Patent Description:
Carbon nanostructures (CNSs) refer collectively to nanosized carbon structures having various shapes, such as nanotubes, nanohairs, fullerenes, nanocones, nanohorns, and nanorods. Carbon nanostructures can be widely utilized in a variety of technological applications because they possess excellent characteristics.

Carbon nanotubes (CNTs) are tubular materials consisting of carbon atoms arranged in a hexagonal pattern and have a diameter of approximately <NUM> to <NUM>. Carbon nanotubes exhibit insulating, conducting or semi-conducting properties depending on their inherent chirality. Carbon nanotubes have a structure in which carbon atoms are strongly covalently bonded to each other. Due to this structure, carbon nanotubes have a tensile strength approximately <NUM> times greater than that of steel, they are highly flexible and elastic, and are chemically stable.

Carbon nanotubes are divided into three types: single-walled carbon nanotubes (SWCNTs) consisting of a single sheet and having a diameter of about <NUM>; double-walled carbon nanotubes (DWCNTs) consisting of two sheets and having a diameter of about <NUM> to about <NUM>; and multi-walled carbon nanotubes (MWCNTs) consisting of three or more sheets and having a diameter of about <NUM> to about <NUM>.

Carbon nanotubes are being investigated for their commercialization and application in various industrial fields, for example, aerospace, fuel cell, composite material, biotechnology, pharmaceutical, electrical/electronic, and semiconductor industries, due to their high chemical stability, flexibility and elasticity.

Carbon nanotubes are generally produced by various techniques, such as arc discharge, laser ablation, and chemical vapor deposition. However, arc discharge and laser ablation are not appropriate for mass production of carbon nanotubes and require high arc production costs or expensive laser equipment. Catalytic Chemical Vapor Deposition (CCVD) of hydrocarbons over metallic catalysts provides, with respect to other methods, higher yields and quality and simplifies the manufacturing process on an industrial scale.

Most researches carried out in the CCVD technology are presently focused on developing new catalysts and reaction conditions for controlling the type (single, double or multi-walled), diameter, length and purity of carbon nanotubes. The structural, physical and chemical properties of carbon nanotubes have been related to their electrical conducting capacity, mechanical strength and thermal, optical and magnetic properties.

<CIT> discloses a large variety of metal oxide systems (such as Co, Fe, Ni, V, Mo and Cu) and catalyst supports (such as Al(OH)<NUM>, Ca(OH)<NUM>, Mg(OH)<NUM>, Ti(OH)<NUM>, Ce(OH)<NUM> and La(OH)<NUM>), for the single-walled and multi-walled carbon nanotube production. The various metals and mixtures of metals in this document were tested for their selectivity properties, i.e. the ability of the catalyst to selectively produce single, double or multi-walled nanotubes with respect to a certain proportion of amorphous carbon or fibers formed simultaneously during the reaction.

<CIT> discloses an impregnated supported catalyst and a carbon nanotube aggregate comprising the impregnated supported catalyst, said impregnated supported catalyst being prepared by sequentially adding a multicarboxylic acid and precursors of first (Co) and second (Fe, Ni) catalytic components to precursors of first (Mo) and second (V) active components to obtain a transparent aqueous metal solution, impregnating an aluminum-based granular support with the transparent aqueous metal solution, followed by drying and calcination, wherein the supported catalyst has a bulk density of <NUM> to <NUM>/cm<NUM>.

<CIT> discloses a catalyst for producing carbon nanotubes, comprising a support and a graphitization metal catalyst supported on the support wherein the graphitization metal catalyst is a multi-component metal catalyst comprising a main catalyst and an auxiliary catalyst, wherein the main catalyst is selected from Co, Fe, and mixtures thereof and the auxiliary catalyst is V, and wherein the catalyst is a supported catalyst obtained by calcining aluminum hydroxide at a primary calcination temperature of <NUM> to <NUM> to form the support, supporting a catalytic metal precursor on the support, and calcining the catalytic metal precursor supported on the support at a secondary calcination temperature of <NUM> to <NUM>.

<CIT> discloses a method for producing carbon nanotubes, comprising primarily calcining support precursor having a BET specific surface area of <NUM><NUM>/g or less at a temperature of <NUM> to <NUM> to form a support, supporting a graphitization metal catalyst on the support, secondarily calcining the catalyst supported on the support at a temperature of <NUM> to <NUM> to prepare the supported catalyst, and bringing the supported catalyst into contact with a carbon source in the gas phase to form carbon nanotubes, wherein the support precursor is aluminum trihydroxide and wherein the graphitization metal catalyst is a binary metal catalyst selected from Co/Mo, Co/V, Fe/Mo and Fe/V.

