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  • B01J20/28042—Shaped bodies; Monolithic structures
  • B01J20/28045—Honeycomb or cellular structures; Solid foams or sponges
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  • B01J20/28078—Pore diameter
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  • B01J20/28078—Pore diameter
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  • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
  • B01J21/18—Carbon
  • B01J21/185—Carbon nanotubes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
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  • C01B32/00—Carbon; Compounds thereof
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  • B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
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  • B01J35/613—10-100 m2/g
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• • B—PERFORMING OPERATIONS; TRANSPORTING
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• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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  • Y10T428/24124—Fibers

Definitions

• the present invention relates to novel materials of the carbon airgel type, which are useful in particular as separation materials, in particular adapted for the filtration of liquid media.
• carbon aerogels Materials commonly referred to as "carbon aerogels” are essentially (and usually exclusively) macroscopic materials made of carbon, which have an extremely porous structure, resulting in a very low bulk density.
• the volume occupied by the pores represents at least 70% of the total volume of the material, with a corresponding bulk density generally less than 0.6 g / cm 3 .
• the aerogels and the alveolar structure two examples of carbon foams of L. Kocon and T. Piquero, in the actuality Chemical, No. 245-246, pp. 119-123 (March-April 2006).
• Carbon aerogels are generally obtained by processes called “replication” ("templating” in English).
• a porous three-dimensional structure of carbon or a precursor of carbon is formed by employing a solid structure or molecular organization of the liquid crystal type as a “mold” of the desired structure.
• This "mold”, said texturing agent ("template” in English) can take different forms depending on the method used.
• texturing processes leading to carbon aerogels there are three main families of texturing processes leading to carbon aerogels:
• the airgel is synthesized by filling with carbon the pores of a porous material used as a mold.
• a porous material used as a mold.
• materials of the alumina, silica or aluminosilicate type are used in the form of a microporous or mesoporous material (zeolite or mesostructures of the MCM-41 type, for example).
• the mold is destroyed, usually by acid attack, whereby the mold cavity completely formed of carbon is recovered.
• the initial introduction of the carbon into the texturizing material can be carried out by direct carbon deposition, generally by chemical vapor deposition, in particular by chemical vapor deposition ("CVD"), or by impregnation of an organic liquid within the pores, which is then calcined to be converted to carbon.
• CVD chemical vapor deposition
• Examples of preparations of aerogels according to this mode are described in particular in the articles "New concepts of elaboration of porous carbon materials" of C. Vix-Guterl, J. Parmentier, P. Delhaés, in L' part chimique, No. 245-246, pp. 124-128 (March-April 2006). and "Synthesis of highly ordered carbon molecular sieves via template-mediated structural transformation” by R. Ryoo, S.-H. Soo, S. Jun, in The Journal of Physical Chemistry B, 103 (37), pp. 7743-7746 (1999).
• the organization of the carbon in the form of an airgel is carried out in a liquid or gelled medium, typically in the form of a liquid crystal.
• the "mold" used is not of a solid nature, but nevertheless prints an organized structure to the synthesized carbon structure.
• the airgel used in this context is generally obtained by directly structuring a carbonaceous mesophase and then by carbonizing the formation medium of the structure, in particular by implementing the techniques described in the articles "High-thermal conductivity, mesophase pitch-derived carbon foams: effect of precursors on structure and properties by J. Klett et al., in Carbon, 38, pp. 153-173 (2000) or "Novel high strength graphitic foams", by T. Beechem, K. Lafdi, in Carbon, 44, pp. 1548-1549 (2002).
• an airgel is formed not by directly structuring carbon as in the preceding variant, but by organic constituents, whereby an organic airgel is obtained, which is then carbonized by heat treatment in order to obtain the carbon airgel.
• Carbon aerogels as obtained by the aforementioned texturizing processes are materials with interesting properties, which have many applications. In general, it can be used in almost every known applications of aerogels-type materials, where can be made use of their high porosity and relatively high surface area. In particular, their chemical inertness vis-à-vis many reagents and their good thermal stability make them interesting materials especially as a catalyst support, especially in a reducing medium. They can also be used as phonic or anti-vibrational insulators, given their relatively good mechanical strength.
• An object of the present invention is to provide suitable carbon aerogels especially as separation materials, capable in particular to lead to interesting results both in terms of efficiency and speed of separation.
