WO2011020970A2 - Supported fe/mo catalyst, process for preparing same and use for the manufacture of nanotubes - Google Patents

Abstract

The invention relates to a catalyst material for preparing nanotubes, especially carbon nanotubes, said material being in the form of solid particles, said particles comprising a porous substrate having a BET specific surface area of greater than 50 m²/g, preferably between 70 and 400 m²/g, covered with at least one catalytic layer of a mixture of iron and molybdenum, said catalytic layer representing from 30% to 70%, preferably from 40 to 55% by weight of the total weight of the catalyst material, the iron content being at least 25%, preferably from 30% to 40% by weight of the total weight of the catalyst material, and the iron/molybdenum weight ratio being between 11 and 1.5, preferably between 5 and 2. The invention also relates to the process for preparing same and to a process for the synthesis of CNTs using this catalyst material.

Description

FE / Mo SUPPORTED CATALYST, PROCESS FOR PREPARING THE SAME, AND USE IN THE MANUFACTURE OF NANOTUBES

The present invention relates to novel supported Fe / Mo catalysts. It also relates to the process for preparing these catalysts and their use for the manufacture of nanotubes, especially carbon nanotubes.

 Many studies have focused on supported transition metal catalysts, in particular for the manufacture of carbon nanotubes (CNTs).

 In recent years, CNTs have been intensively researched to replace powdered carbon black powder, which is difficult to handle in all its applications. The CNTs furthermore have the advantage of conferring improved mechanical properties and electrical and / or thermal conduction properties on any composite material containing them, at least equal to those of the pulverulent carbon black, at lower contents. Their good mechanical properties and especially resistance to elongation are related in part to their very high aspect ratios (length / diameter).

 They consist of one or more graphitic sheets arranged concentrically about a longitudinal axis. For nanotubes composed of a single sheet, we speak of SWNT (acronym for Single Wall Nanotubes) and for nanotubes composed of several concentric layers, this is called MWNT (acronym for Multi Wall Nanotubes). SWNTs are generally more difficult to manufacture than MWNTs.

Carbon nanotubes can be manufactured using various processes such as electrical discharge, laser ablation, chemical vapor deposition (CVD in abbreviation) or physical vapor deposition (PVD abbreviation).

 According to the Applicant, the most promising method of manufacturing CNTs in terms of CNT quality, reproducibility of CNT characteristics, and productivity is the CVD process. This process involves injecting a source of carbon-rich gas into a reactor containing a high temperature metal catalyst. In contact with the metal, the gas source decomposes into graphitic plane NTC and hydrogen. In general, the catalyst consists of a catalytic metal such as iron, cobalt, nickel, supported by a solid substrate, in the form of grains, and chemically inert, such as alumina, silica, magnesia or still carbon.

 The gaseous carbon sources generally used are methane, ethane, ethylene, acetylene or benzene.

 By way of example of documents describing this CVD process, mention may be made of WO 86/03455 of Hyperion Catalysis International Inc. which can be considered as one of the basic patents on the synthesis of CNTs. This document describes carbon fibrils (former NTC designation) almost cylindrical, whose diameter is between 3.5 and 70 nm and whose aspect ratio is greater than or equal to 100, and their method of preparation.

CNTs are synthesized by contacting a catalyst containing iron (e.g. Fe3θ _4, Fe on a carbon support, Fe on an alumina carrier or Fe on a carbon fibril support) with a rich gaseous compound carbon, such as a hydrocarbon, in the presence of another gas capable of reacting with the carbon-rich gaseous compound. The synthesis is carried out at a chosen temperature in the range from 85O ⁰ C to 1200 ⁰ C. The catalyst is prepared by dry impregnation, precipitation or wet impregnation.

 Other documents describe improvements in this process, such as the use of a continuous fluidized bed of catalyst, which makes it possible to control the state of aggregation of the catalyst and of the carbonaceous materials formed (see, for example, WO 02 / 94713A1 on behalf of Tsinghua University and FR 2 826 646 INPT).

Many studies have also focused on improving the catalyst, especially by combining different catalytic metals. Thus, US 2001/00036549 of Hyperion Catalysis International Inc. has described supported bimetallic catalysts of Fe / Mo and Fe / Cr type and has shown that a molybdenum doping of the order of 1 to 2% by weight made it possible to doubling the productivity compared to a monometallic iron catalyst, in a temperature range of 500 ⁰ C to 1500 ⁰ C, but doping above 2.5% lowered productivity. We can also mention the US patent application 2008/0003169 which describes Fe / Mo / alumina type catalysts for good productivity. However, in this case, the catalyst has a structure different from that of a supported catalyst since it is obtained by coprecipitation, on the one hand, of a solution of iron salts and molybdenum salts and, on the other hand, a solution of aluminum salts.