<CIT> discloses a method for synthesizing a supported catalyst with a view to the production of multi-walled carbon nanotubes comprising the following steps: mixing an Al(OH)<NUM> powder having a particle size lower than about <NUM> with an aqueous solution of an iron and cobalt salt, the whole forming a paste; drying said paste until a powder with a moisture level lower than about <NUM>% by weight is obtained; selecting the particle-size fraction of said supported catalyst that is lower than about <NUM>; and producing nanotubes using the supported catalyst having a particle size lower than about <NUM>.

<CIT> discloses a method for producing carbon nanotube aggregates comprising heat treating a carrier precursor comprising a layered metal hydroxide and a non-layered metal hydroxide to form a porous carrier; supporting a catalyst metal or a catalyst metal precursor on the carrier to form a supported catalyst; and forming a carbon nanotube aggregate in which the supported catalyst and the carbon-containing compound are brought into contact with each other under a heating region to form a bundle and an entangled carbon nanotube aggregate. The catalyst metal combines an element selected from iron, cobalt, and nickel, an element selected from titanium, vanadium, and chromium, and an element selected from molybdenum (Mo) and tungsten (W).

<CIT> discloses a method for producing a carbon nanotube aggregate, comprising:.

The graphitization catalyst is a catalyst containing only iron (Fe) or a binary or multi-component catalyst comprising one or more metals selected from cobalt (Co), molybdenum (Mo), and vanadium (V).

<CIT> discloses a method for producing a carbon nanotube aggregate, comprising calcining aluminum hydroxide at a primary calcination temperature of <NUM> to <NUM> to form a support; supporting a catalytic metal precursor on the support; calcining the catalyst-containing support at a secondary calcination temperature of <NUM> to <NUM> to obtain a supported catalyst; and bringing the supported catalyst into contact with a carbon-containing compound under heating to react with each other, wherein the primary calcination temperature, the secondary calcination temperature, the amount of the catalyst supported or the reaction time is controlled such that the carbon nanotube aggregate has a bulk density of <NUM>/m<NUM> or more. The catalytic metal comprises Fe, Co, Mo, V or a combination of two or more thereof. The graphitization metal catalyst may be a composite catalyst consisting of a main catalyst and an auxiliary catalyst. In this case, the main catalyst may include iron (Fe) or cobalt (Co) and the auxiliary catalyst may be molybdenum (Mo), vanadium (V) or a combination thereof. An organic acid is added in a molar ratio of <NUM>:<NUM> to <NUM>:<NUM> relative to the catalytic metal for the preparation of the supported catalyst.

Carbon nanotubes have attracted attention as potential electrode materials in lithium batteries.

Typical lithium-ion batteries utilize carbon anodes (negative electrode) and lithiated transition metal oxide cathodes (positive electrode) situated on opposite sides of a microporous polymer separator.

A lithium-ion cell begins life with all of the lithium in the cathode and upon charging, a percentage of this lithium is moved over to the anode and intercalated within the carbon anode.

A failure in lithium-ion batteries is the result of a formation of dendrites within the battery. Dendrites are microscopic metal deposits that can form within the cell. Dendrite formation generally begins in the anode and creates an internal shortcut when it extends through the separator to the cathode.

When iron impurities from any electrode dissolve in the electrolyte, there is a significant risk that these impurities migrate on the anode side and initiate dendrite growth by deposition. Because of this, iron-free materials are required as electrode material.

When using MWCNT's as electrode material, the risk of battery failure caused by those dendrites arises.

Consequently, MWCNT's comprising interstitial iron-components obtained by a process using a catalytic system comprising an iron-based graphitization catalyst should be avoided.

Therefore, a need exists for MWCNT, produced by a CCVD-process of hydrocarbons over iron-free metallic catalysts with improved selectivity and productivity.

The aim of the present invention is to disclose an iron-free catalyst for the preparation of MWCNT and a method for its preparation, as well as the use of those carbon nanotubes in batteries.

The present invention discloses an iron-free supported catalyst as defined in claim <NUM>.

