to T1.

[0070] The sublimator 1 is heated to the temperature Ts and the pressure is fixed at the value Pa in the whole apparatus by introducing carrier gases 3, 5 and 6 (total flow rate Q). The deposition then starts and lasts for a time tc.

[0071] At the end of the deposition, the temperature is restored to ambient temperature by slow cooling and the vacuum pump 9 is switched off. Once the system has returned to ambient temperature and atmospheric pressure, the catalytic granular composition is removed from the column 7 under an inert gas atmosphere (for example nitrogen); it is then ready to be used for the production of the nanotubes.

[0072] Two variants of the growth reactor 30, of different diameters, were used in the examples for growing the nanotubes.

[0073] In the first variant, shown in FIG. 1, the growth reactor 30 consists of a quartz fluidisation column (2.5 cm diameter) 10 equipped in its middle part with a distributing plate (quartz fritted plate) 11 on which is placed the powder of the catalytic granular composition. The column 10 may be heated to the desired temperature by means of an external heater 12 that can slide vertically along the fluidisation column 10. In the procedure that is employed this heater 12 is arranged either at an upper position, where it does not heat the fluidised bed, or at a lower position, where it heats the bed. The gases 13 (neutral gas such as helium, carbon source, and hydrogen) are stored in cylinders and are added to the fluidisation column by means of flow regulators 14.

[0074] In the upper part, the fluidisation column 10 is connected in a gas-tight manner to a trap 15 intended to collect any fine particles of catalytic granular composition or a mixture of catalytic granular composition and nanotubes.

[0075] The height of the column 10 is adapted so as to contain, during operation, the fluidised bed of the catalyst granules. In particular, the height is at least equal to 10 to 20 times the initial height of the bed of catalyst granules measured in the absence of the gaseous feed, and should correspond to the heated zone. In the examples a column 10 to 70 cm in total height is chosen, which is heated over 60 cm of its height by the heater 12.

[0076] In the second variant (not shown) the growth reactor consists of a stainless steel fluidisation column (5 cm diameter and 1 m total height, heated over the whole height) provided at its base with a distributor plate (stainless steel) on which is placed the catalyst powder. The column may be heated to the desired temperature by means of two fixed heaters and the said desired temperature is controlled by a thermocouple dipping into the fluidised bed. The gases (neutral gas, carbon source and hydrogen) are stored in cylinders and are fed to the fluidisation column by means of flow regulators.

[0077] FIG. 2 shows a variant of a process according to the invention in which the catalytic granular composition is prepared, according to the invention, continuously in the deposition reactor 20, removed continuously from this deposition reactor 20 through a line 25a via which it is introduced to an intermediate buffer reservoir 26, from which it is fed continuously, through a line 25b, to the growth reactor 30 where the nanotubes are produced. The deposition reactor 20 is fed continuously with support granules through a line 19 from a reservoir 18. The powder of catalyst granules on which the nanotubes are attached is removed continuously from the growth reactor 30 through an extraction line 27 that terminates in a buffer reservoir 28. The nanotubes may then be separated from the support granules and metallic particles in a known manner, following which they are stored in a storage reservoir 29.

[0078] In the variants shown in the Figures, a growth reactor 30 different from the deposition reactor 20 is employed. By way of variation (not shown), the deposition reactor 20 may then be used for growing the nanotubes in a subsequent stage. However, this latter variant means that the two stages have to be successively carried out with different operating parameters, and there is a risk of interference in the growth reaction, particularly in its initial phase, due to residual byproducts from the deposition phase.

[0079] The procedure for implementing the examples relating to the production of nanotubes according to the invention is as follows:

[0080] a mass Mc of catalyst (catalytic granular composition according to the invention) is added to the fluidisation column 10 under an inert gas atmosphere.

[0081] With the heater 12 in its low position with respect to the catalytic bed, its temperature is raised to the desired value Tn for the synthesis of the nanotubes, either under an inert gas atmosphere or under an atmosphere of a mixture of inert gas and hydrogen (reactive gas).