 The Applicant has proposed in his patent application WO 2006/082325 a new type of supported catalyst that can combine several types of metals. However, this document focuses only on examples of Fe / alumina catalyst.

Furthermore, document US 2004/162216 discloses various types of catalysts for the production of carbon nanotubes. A first type is obtained by placing in contact with a catalytic material with a carboxylate.

It leads to a catalyst containing well less than 25% of iron (Examples I-VI I and IX) or whose substrate (magnesia

MagChem ^® by MARTIN MARIETTA) has a particle size of at most 50 μm and is used as a paste. Another type of catalyst is formed by co-precipitation of catalytic metals with aluminum or magnesium compounds

(Example X) This complex process induces a filling of the porosity of the support and leads to a catalyst whose active phase is diluted throughout the support. It is not therefore a supported catalyst. Yet another type of catalyst consists of particular compositions deposited on carbon fiber aggregates (Example XI). Again, the percentage of iron in the catalyst is well below 25% by weight.

 Other examples of catalysts suitable for the preparation of carbon nanotubes, which comprise alumina impregnated with iron and molybdenum and contain less than 25% by weight of iron, are described (catalysts A and B) in the document WO 02/081371.

 Finally, the document WO 2008/100325 discloses (Example 2) a process for manufacturing a catalyst intended for the preparation of carbon nanotubes, without mentioning the quantities of the impregnation solutions used and therefore the percentage by weight of iron. in the catalyst.

Despite these various developments, there is still a need for new catalysts, which can further improve the productivity of CNT synthesis reactions at about 650 ⁰ C, in which they are used.

The present inventors have found that a very particular Fe / Mo type catalyst supported on porous alumina, allowed to obtain a very high productivity at a CNT synthesis reaction temperature of 620 to 680 ⁰ C, the productivity being calculated as follows:

 (ÇTc / ÇCatalyst)

g where _c is the mass of carbon formed and g _{y C} atai sor is the mass of catalyst used.

The invention thus aims at providing a supported catalyst material, for the preparation of nanotubes, in particular carbon, said material being in the form of solid particles, said particles comprising a porous substrate having a BET specific surface area greater than 50 m ² / g, preferably between 70 and 400 m ² / g, in the form of particles having a larger dimension of between 75 and 500 microns, supporting a catalytic layer of a mixture of iron and molybdenum, said catalytic layer representing from 30% to 70% by weight. %, preferably from 40 to 55% by weight of the total mass of the catalyst material, the iron content being at least 25%, preferably from 30% to 40% of the total mass of the catalyst material, and the ratio iron / molybdenum mass being between 11 and 1.5, preferably between 5 and 2.

 It is understood, in the present description, that by "iron" and "molybdenum", reference is made to these metals in the elemental state, that is to say in the oxidation state 0, or the oxidized state. However, it is preferred that these metals are primarily in the elemental state.

The substrate is a substrate that is chemically inert with respect to molybdenum, iron and the carbon source, under the operating conditions of the CNT synthesis method by the CVD technique; it represents from 30% to 70% by weight of the total mass of the catalyst. Advantageously, this substrate may be inorganic. It is especially chosen from alumina, an activated carbon, silica, a silicate, magnesia, titanium oxide, zirconia, a zeolite or carbon fibers. According to a mode of Advantageously, the substrate is alumina, in particular of the gamma or theta type.

 The porosity of the substrate can be measured by nitrogen adsorption, a method well known to those skilled in the art.

 The macroscopic shape of the substrate particles, and particles of catalyst material, can be globally substantially spherical or not. The invention also applies to grains of macroscopic shape more or less flattened (flakes, discs ...) and / or elongated (cylinders, rods, ribbons ...).

 According to the invention, the shape and size of the particles are adapted to allow the formation of a fluidized bed of the catalyst material. In practice, to ensure correct productivity, it is preferred that the substrate particles have a larger dimension of between 75 and 150 microns, more preferably between 80 and 100 microns, as measured by laser granulometry. The use of particles of this size makes it possible to use the substrate in solid form, in the dry state, in an impregnation process and thus to obtain a catalyst material having a higher iron content than the catalysts obtained by liquid impregnation, in particular. The catalyst grains thus obtained can also be used in a fluidized bed, and thus more easily lead to the production of multi-walled carbon nanotubes.