The present invention further discloses a method for the production of the iron-free supported catalyst comprising the steps of:.

Preferred embodiments of the method for the production of iron-free supported catalyst of the present invention disclose one or more of the following features:.

The present invention further discloses a method for producing multi-walled carbon nanotubes from the iron-free supported catalyst, comprising the steps of:.

Preferred embodiments of the method for the production of multi-walled carbon nanotubes of the present invention disclose one or more of the following features:.

The present invention further discloses multi-walled carbon nanotubes obtained by the method for the production of multi-walled carbon nanotubes of the present invention, comprising between <NUM> and <NUM> % by weight, preferably <NUM> and <NUM> % by weight of the iron-free supported catalyst, said iron-free supported catalyst being obtained by the method for the preparation of iron-free supported catalyst of the present invention.

The present invention further discloses a polymer matrix comprising said multi-walled carbon nanotubes, obtained by the method of the invention.

The present invention further discloses the use of said multi-walled carbon nanotubes, obtained by the method of the invention, in batteries.

The present invention discloses a supported iron-free catalyst giving rise to increased selectivity in multi-walled nanotube production with specific characteristics, said improved multi-walled selectivity being obtained at a high yield while catalyst consumption is reduced. The present invention also discloses an economically attractive process for obtaining said supported catalyst.

By iron-free catalyst, the present invention means that the iron content is reduced as much as possible, with the exception of unavoidable traces. Nevertheless the iron content within the overall transition metal content is less than <NUM> ppm, preferably less than <NUM> ppm, more preferably less than <NUM> ppm, most preferably less than <NUM> ppm.

In a first embodiment of the present invention, the supported catalyst is an iron-free two-component catalyst comprising a first cobalt-based catalytic component and a second vanadium-based catalytic component, both preferably in the form of oxide, and supported on a support comprising aluminum oxide (Al<NUM>O<NUM>) and/or aluminum hydroxide (Al(OH)<NUM>) and aluminum oxide hydroxide (AlO(OH)) (further called the "support element").

In a second embodiment of the present invention, the supported catalyst is an iron-free three-component graphitization catalyst comprising a first cobalt-based catalytic component, a second vanadium-based catalytic component and a molybdenum-based catalytic component, all preferably in the form of oxide, and supported on a support comprising aluminum oxide and/or aluminum hydroxide and aluminum oxide hydroxide (further called the "support element").

Preferably, the support precursor is aluminum hydroxide, more preferably gibbsite or bayerite.

Preferably, the support precursor is characterized by a volume median particle diameter (D<NUM>) of less than <NUM> and a specific surface area of less than <NUM><NUM>/g.

Preferably, the support precursor is gibbsite, characterized by a specific surface area between <NUM> and <NUM><NUM>/g, preferably between <NUM> and <NUM><NUM>/g.

Preferably, the cobalt-based catalytic precursor of the graphitization catalyst is obtained from a cobalt-based precursor, said precursor being a cobalt salt, a cobalt oxide or a cobalt compound such as Co(NO<NUM>)<NUM>. <NUM><NUM>O; Co<NUM>(CO)<NUM> and Co(Oac)<NUM>. <NUM><NUM>O.

Preferably, the vanadium-based catalytic precursor of the graphitization catalyst is obtained from a vanadium-based precursor, said precursor being a vanadium salt, a vanadium oxide or a vanadium compound such as NH<NUM>VO<NUM>.

Preferably, the molybdenum-based catalytic precursor of the graphitization catalyst is obtained from a molybdenum-based precursor, said precursor being a molybdenum salt, a molybdenum oxide or a molybdenum compound such as (NH<NUM>)<NUM>Mo<NUM>O<NUM>. <NUM><NUM>O; Mo(CO)<NUM> or (NH<NUM>)<NUM>MoS<NUM>.

The present invention also discloses a method for the production of said supported catalyst comprising the steps of:.

The polycarboxylic acids used in the method of the present invention are preferably selected from the group consisting of dicarboxylic acids, tricarboxylic acids, tetracarboxylic acids and mixtures thereof. Examples of such multicarboxylic acids include oxalic acid, succinic acid, tartaric acid, malic acid, fumaric acid, itaconic acid, citraconic acid, mesaconic acid, citric acid, <NUM>-butene-<NUM>,<NUM>,<NUM>-tricarboxylic acid and <NUM>,<NUM>,<NUM>,<NUM>-butanetetracarboxylic acid.