[0082] When this temperature is reached, the carbon source, the hydrogen and a neutral gas supplement are added to the column 10. The overall flow rate QT ensures a bubbling regime of the bed at the temperature Tn, without any leakage.

[0083] The growth of the nanotubes then starts and lasts for a time tn.

[0084] At the end of the growth stage the heater 12 is placed in the high position with respect to the catalytic bed, the gas flow rates corresponding to the carbon source and hydrogen are stopped, and the temperature is slowly restored to ambient temperature.

[0085] The procedure is similar in the case of reactors with fixed heaters.

[0086] The carbon nanotubes associated with the metallic particles and fixed to the support granules are removed from the growth reactor 30 and stored without any particular precautions. The carbon nanotubes may then be separated from the metallic particles and support granules so that they can be obtained in the pure state, for example by dissolution with acid as described in WO 01/94260.

[0087] The amount of carbon deposited is measured by weighing and by gravimetric thermal analysis.

[0088] The nanotubes produced in this way are analysed by transmission electron microscopy (TEM) and scanning electron microscopy (SEN) for the size and dispersion measurements, and by X-ray crystallography and Raman spectroscopy in order to evaluate the crystallinity of the nanotubes.

EXAMPLES

[0089] Preparation of the Catalysts

Comparison Example 1

[0090] A catalyst containing 2.6% Fe/Al2O3 is prepared by a known method of liquid impregnation of metallic salts. The iron precursor is hydrated iron nitrate Fe(NO3)3,9H2O. The support granules of alumina have a mean grain size of 120&mgr;, a density of 1.19 g/cm3 and a specific surface of 155 m2/g. The carrier gas is nitrogen. The implementation of the preparation of the catalyst is as follows:

[0091] The support is a mesoporous alumina. 100 g of this support are dehydrated in vacuo for 120 minutes. The appropriate amount of salt in order to obtain 2.6% Fe/Al2O3 is contacted with the alumina in 250 cm3 of deaerated ethanol. After 3 hours' contact time, the solvent is evaporated and the catalyst is dried overnight under reduced pressure (0.1 Torr). The catalyst is then calcined at 500° C. for 2 hours, following which it is reduced by a mixture of nitrogen and hydrogen (80/20 by volume) for 2 hours at 650° C.

[0092] The product obtained has a mean dimension of the metallic particles equal to 13 nm and the variation of the dimensions of the metallic particles with respect to this value is, for at least 98% of the particles, at most of the order of 11 nm.

Example 2

[0093] A catalyst containing 2.6% Fe/Al2O3 is prepared in accordance with the process according to the invention, in the deposition reactor 20, as described hereinbefore but without using water to activate the decomposition of the precursor. The organometallic precursor used is the complex Fe(CO)5, while the support granules and the carrier gas that are used are the same as in Example 1. The various parameters are adjusted as follows:

[0094] Ma=9.11 g,

[0095] Ms=100 g,

[0096] Tl=220° C.,

[0097] Pa=22 Torr,

[0098] Ts=35° C.,

[0099] Q=82 cm3/min,

[0100] tc=15 min.

[0101] The product obtained (catalytic granular composition according to the invention) comprises metallic particles deposited on the granules. The dimension of the metallic particles after heating under nitrogen at 750° C. for 5 minutes is equal to 4 nm, and the variation of the dimensions of the metallic particles with respect to this value is, for at least 97% of the particles, at most of the order of 3.5 nm.

Example 3

[0102] A catalyst containing 1.3% of Fe/Al2O3 is prepared according to the invention. The carrier gas is nitrogen. The organometallic precursor, the support granules and the carrier gas used are the same as in Example 2. The various parameters are adjusted as follows:

[0103] Ma=7.12 g,

[0104] Ms=150 g,

[0105] Me=10 g,

[0106] Tl=220° C.,

[0107] Pa=26 Torr,

[0108] Ts=35° C.,

[0109] Q=82 cm3/min,

[0110] tc=7 min.

[0111] The product obtained has a mean dimension of the particles equal to 3 nm and the variation of the dimensions of the metallic particles with respect to this value is, for at least 98% of the particles, at most of the order of 2.5 nm.