Furthermore, according to one embodiment of the invention, the catalyst material is in the form of spherical particles having a unimodal particle size distribution, the equivalent diameter of the particles being between 80% and 120% of the average particle diameter of the catalyst material. . According to other forms In one embodiment, the particles of catalyst material may have a broad or bimodal particle size distribution, with an equivalent diameter ranging from 20 to 600%.

 Advantageously, the catalyst material according to the invention comprises particles of alumina supporting a catalytic layer, the mass percentages of the various constituents being 32 for iron, 12 for molybdenum and 56 for alumina, relative to total mass of the catalyst material.

 The invention extends to a process for preparing the catalyst material described above, by impregnating the dry substrate with an impregnating solution comprising an iron salt and a molybdenum salt. The solution may be an alcoholic or aqueous solution. The iron salt may be iron nitrate, and in particular iron nitrate nonahydrate. The molybdenum salt may be ammonium molybdate, and in particular ammonium molybdate tetrahydrate. Advantageously, the impregnation solution is an aqueous solution of iron nitrate nonahydrate and ammonium molybdate tetrahydrate.

The impregnation step is preferably carried out under a dry gas sweep, preferably under a sweep of air. It is carried out at a temperature measured in situ ranging from 100 to 150 ^° C., preferably approximately 120 ^° C. Advantageously, the quantity of impregnation solution, at any time, in contact with the substrate is generally just sufficient to ensuring the formation of a film or a layer on the surface of the substrate particles.

As indicated above, this impregnation step makes it possible to obtain a catalyst material containing more than 25% by weight of iron. It is also easier to implement than the co-precipitation or liquid impregnation processes of the art. and does not require the use of carboxylate, or additional filtration step.

The process for preparing the catalytic material according to the invention also comprises a drying step at a temperature ranging, for example, from 150 to 250 ^° C., measured in situ, advantageously followed by a denitrification step, preferably under an inert atmosphere at a temperature ranging from 350 to 45O ⁰ C, measured in situ.

 The invention also extends to a catalyst material obtained by a process according to the invention as defined above.

 The invention also extends to a method for manufacturing nanoparticles of material chosen from silicon, carbon, boron, and a mixture of these elements, optionally associated with nitrogen or doped with nitrogen, characterized in that at least one catalyst material according to the invention is used.

 Advantageously and according to the invention, it is a reaction for the selective production of carbon nanotubes by thermal decomposition of a source of gaseous carbon. Thus, the invention more particularly relates to a process for selectively producing carbon nanotubes by decomposition of a carbon source in the gaseous state, comprising the following steps:

 a) introducing, in particular the fluidized bed, into a reactor, a catalyst material as defined above,

b) heating said catalyst material at a temperature ranging from 620 to 68O ⁰ C, preferably about 65O ⁰ C;

c) contacting a carbon source (alkane or alkene), preferably ethylene, with the catalyst material of step b), to form on the surface of said catalyst material carbon nanotubes and hydrogen by catalytic decomposition of said carbon source;

 (d) the recovery of the carbon nanotubes produced in (c).

 The carbon source is an alkane such as methane or ethane or preferably an alkene which may be selected from the group consisting of ethylene, isopropylene, propylene, butene, butadiene, and mixtures thereof. This source of carbon may be of renewable origin as described in patent application EP 1 980 530. The alkene preferably used is ethylene.

 Advantageously and according to the invention, the carbon source, and preferably ethylene, is mixed in step c) with a stream of hydrogen.

 The carbon / hydrogen source ratio can in this case be between 90/10 and 60/40, preferably between 70/30 and 80/20. Advantageously, step c) is carried out with an ethylene / hydrogen mixture in a ratio of 75/25.

 The different steps are preferably carried out simultaneously and continuously in the same reactor.

 In addition, this process may comprise other steps (preliminary, intermediate or subsequent), as long as they do not adversely affect the production of carbon nanotubes.

 Thus, advantageously, the catalyst material is reduced in situ in said reactor, between steps a) and b). Thus, the catalytic layer is not oxidized at the moment when the catalyst material is used for the synthesis of CNTs.

If necessary, a step of grinding the nanotubes in situ or ex situ may be considered, before or after step d). It is also possible to provide a step of chemical and / or thermal purification of the nanotubes before or after step d).

 The productivity obtained with the process of the invention is particularly high, since it is always greater than 15, even greater than 25. In addition, the carbon nanotubes formed are less likely to agglomerate than in the processes described in US Pat. prior art.