By salts of polycarboxylic acid, the present invention means the polycarboxylic acid wherein at least one carboxylic acid group is converted into an ammonium or alkali metal salt.

Preferably, the polycarboxylic acid is citric acid or malic acid; preferably the polycarboxylic acid salt is the ammonium salt.

Preferably, one or more polycarboxylic acid(s) and/or salt(s) thereof are added in such an amount that the resulting aqueous solution comprises between <NUM> and <NUM>%, more preferably between <NUM> and <NUM> % of polycarboxylic acid(s) and/or salt(s).

Preferably, the polycarboxylic acid used in the method of the present invention is a blend of citric acid and malic acid wherein the mole ratio malic acid / citric acid is between <NUM> and <NUM>, preferably between <NUM> and <NUM>.

In the method of the first embodiment, <NUM> of support precursor is added to an aqueous mixture obtained from mixing an aqueous solution comprising <NUM> to <NUM> of vanadium-based precursor in between <NUM> to <NUM> of water and between <NUM> and <NUM> of cobalt-based precursor as a powder or as an aqueous mixture comprising up to <NUM> of water.

In the method of the second embodiment, <NUM> of support precursor is added to an aqueous mixture obtained from mixing an aqueous solution comprising <NUM> to <NUM> of vanadium-based precursor and <NUM> to <NUM> molybdenum precursor in between <NUM> to <NUM> of water and between <NUM> and <NUM> of cobalt-based precursor as a powder or as an aqueous mixture comprising up to <NUM> of water.

In the method according to the present invention:.

After both heating cycles, the support precursor is converted into a calcinated product, i.e. the support, comprising one or more components selected from the group consisting of hydroxides, oxide hydroxides and oxides while the catalyst precursors are converted into oxides, wherein the graphitization catalyst is preferably present as a mixed oxide.

The type of the heat source used in both heating cycles is not limited and may be, for example, induction heating, radiant heating, laser, IR, microwave, plasma, UV or surface plasmon heating.

The inventors have observed that the BET of the support precursor, Al(OH)<NUM>, is an important parameter for obtaining an iron-free supported catalyst enabling the production of MWCNT at a high carbon yield.

In the method according to the present invention the BET of the Al(OH)<NUM> support precursor is comprised between <NUM> and <NUM><NUM>/g, preferably between <NUM> and <NUM><NUM>/g.

The conversion of gibbsite to boehmite, studied by X-Ray Diffraction, is for example described by <NPL>.

The qualitative and quantitative analysis of aluminum oxide hydroxide (boehmite) in aluminum oxide (bauxite) by X-Ray Diffraction is for example described by <NPL>.

The inventors have experienced that the presence of aluminum oxide hydroxide in the iron-free supported catalyst, can be easily identified with certainty by X-Ray Diffraction, yet this quantification is subject to uncertainty and thus should be limited to an estimation of the weight percentage of AlO(OH) on the total of Al<NUM>O<NUM>, Al(OH)<NUM> and AlO(OH).

That aside, the inventors have observed that aluminum oxide hydroxide is present in an amount of at least <NUM>% by weight, preferably of at least <NUM>% by weight, more preferably of at least <NUM>% by weight, most preferably of at least <NUM>% by weight and even of at least <NUM>% by weight of the total of Al<NUM>O<NUM>, Al(OH)<NUM> and AlO(OH).

In the method of the present invention, the first heating cycle, intended to dry the paste, may be replaced by alternative drying methods well known in the art, or combinations thereof. Among these, flash drying or spray drying are widely used.

A typical supported catalyst according to the present invention is represented by the formula (CovVw)Oy. (support)z or (CovVwMox)Oy. (support)z.

The iron-free two component graphitization catalyst is characterized in that:.

The iron-free two component graphitization catalyst is further characterized in that the mass ratio of cobalt to vanadium is comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM>.

The iron-free three component graphitization catalyst is characterized in that:.

The iron-free three component graphitization catalyst is further characterized in that the ratio of cobalt mass to the combined vanadium and molybdenum mass is comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM>.

For the preparation of MWCNT, the supported iron-free catalyst is brought into contact with a carbon source in the gas phase.

The use of the supported catalyst allows for growth of the carbon nanotubes by chemical vapor synthesis through decomposition of the carbon source, leading to the production of the carbon nanotube aggregate.