Example 4

[0112] This example relates to the preparation of a catalyst containing 2.5% Fe/Al2O3. The organometallic precursor, the support granules and the carrier gas used are the same as in Example 2. The various parameters are adjusted as follows:

[0113] Ma=17.95 g,

[0114] Ms=200 g,

[0115] Me=25 g,

[0116] Tl=220° C.,

[0117] Pa=20 Torr,

[0118] Ts=35° C.,

[0119] Q=82 cm3/min,

[0120] tc=18 min.

[0121] The product obtained has a mean dimension of the metallic particles equal to 4 nm and the variation of the dimensions of the metallic particles with respect to this value is, for at least 98% of the particles, at most of the order of 3.5 nm.

Example 5

[0122] This example relates to the preparation of a catalyst containing 3.5% Fe/Al2O3. The organometallic precursor, the support granules and the carrier gas used are the same as in Example 2. The various parameters are adjusted as follows:

[0123] Ma=12.27 g,

[0124] Ms=100 g,

[0125] Me=25 g,

[0126] Tl=220° C.,

[0127] Pa=24 Torr,

[0128] Ts=35° C.,

[0129] Q=82 cm3/min,

[0130] tc=20 min.

[0131] The product obtained has a mean dimension of the particles equal to 5 nm and the variation of the dimensions of the metallic particles with respect to this value is, for at least 98% of the particles, at most of the order of 4.5 nm. A histogram of the sizes of particles is given in FIG. 3.

[0132] In this figure the mean dimension of the particles is plotted along the x axis and their number is plotted along the y axis.

Example 6

[0133] This example relates to the preparation of a catalyst containing 5.65% Fe/Al2O3. The organometallic precursor, the support granules and the carrier gas used are the same as in Example 2. The various parameters are adjusted as follows:

[0134] Ma=9.89 g,

[0135] Ms=100 g,

[0136] Me=15 g,

[0137] Tl=220° C.,

[0138] Pa=23 Torr,

[0139] Ts=35° C.,

[0140] Q=82 cm3/min,

[0141] tc=23 min.

[0142] The product obtained has a mean dimension of the particles equal to 6 nm and the variation of the dimensions of the metallic particles with respect to this value is, for at least 98% of the particles, at most of the order of 5.5 nm.

[0143] The results of Examples 1 to 6 are summarised in the following Table I. 1 TABLE I Size of the Metallic Example Precursor Method % Fe Particles (nm) 1 Fe(NO3)3, 9H2O impregnation 2.6 13 ± 11   2 Fe(CO)5  CVD* 2.6 4.5 ± 4    3 Fe(CO)5 CVD 1.3 3 ± 2.5 4 Fe(CO)5 CVD 2.5 4 ± 3.5 5 Fe(CO)5 CVD 3.50 5 ± 4.5 6 Fe(CO)5 CVD 5.65 6 ± 5.5 *Preparation without addition of water

[0144] Production of the Nanotubes

Comparison Example 7

[0145] Multi-walled nanotubes are produced using the catalyst of comparison example 1 containing 2.6% Fe/Al2O3. In this test the amount of catalyst was intentionally reduced so as not to obtain large yields and, more particularly, so as to be better able to determine the influence of the method of preparation of the catalyst. The various parameters are adjusted as follows:

[0146] Mc=5 g,

[0147] Tn=750° C.

[0148] QT=320 cm3/min,

[0149] Amount of carbon added=3 g,

[0150] tn=60 min.

[0151] Under these conditions, the amount of carbon deposited is 0.16 g, which should be compared with the result obtained in test 5 of Example 12 (same percentage of iron and identical conditions), namely 1.57 g. The height of the bed remains substantially the same, whereas it changes from about 1 cm to 8.7 cm in test 5 of Example 12. The SEM and TEM analyses show that the multi-walled nanotubes comprise only a part of the deposit and that the encapsulated particles are in this case extremely numerous. Thus, only a catalyst composition according to the invention permits the selective production of multi-walled nanotubes of homogeneous mean dimensions.