The invention also extends to carbon nanotubes that can be obtained according to the method described above. It is advantageously multi-walled nanotubes, comprising for example from 5 to 15, and preferably from 7 to 10, graphene sheets wound concentrically. The nanotubes obtained according to the invention usually have a mean diameter ranging from 0.1 to 200 nm, preferably from 0.1 to 100 nm, more preferably from 0.4 to 50 nm and better still from 1 to 30 nm and advantageously a length of more than 0.1 microns and advantageously from 0.1 to 20 microns, for example about 6 microns. Their length / diameter ratio is advantageously greater than 10 and most often greater than 100. Their specific surface area is for example between 100 and 600 m ² / g and their apparent density may especially be between 0.01 and 0.5 g. / cm ³ and more preferably between 0.07 and 0.2 g / cm ³ .

The invention also relates to the use of nanotubes, which can be obtained as described above, in composite materials to impart improved electrical and / or thermal conduction properties and / or mechanical properties, in particular resistance to corrosion. elongation, improved. In particular, CNTs can be used in macromolecular compositions intended for the packaging of electronic components or the manufacture of fuel lines (fuel oil) or coatings or paints (antistatic coating), or in thermistors or electrodes for supercapacities or for the manufacture of structural parts for aeronautical, nautical or automotive fields.

 Other objects, features and advantages of the invention appear on reading the following description of its exemplary embodiments which refers to the appended figures in which:

FIG. 1 is a graph obtained from Examples 5 to 7 and representing the productivity expressed as the ratio between the carbon mass of the CNTs and the catalyst mass gc / gcatai _yS eur as a function of the temperature,

 FIG. 2 is a graph obtained from example 5 and representing the productivity as a function of the molybdenum content,

 FIG. 3 is a microscopic photograph showing nanotubes obtained in example 5.

 The invention will be described in more detail with reference to the following examples which are given purely by way of illustration and in no way limitative.

EXAMPLES Example 1: Preparation of a Catalyst 32Fel2Mo / Al ₂ O ₃

In an IL reactor equipped with a jacketed jacket heated to 120 ^° C., 100 g of Puralox ^® SCCa-5/150 alumina having a median diameter of about 85 μm and a specific surface area of 160 m ² / g are introduced, and we sweep in the air. By means of a pump, 600 ml of a solution of iron nitrate and ammonium molybdate containing 675 g / l of iron nitrate nonahydrate and 65 g / l of ammonium molybdate tetrahydrate are then continuously injected. The duration of addition is fixed at 25h. The catalyst is then heated in situ at 20 ^° C. under a dry air sweep for 8 hours and then placed in a muffle furnace at 400 ^° C. for 8 hours. A catalyst containing 32% iron and 12% molybdenum is obtained, the percentages being percentages by weight relative to the mass of catalyst.

Example 2 Preparation of a Catalyst 32Fe6Mo / Al ₂ θ ₃

A catalyst is prepared under the same conditions as in Example 1 but adjusting the amount of ammonium molybdate to obtain a catalyst with 32% by weight of iron and 6% by weight of molybdenum.

Example 3 Preparation of a Catalyst 32Fe3Mo / Al ₂ O ₃

A catalyst is prepared under the same conditions as in Example 1, but by adjusting the amount of ammonium molybdate to obtain a catalyst with, in masses, 32% iron and 3% molybdenum.

Example 4 (Comparative): Preparation of a 32Fe / Al ₂ O ₃ Catalyst

 A catalyst is prepared under the same conditions as in Example 1, but without adding ammonium molybdate and adjusting the amount of solution injected to obtain a 32% iron catalyst.

Example 5 Catalytic Test at 650 ^° C.

With each of the catalysts of Examples 1 to 4 above, a catalytic test is carried out by putting a mass of about 2.3 g of catalyst layer in a reactor of 5 cm in diameter and 1 m in effective height. The mixture is heated at 65 ^° C. under 2.66 L / min of nitrogen for 30 minutes and then a reduction stage is maintained for 30 minutes under 2 L / min of nitrogen and 0.66 L / min of hydrogen. Once this stage is over, an ethylene flow rate of 2 L / min and 0.66 L / min of hydrogen. After 60 minutes, the heating was stopped and the reactor was cooled under a nitrogen flow of 2.66 L / min. The amount of product formed is evaluated by calculating the mass remaining after a calcination of about 2 g of the composite at 800 ^° C. for 6 hours. At the same time, an estimate of the quality of the nanotubes is made by microscopy.

 The results are shown in Table 1 below:

 TABLE 1

The productivity of the catalysts according to the invention is improved with respect to the catalyst of the prior art. Molybdenum enrichment improves productivity and activity, as illustrated in Figure 2.

In FIG. 3, it can be seen that the CNTs formed have a large length and that there is no aggregation of the catalyst grains. Example 6 Catalytic Test at 700 ^° C.