According to the chemical vapor synthesis, the iron-free graphitization catalyst is charged into a reactor and the carbon source in the gas phase is then supplied to the reactor at ambient pressure and high temperature to produce the carbon nanotube aggregate in which the carbon nanotubes are grown on the supported catalyst. As described above, the carbon nanotubes are grown by thermal decomposition of a hydrocarbon as carbon source. The thermally decomposed hydrocarbon is infiltrated and saturated in the graphitization catalyst and carbon is deposited from the saturated graphitization catalyst to form hexagonal ring structures.

The chemical vapor synthesis can be performed in such a manner that the supported catalyst is fed into a reactor and at least one carbon source selected from the group consisting of C<NUM>-C<NUM> saturated hydrocarbons, C<NUM>-C<NUM> unsaturated hydrocarbons, C<NUM>-C<NUM> alcohols and mixture thereof, optionally together with a reducing gas (e.g., hydrogen) and a carrier gas (e.g., nitrogen), is introduced into the reactor at a temperature equal to or higher than the thermal decomposition temperature of the carbon source in the gas phase to a temperature equal to or lower than the melting point of the graphitization catalyst, for example, at a temperature comprised between <NUM> and about <NUM>, preferably between <NUM> and <NUM>, more preferably between <NUM> and <NUM>. Carbon nanotubes may be grown for <NUM> minute to <NUM> hours, preferably <NUM> minute to <NUM> minutes after the carbon source is introduced into the supported catalyst.

Preferably, the space time, defined as the weight of supported catalyst in grams divided by the flow of reactant stream in mole/h, at standard temperature and pressure conditions, is comprised between <NUM> and <NUM>. h/mole, preferably between <NUM> and <NUM>. h/mole during a period comprised between <NUM> and <NUM> minutes, preferably between <NUM> and <NUM> minutes.

The type of the heat source for the heat treatment in the method for preparing the MWCNT is not limited and may be, for example, induction heating, radiant heating, laser, IR, microwave, plasma, UV or surface plasmon heating.

Any carbon source that can supply carbon and can exist in the gas phase at a temperature of <NUM> or more may be used without particular limitation for the chemical vapor synthesis. The gas-phase carbonaceous material may be any carbon-containing compound but is preferably a compound consisting of up to <NUM> carbon atoms, more preferably a compound consisting of up to <NUM> carbon atoms. Examples of such gas-phase carbonaceous materials include, but are not limited to, carbon monoxide, methane, ethane, ethylene, methanol, ethanol, acetylene, propane, propylene, butane, butadiene, pentane, pentene, cyclopentadiene, hexane, cyclohexane, benzene, and toluene. These gas-phase carbonaceous materials may be used alone or as a mixture thereof. The mixed gas of reducing gas (e.g. hydrogen) and carrier gas (e.g. nitrogen) transports the carbon source, prevents carbon nanotubes from burning at high temperature, and assists in the decomposition of the carbon source.

The iron-free catalyst according to the present invention allows for the production of MWCNT at a carbon yield comprised between <NUM> and <NUM>% by weight, preferably between <NUM> and <NUM>% by weight, more preferably between <NUM> and <NUM>% by weight.

The carbon yield, in % by weight, is defined as: <MAT> wherein mtot is the total weight of product after reaction and mcat is the weight of the catalyst used for the reaction.

The following illustrative examples are merely meant to exemplify the present invention but they are not intended to limit or otherwise define the scope of the present invention.

<NUM> parts by weight of water, at <NUM>, comprising <NUM> parts by weight of citric acid and <NUM> parts by weight of malic acid, were added to <NUM> parts by weight of ammonium metavanadate, and mixed during <NUM> minutes using a paddle mixer, resulting in a first aqueous solution.

Similarly, <NUM> parts by weight of water, at <NUM>, were added to <NUM> parts by weight of cobalt(II) acetate tetrahydrate and mixed during <NUM> minutes using a paddle mixer, resulting in a second aqueous solution.

The second aqueous solution was added to the first aqueous solution and mixed during <NUM> minutes using a paddle mixer.

To the mixture of the first and the second aqueous solution, <NUM> parts by weight of aluminum hydroxide (Apyral® <NUM> SM - Nabaltec), with specific surface area (BET) of <NUM><NUM>/g, was added and mixed during <NUM> minutes using a paddle mixer.