Example 8

[0152] Multi-walled nanotubes are produced using the catalyst of Example 2 containing 2.6% Fe/Al2O3 prepared without using water to activate the decomposition of the precursor. In this test, the amount of catalyst was intentionally reduced so as not to obtain high yields, and more specifically so as to be better able to determine the influence of the activation of the catalyst by water. The various parameters are adjusted as follows:

[0153] Mc=5 g,

[0154] Tn=750° C.

[0155] QT=320 cm3/min,

[0156] Amount of carbon added=3 g,

[0157] tn=60 min.

[0158] Under these conditions, the amount of carbon deposited is 0.88 g, which should be compared with the result obtained in test 5 of Example 12 (same percentage of iron and identical conditions except for the addition of water), namely 1.57 g.

[0159] The activation of the catalyst by water thus promotes a high yield of nanotubes.

[0160] The TEM and SEM analyses show that the multi-walled nanotubes constitute the only product of the deposition reaction.

Example 9

[0161] Nanotubes are produced using the catalyst of Example 4 containing 2.5% Fe/Al2O3 and ethylene, and using a stainless steel reactor of 5 cm internal diameter. Five tests were carried out under the same conditions so as to verify the reproducibility of the results.

[0162] The various parameters are adjusted as follows:

[0163] Mc=100 g,

[0164] Tn=650° C.

[0165] QT=1200 cm3/min,

[0166] Amount of carbon added=30 g,

[0167] tn=120 min.

[0168] Under these conditions, the amount of carbon deposited is 27±0.2 g in all the tests carried out, i.e. a yield of 90% with respect to the added carbon. The SEM and TEM analyses show that the multi-walled nanotubes constitute the only product of the reaction. The pyrolytic carbon or the encapsulated metal particles are largely absent from the deposit. TEM micrographs of the nanotubes obtained are shown in FIGS. 4 and 5. In FIG. 4 the scale represented by the continuous line is 400 nm. In FIG. 5 the scale represented by the continuous line is 20 nm. The external diameter of the nanotubes is 20±5 nm and their internal diameter is 4±2 nm, which corresponds substantially to the mean dimension of the metallic particles. The X-ray and Raman analyses of the nanotubes obtained show the good degree of graphitisation of the latter; this can also be seen in FIG. 5, where the planes of the graphite can be observed.

Example 10

[0169] Nanotubes are produced using the catalyst of Example 4 containing 2.5% Fe/Al2O3 and ethylene, and using a stainless steel reactor of 5 cm internal diameter.

[0170] The various parameters are adjusted as follows:

[0171] Mc=100 g,

[0172] Tn=650° C.

[0173] QT=1200 cm3/min,

[0174] Amount of carbon added=45 g,

[0175] tn=180 min.

[0176] Under these conditions, the amount of carbon deposited is 44 g, i.e. a yield of 97% with respect to the added carbon. The SEM and TEM analyses show that the multi-walled nanotubes constitute the only product of the reaction.

Example 11

[0177] A series of tests was carried out in a reactor of 2.5 cm diameter so as to investigate the influence of the amount of metal on the preparation of multi-walled nanotubes using the catalysts of Examples 3 to 6 and a catalyst containing 0.5% of iron prepared in a similar manner, and with ethylene as carbon source. In these tests the amount of catalyst was intentionally reduced so as not to obtain large yields, and specifically so as to be able better to determine the influence of the amount of metal.

[0178] The various parameters are adjusted as follows:

[0179] Mc=5 g,

[0180] Tn=750° C.

[0181] QT=320 cm3/min,

[0182] Amount of carbon added=3 g,

[0183] tn=60 min.

[0184] The tests 1 to 5 of this example are summarised in the following Table II. 2 TABLE II Height of Bed Deposited after Deposition Test % Fe Carbon (g) (cm) TEM Observation 1 0.5 0.52 3.2 multi-walled nanotubes 1 1.3 1.13 4 multi-walled nanotubes 2 2.5 1.90 6.2 multi-walled nanotubes 3 3.5 2.29 8.6 multi-walled nanotubes 4 5.65 1.37 3 nanotubes + particles of encapsulated iron

[0185] The TEM and SEM analyses show that the multi-walled nanotubes constitute the only product or virtually the only product of the deposition reaction. The pyrolytic carbon or the particles of encapsulated metal are particularly absent in tests 1 to 5. In test 1, since the concentration of iron is low (0.5%) the yield is greatly affected. In test 5, since the concentration of iron is high the size of the iron particles is large and the formation of particles of encapsulated iron can be seen.