With the catalyst of Example 1, a catalytic test is carried out as in Example 5, but at a temperature of 700 ^° C. Example 7 Catalytic Test at 600 ^° C.

With the catalyst of Example 1, a catalytic test is carried out as in Example 5, but at a temperature of 600 ^° C.

 The results obtained in Examples 6 and 7 are shown in Table 2 below and in Figure 1.

TABLE 2

The productivity of the catalysts according to the invention is dependent on the reaction temperature and is maximum at 65 ^° C., as also illustrated in FIG.

 The nanotubes obtained can then be introduced into a polymer matrix in order to produce composite materials with improved mechanical and / or thermal and / or conductive properties.

Claims

1. Supported catalyst material for the preparation of nanotubes, in particular carbon, said material being in the form of solid particles, said particles comprising a porous substrate with a BET specific surface area greater than 50 m ² / g, preferably between 70 and 400 m ² / g, in the form of particles having a larger dimension of between 75 and 500 microns, supporting a catalytic layer of a mixture of iron and molybdenum, said catalytic layer representing from 30% to 70%, preferably from 40 to 55% by weight of the total mass of the catalyst material, the iron content being at least 25%, preferably 30% to 40% by weight of the total mass of the catalyst material, and the mass ratio iron / molybdenum being between 11 and 1.5, preferably between 5 and 2.  2. Catalyst material according to claim 1, in which the substrate is chosen from alumina, an active carbon, silica, a silicate, magnesia, titanium oxide, zirconia, a zeolite or carbon fibers, preferably the substrate is alumina.  3. Catalyst material according to claim 1 or 2, characterized in that the substrate particles have a larger dimension of between 80 and 100 microns.  4. Catalyst material according to any one of claims 1 to 3, characterized in that the substrate is alumina and the mass percentages of the various constituents are 32 for iron, 12 for molybdenum and 56 for alumina , relative to the total mass of catalyst material. 5. Process for the preparation of the catalyst material according to any one of claims 1 to 4, by impregnation of the dry substrate with an impregnation solution of iron salt and molybdenum salt, preferably iron nitrate. and ammonium molybdate, said impregnation being preferably carried out under dry gas scavenging. 6. Process according to claim 5, characterized in that the impregnation is carried out at a temperature, measured in situ, ranging from 100 to 150 ^° C., preferably at about 120 ^° C.  A process according to any one of the preceding claims wherein the amount of impregnating solution at all times in contact with the substrate is just sufficient to provide for the formation of a film on the surface of the substrate particles. 8. Method according to one of claims 5 to 7, which further comprises a drying step at a temperature ranging from 150 to 250 ^° C., measured in situ, optionally followed by a denitrification step, preferably under an inert atmosphere. at a temperature ranging from 350 to 45O ⁰ C, measured in situ.  9. A process for manufacturing carbon nanotubes, comprising the following steps: a) the introduction, in particular the fluidized bed, into a reactor, of a catalyst material as defined in any one of claims 1 to 4; or prepared according to any one of claims 5 to 8, b) heating said catalyst material at a temperature ranging from 620 to 68O ⁰ C, preferably about 65O ⁰ C;  c) contacting a source of carbon, preferably ethylene, with the catalyst material of step b), to form on the surface of said catalyst carbon nanotubes and hydrogen by catalytic decomposition of said carbon source; (d) the recovery of the carbon nanotubes produced in (c). 10. The method of claim 9, characterized in that the carbon source is an alkane such as methane or ethane, an alkene such as ethylene, or mixtures thereof.  11. The method of claim 9 or 10, characterized in that the carbon source is mixed in step c) with a flow of hydrogen.  12. The method of claim 11, wherein the carbon / hydrogen source ratio is between 90/10 and 60/40, preferably between 70/30 and 80/20.  The process according to claim 12, wherein the carbon source is ethylene and the ethylene / hydrogen ratio is 75/25.  14. Process according to any one of claims 9 to 13, characterized in that the catalyst material is reduced in situ in said reactor.  15. Carbon nanotubes obtainable by the method of any one of claims 9 to 14.  16. Use of carbon nanotubes according to claim 15, in composite materials to impart improved electrical and / or thermal conduction properties and / or improved mechanical properties, especially resistance to elongation.  17. The use of carbon nanotubes according to claim 16, in macromolecular compositions intended for the packaging of electronic components or for the manufacture of fuel lines (fuel oil) or antistatic coatings or paints (coating), or in thermistors or electrodes for supercapacities or for the manufacture of structural parts in the aeronautical, nautical or automotive fields.