The paste thus obtained was then transferred to ceramic crucibles with a large opening and subjected to a heating process, wherein the paste was heated to <NUM> with a heating gradient of <NUM>/min and an air flow of <NUM><NUM>/h and maintained at <NUM> for <NUM> hours.

After <NUM> hours at <NUM>, the paste was further heated to a temperature of <NUM> with a heating gradient of <NUM>/min. and maintained at <NUM> for <NUM> hours while maintaining an air flow of <NUM><NUM>/h.

The solid material thus obtained was cooled down to room temperature and ground, by means of a conical grinder, to a powder characterized by a volume median particle diameter (D<NUM>) of <NUM>.

Example <NUM> was repeated with the exception that <NUM> parts by weight of water, at <NUM>, comprising <NUM> parts by weight of citric acid and <NUM> parts by weight of malic acid, were added to <NUM> parts by weight of ammonium metavanadate and <NUM> parts by weight of ammonium heptamolybdate tetrahydrate, resulting in a first aqueous solution. The second aqueous solution is obtained by adding <NUM> parts by weight of water, at <NUM>, to <NUM> parts by weight of cobalt(II) acetate tetrahydrate.

To the mixture of the first and the second aqueous solution, <NUM> parts by weight of aluminum hydroxide (Apyral® <NUM> SM - Nabaltec), with a specific surface area (BET) of <NUM><NUM>/g, was added and mixed during <NUM> minutes using a paddle mixer.

Examples <NUM> to <NUM> are prepared using the process conditions of example <NUM>, i.e. temperature and time period of mixing, drying and calcinating conditions (temperature, heating gradient, time, air flow), and grinding conditions for obtaining a D<NUM> of about <NUM>, with the exception that the cobalt-based precursor is added as a powder to the aqueous solution comprising the vanadium-based precursor and the optional molybdenum-based precursor, said aqueous solution comprising <NUM> parts by weight of water.

Example <NUM> is a comparative example wherein the support precursor is calcinated before being added to, and mixed with, the aqueous mixture comprising the cobalt-based precursor, the vanadium-based precursor and the molybdenum-based precursor. The support precursor is first impregnated with water and stirred for <NUM> hours at <NUM>, before being dried at <NUM> and <NUM> mbar. Subsequently the dried support precursor is calcinated at a temperature of <NUM> during <NUM> hours under a nitrogen atmosphere, whereupon the calcinated support is added to the aqueous mixture of catalyst precursors. The aqueous mixture comprising the cobalt-based precursor, the vanadium-based precursor and the molybdenum-based precursor is obtained from adding the cobalt-based precursor, as a powder, to the aqueous solution comprising the vanadium-based precursor, the molybdenum-based precursor and <NUM> parts by weight of water. The resulting paste was heated to <NUM> with a heating gradient of <NUM>/min and an air flow of <NUM><NUM>/h and maintained at <NUM> for <NUM> hours. Subsequently the paste was further heated to a temperature of <NUM> with a heating gradient of <NUM>/min and maintained at <NUM> for <NUM> hours while maintaining an air flow of <NUM><NUM>/h. Diffraction peaks corresponding to boehmite, AlO(OH), were not detected.

In table <NUM> the quantities of catalyst precursors, the support precursor and the polycarboxylic acid(s) and/or the salts thereof, in parts for <NUM> parts by weight of water, are reported for examples <NUM> to <NUM>.

<NUM> of the iron-free supported graphitization catalyst of examples <NUM> to <NUM> were spread in a quartz vessel which subsequently was brought in the center of a quartz tube-type reactor with an inlet and an outlet.

The center of the quartz tube reactor where the vessel comprising the catalyst is located was heated to a temperature of <NUM>.

Subsequently ethylene gas, nitrogen and hydrogen were allowed to flow through the quartz tube reactor at a flow rate of <NUM>/min (C<NUM>H<NUM>); <NUM> I/min (N<NUM>) and <NUM> I/min (H<NUM>) during <NUM> minutes.

In table <NUM>, carbon yield (column <NUM>) is given for the MWCNT (examples A to H) (column <NUM>) prepared using the catalysts of examples <NUM> to <NUM> (column <NUM>).