Example 12

[0186] A series of tests was carried out in a reactor of 2.5 cm diameter so as to investigate the influence of the temperature on the preparation of multi-walled nanotubes using the catalyst of Example 4 containing 2.5% Fe/Al2O3 and ethylene as carbon source. In these tests the amount of catalyst was intentionally reduced so as not to obtain high yields, and so as to be better able to determine the influence of the temperature.

[0187] The various parameters are adjusted as follows:

[0188] Mc=5 g,

[0189] Tn=variable from 500 to 850° C.

[0190] QT=320 cm3/min,

[0191] Amount of carbon added=3 g,

[0192] tn=60 min.

[0193] The tests 1 to 6 of this example are summarised in Table III. 3 TABLE III Deposited Height of Bed Temp. Carbon after Deposition Test (°C.) (g) (cm) TEM Observation 1 500 0.05 1.9 multi-walled nanotubes 2 600 1.05 4.4 multi-walled nanotubes 3 650 1.13 5.5 multi-walled nanotubes 4 700 1.29 4.7 multi-walled nanotubes 5 750 1.57 8.7 multi-walled nanotubes 6 850 1.86 4.7 nanotubes + pyrolytic carbon + particles of encapsulated iron

[0194] The TEM and SEM analyses show that the multi-walled nanotubes constitute the only product or virtually the only product of the deposition reaction. The pyrolytic carbon or the particles of encapsulated metal are particularly absent in tests 1 to 5. In test 1, the temperature is too low for the reaction to proceed properly. In test 6, the temperature is too high and a thermal decomposition of the ethylene leads to the formation of pyrolytic carbon.

Example 13

[0195] This example relates to the preparation of nanotubes using the catalyst of Example 4 containing 2.5% Fe/Al2O3 and ethylene, and using a stainless steel growth reactor of 5 cm internal diameter.

[0196] The various parameters are adjusted as follows:

[0197] Mc=100 g,

[0198] Tn=650° C.,

[0199] QT=1405 cm3/min,

[0200] Amount of carbon added=48.5 g,

[0201] tn=120 min.

[0202] Under these conditions, the amount of carbon deposited is 46.2 g, i.e. a yield of 95% with respect to the added carbon. The TEM and SEM analyses show that the multi-walled nanotubes constitute the only product of the reaction.

Example 14

[0203] This example relates to the preparation of nanotubes using a catalyst containing 0.5% Fe/Al2O3 prepared according to the procedure described in Example 4 and ethylene, and using a stainless steel growth reactor of 5 cm internal diameter.

[0204] The various parameters are adjusted as follows:

[0205] Mc=100 g,

[0206] Tn=650° C.,

[0207] QT=1405 cm3/min,

[0208] Amount of carbon added=48.5 g,

[0209] tn=120 min.

[0210] Under these conditions, the amount of carbon deposited is 20.4 g, i.e. a yield of 42% with respect to the added carbon. The TEM and SEM analyses show that the multi-walled nanotubes constitute the only product of the reaction. This example confirms the poor performances of the catalyst containing 0.5% of iron.

Example 15

[0211] This example relates to the purification of nanotubes produced using a catalyst containing 2.5% Fe/Al2O3 and ethylene, and using a stainless steel growth reactor of 5 cm internal diameter according to the procedure described in Example 9. The solid powder leaving the reactor is added to a 2 l capacity flask in the presence of 500 ml of water and 500 ml of 98% sulfuric acid.

[0212] The various parameters are adjusted as follows:

[0213] M (nanotube powder+catalyst)=75 g,

[0214] V(H2O)=500 ml,

[0215] V(H2SO4, 98%)=500 ml,

[0216] T=140° C.,

[0217] tn=120 min.

[0218] After dissolving the alumina for 2 hours with acid, the solution is filtered, the nanotubes are washed several times with water and then dried in a stove. The dry product (thermogravimetric analysis) consists of 97% by weight of carbon nanotubes and 3% of iron.

Claims

1. A process for the selective production of ordered carbon nanotubes by decomposition of a source of carbon in the gaseous state in contact with at least one solid catalyst in the form of metallic particles comprising at least one transition metal carried on granules of solid support, so-called catalyst granules, capable of being able to form a fluidised bed, the metallic particles having a mean dimension between 1 nm and 10 nm as measured after activation by heating to 750° C., in which a fluidised bed of the catalyst granules is formed in a reactor, the so-called growth reactor (30), and the carbon source is added continuously to the growth reactor (30) in contact with the catalyst granules under conditions capable of ensuring the fluidisation of the bed of catalyst granules, the decomposition reaction and the formation of nanotubes, wherein:

the catalyst granules of each catalyst are produced beforehand by deposition of metallic particles on support granules in a fluidised bed of the support granules formed in a reactor, the so-called deposition reactor (20), fed with at least one precursor capable of forming the metallic particles, and so as to obtain catalyst granules comprising a proportion by weight of the metallic particles of between 1% and 5%,

the catalyst granules are then placed in the growth reactor (30) without contact with the external atmosphere, followed by the formation of the fluidised bed of the catalyst granules and the formation of nanotubes in the growth reactor (30).

2. A process as claimed in claim 1, wherein the catalyst granules are produced having a mean dimension of the metallic particles of between 2 nm and 8 nm, and in which, for at least 97% by number of the metallic particles, the difference between their dimension and the mean dimension of the metallic particles is less than or equal to 5 nm.

3. A process as claimed in claim 1, wherein the catalyst granules are produced with a mean dimension of the particles of the order of 4 nm to 5 nm, and in which, for at least 97% by number of the metallic particles, the difference between their dimension and the mean dimension of the metallic particles is of the order of 3 nm.

4. A process as claimed in claim 1, wherein the catalyst granules are produced with a dimension of the metallic particles of less than 50 nm.

5. A process as claimed in claim 1, wherein the fluidised bed is situated in the growth reactor (30) at a temperature between 600° C. and 800° C.

6. A process as claimed in claim 1, wherein the metallic particles consist in an amount of at least 98% by weight of at least one transition metal and are substantially free of non-metallic elements apart from traces of carbon and/or oxygen and/or hydrogen and/or nitrogen.

7. A process as claimed in claim 1, wherein the metallic particles consist of a pure metallic deposit of at least one transition metal.

8. A process as claimed in claim 1, wherein the catalyst granules are produced with a mean dimension between 10&mgr; and 1000&mgr;.

9. A process as claimed in claim 1, wherein the difference between the dimension of the catalyst granules and the mean dimension of the produced catalyst granules is less than 50% of the value of the said mean dimension.

10. A process as claimed in claim 1, wherein the support has a specific surface greater than 10 m2/g.

11. A process as claimed in claim 1, wherein the support is a porous material having a mean pore size greater than the mean dimension of the metallic particles.

12. A process as claimed in claim 1, wherein the support is chosen from alumina, an activated carbon, silica, a silicate, magnesia, titanium dioxide, zirconia, a zeolite or a mixture of granules of several of these materials.

13. A process as claimed in claim 1, wherein the metallic particles consist of pure iron deposited in the dispersed state on alumina granules.

14. A process as claimed in claim 1, wherein the deposition reactor (20) and the growth reactor (30) are different.

15. A process as claimed in claim 14, wherein the deposition reactor (20) and the growth reactor (30) are joined by at least one gas-tight line (25a, 26, 25b) and wherein the growth reactor (30) is fed with catalyst granules through this line (25).

16. A process as claimed in claim 1, wherein the catalyst granules are produced by chemical deposition in the vapour phase of the metallic particles on the support granules in a fluidised bed of the support granules in the deposition reactor (20).

17. A process as claimed in claim 1, wherein the deposition of the particles on the support granules is carried out at a temperature between 200° C. and 300° C.