As clearly appears from table <NUM>, the iron-free supported catalysts according to the present invention (examples <NUM> to <NUM>) give rise to a MWCNT (examples A to G) with a carbon yield of at least <NUM>%, contrary to the MWCNT obtained from a process using an iron-free supported catalyst (example <NUM>), wherein the support precursor is calcinated before impregnation with the catalyst precursors. MWCNT with the highest carbon yield are obtained from iron-free supported catalysts, prepared from Al(OH)<NUM> support precursor, characterized by a BET comprised between <NUM> and <NUM><NUM>/g. The iron-free supported catalyst of example <NUM> (= comparative example) gives rise to a MWCNT (example H) with a carbon yield of <NUM>%, though said supported catalyst is prepared from Al(OH)<NUM> support precursor with a BET of <NUM> - <NUM><NUM>/g. For the iron-free supported catalyst of example <NUM> (= comparative example), diffraction peaks corresponding to boehmite, AlO(OH), were not detected.

The inventors have surprisingly observed that a calcination temperature, of the dried mixture of aluminum hydroxide and the catalytic precursors, comprised between <NUM> and <NUM> results in multi-wall carbon nanotubes with a high carbon yield, contrary to multi-wall carbon nanotubes resulting from a supported catalysts obtained from the same dried mixture but calcinated at a temperature above <NUM>.

The inventors have observed as well that also the drying method has an influence, although to a lesser extent, on carbon yield of the final multi-wall carbon nanotubes.

The influence of the calcination temperature is reflected by the ratios of the intensity of diffraction peaks in the XRD pattern of the supported catalyst, recorded in the 2θ range of <NUM>° to <NUM>°.

In the XRD pattern, a diffraction peak with maximum intensity at a 2θ angle of <NUM>° to <NUM>° is defined as "a". When the intensity of the diffraction peak at a 2θ angle of <NUM>° to <NUM>° is defined as "b" and the intensity of the diffraction peak at a 2θ angle of <NUM>° to <NUM>° is defined as "c", multi-wall carbon nanotubes with high carbon yield are prepared when using the iron-free supported catalyst where both conditions of intensity ratios (b/a and c/a), being the ratio b/a comprised between <NUM> and <NUM> and the ratio c/a comprised between <NUM> and <NUM>, are met.

In table <NUM>, the value of the 2θ angle, the net intensity at said 2θ angle and the intensity ratios b/a and c/a of the supported catalyst, obtained from different drying methods and calcination temperatures, are reported.

In table <NUM>, the carbon yield, in % by weight of MWCNT of example B, obtained from the iron-free supported catalyst of example <NUM> is reported for drying the catalyst precursor:.

The inventors have observed that calcination of the dried mixture of aluminum hydroxide and the catalytic precursors, at a temperature of <NUM>, results in multi-wall carbon nanotubes with lower carbon yield; at a calcination temperature of <NUM>, the intensity ratio (b/a) is not met. Diffraction peaks corresponding to boehmite (AlO(OH)) are not detected.

A reduced carbon yield, in % by weight, relative to the carbon yield, in % by weight, of the MWCNT of example B (Carbon Yield = <NUM>%), was obtained when repeating example B using the iron-free three component graphitization catalyst of example <NUM>, but calcinated for <NUM> hours at <NUM> and <NUM> respectively. As such, a reduction of about <NUM> % carbon yield, relative to carbon yield of example B, was observed for the catalyst of example <NUM>, but calcinated for <NUM> hours at <NUM> (Carbon Yield = <NUM>%), while a reduction of <NUM>% carbon yield, relative to carbon yield of example B, was observed for the catalyst of example <NUM> but calcinated for <NUM> hours at <NUM> (Carbon Yield = <NUM>%).

The spray dried iron-free three component graphitization catalyst of example <NUM>, calcinated during <NUM> hour at <NUM>, resulted in MWCNT at a carbon yield of <NUM>%.

Claim 1:
Iron-free supported catalyst for the selective conversion of hydrocarbons to carbon nanotubes, said catalyst comprising cobalt and vanadium as active catalytic metals in any oxidation state on a catalyst support comprising at least <NUM>% by weight of aluminum oxide hydroxide based on the total of aluminum hydroxides and/or aluminum oxides and aluminum oxide hydroxide, as determined by X-Ray diffraction, wherein:
- the mass ratio of cobalt to vanadium is between <NUM> and <NUM>;
- the mass ratio of cobalt to aluminum is between <NUM><NUM>-<NUM> and <NUM><NUM>-<NUM>;
- the mass ratio of vanadium to aluminum is between <NUM><NUM>-<NUM> and <NUM><NUM>-<NUM>.