18. A process as claimed in claim 1, wherein the fluidised bed of the support granules in the deposition reactor (20) is fed with at least one organometallic precursor.

19. A process as claimed in claim 18, wherein Fe(CO)5 is used as organometallic precursor.

20. A process as claimed in claim 1, wherein the precursor(s) is continuously diluted in the vapour phase in a gaseous mixture that is continuously fed to the deposition reactor (20) under conditions suitable for ensuring the fluidisation of the support granules.

21. A process as claimed in claim 20, wherein the gaseous mixture comprises a neutral gas and at least one reactive gas.

22. A process as claimed in claim 21, wherein steam is used as reactive gas.

23. A process as claimed in claim 1, wherein the fluidised bed of the catalyst granules is formed in a cylindrical growth reactor (30) of diameter greater than 2 cm and having a wall height capable of containing 10 to 20 times the volume of the initial, non-fluidised bed of the catalyst granules as measured in the absence of any gaseous feed.

24. A process as claimed in claim 1, wherein a fluidised bed of the catalyst granules is formed in the growth reactor (30) under a bubbling regime that is at least substantially free of leakage.

25. A process as claimed in claim 1, wherein in order to form the fluidised bed of catalyst granules in the growth reactor (30):

a bed of catalyst granules is formed in the bottom of the growth reactor (30),

the growth reactor (30) is fed from underneath the bed of catalyst granules with at least one gas whose velocity is greater than the minimum velocity of fluidisation of the bed of catalyst granules and less than the minimum velocity for the occurrence of a plunger-type régime.

26. A process as claimed in claim 1, wherein in order to form the fluidised bed of the catalyst granules in the growth reactor (30), the growth reactor (30) is fed from underneath the catalyst granules with the carbon source in the gaseous state and with at least one neutral carrier gas.

27. A process as claimed in claim 1, wherein the growth reactor is fed with at least one carbon-containing precursor forming the carbon source, with at least one reactive gas and with at least one neutral gas, which are mixed before being introduced into the growth reactor (30).

28. A process as claimed in claim 1, wherein the carbon source comprises at least one carbon-containing precursor chosen from hydrocarbons.

29. A process as claimed in claim 1, wherein the growth reactor (30) is fed with hydrogen as reactive gas.

30. A process as claimed in claim 27, wherein the molar ratio of the reactive gas(es) to the carbon-containing precursor(s) is greater than 0.5 and less than 10, and in particular is of the order of 3.

31. A process as claimed in claim 27, wherein the growth reactor (30) is fed at a flow rate of carbon-containing precursor(s) of between 5% and 80%, in particular of the order of 25%, of the overall gaseous flow rate.

32. A process for the preparation of a catalytic granular composition comprising metallic particles containing at least one transition metal carried on solid support granules, so-called catalyst granules, in which there is effected a chemical deposition in the vapour phase of the metallic particles on the support granules, wherein the deposition of the metallic particles on the support granules is carried out in a fluidised bed of the support granules fed with at least one precursor capable of forming the metallic particles, and wherein the support granules are chosen and the parameters of the deposition are adjusted so that:

the catalyst granules are capable of being able to form a fluidised bed,

the proportion by weight of the metallic particles is between 1% and 5%,

the metallic particles have a mean particle dimension between 1 nm and 10 nm as measured after activation by heating to 750° C.

33. A process as claimed in claim 32, wherein the deposition is carried out in the form of a chemical deposition in the vapour phase.

34. A process as claimed in claim 32, wherein the deposition is carried out at a temperature between 200° C. and 300° C.

35. A process as claimed in claim 32, wherein the fluidised bed of the support granules is fed with at least one organometallic precursor.

36. A process as claimed in claim 32, wherein Fe(CO)5 is used as organometallic precursor.

37. A process as claimed in claim 32, wherein the precursor(s) is continuously diluted in the vapour state in a gaseous mixture that is continuously fed to a deposition reactor (20) under conditions capable of ensuring the fluidisation of the support granules.

38. A process as claimed in claim 37, wherein the gaseous mixture comprises a neutral gas and at least one reactive gas.

39. A process as claimed in claim 38, wherein steam is used as reactive gas.