• the present invention relates to carbon aerogels of a new type, which are obtained by coagulation of carbon nanotubes in the form of a cellular structure. More specifically, the aerogels of the invention are obtained according to a particular method, which constitutes another particular object of the invention. This process comprises the following steps:
• a dispersion of carbon nanotubes in water is made by employing a dispersing surfactant (generally ionic, and preferably anionic);
• a foam is made from the aqueous dispersion of nanotubes obtained in step (A), by expansion of the dispersion under the effect of a gas in the presence of a foaming agent;
• step (C) the foam obtained in step (B) is frozen and then the water is removed by sublimation, generally by freeze-drying.
• the method of the invention may comprise, after steps (A) to (C), an additional step (D) in which the material obtained at the end of step (C) is heat treated, typically at a temperature greater than or equal to
• 400 ° C. for example at least 600 ° C., in particular at least 800 ° C., or even at least 1000 ° C.
• step (D) makes it possible in particular to eliminate (by pyrolysis) at least a portion, and preferably all, surfactants and foaming agents used in steps (A) and (B).
• step (C) is advantageously carried out at a temperature greater than or equal to 1200 ° C., which makes it possible to ensure total carbonization of the organic species initially present.
• step (D) may also allow mechanical consolidation of the structure obtained after lyophilization of step (C).
• step (D) it is advantageous for step (D) to be carried out in the presence of organic compounds (which is generally the case: surfactants and / or foaming agents of steps (A) and (B)). are most often organic compounds) and at a temperature advantageously greater than 800 ° C, preferably greater than 1000 0 C for example greater than or equal to 1200 0 C.
• the organic compounds are likely to act as precursors organic by pyrolytically conducting to the formation of carbon capable of binding the carbon nanotubes together.
• a material is obtained based on agglomerated (coagulated) carbon nanotubes, generally monolithic, whose structure reproduces substantially that of the walls of the foam made in step (B).
• the carbon aerogels thus obtained have a very specific structure, with essentially open porosity, which generally comprises at the same time:
• a "macroporosity” consisting of alveoli (or “macropores”) having an average size greater than 40 microns, more generally greater than or equal to
• this nanoporosity essentially localized within the cell walls of the aforementioned macroporosity, this nanoporosity comprising pores with an average dimension of less than 60 nm, essentially corresponding to the inter-nanotube interstices present in the walls of the foam formed in the step (B).
• This particular structure of the aerogels of the invention can in particular be demonstrated on clichés of the materials obtained by scanning electron microscopy (SEM), examples of which are given in the attached figures.
• SEM scanning electron microscopy
• the images obtained by scanning electron microscopy most often show that the walls of the resulting structure comprise substantially aligned nanotubes (which is most likely due, in particular in part, to shear phenomena resulting from water runoff (or drainage) which takes place during the formation of foam of step (B) of the process).
• Scanning electron microscopy also makes it possible to measure the pore dimensions present.
• an airgel as obtained according to the invention has an apparent density most often less than or equal to 0.25 g / cm 3 , generally less than or equal to 0.2 g / cm 3 , and typically around of 0.2 g / cm 3 (which represents only 1/10 of the graphite carbon density), for example of the order of 0.18 g / cm 3 to 0.22 g / cm 3 .
• This very low bulk density reflects a very high pore volume for the aerogels of the invention, where, most often, the volume occupied by the pores represents at least 80% of the total volume of the material, generally at least 90%, or even minus 95%.
• the porosity of the aerogels of the invention is in general an essentially open porosity. This specificity can be highlighted in particular by material density measurements by helium pycnometry. In this type of measurement, the tested airgel is immersed in helium gas, which fills all the accessible pores, which makes it possible to determine the density of the material occupying the volume not filled by helium. In the case of the airgel of the invention, there is then a density of the order of that of graphite carbon (of the order of 2), which indicates that substantially (and generally almost exclusively) all the pores of the material are accessible and that substantially all the internal spaces of the nanotubes remain accessible.
• the aerogels of the invention most often have a high specific surface area, generally between 25 and 300 m 2 / g, this specific surface area being advantageously greater than 30 m 2 / g, preferably at least 35m 2 / g.
• specific surface refers to the BET specific surface area, as determined by nitrogen adsorption, according to the well-known method called BRUNAUER - EMMET - TELLER which is described in The Journal of the American Chemical Society, volume 60, page 309 (1938), and corresponding to the international standard ISO 5794/1 (Appendix D).
• the particular structure of the carbon aerogels obtained according to the invention makes them particularly suitable as separation materials, especially for performing solid / liquid separations.
• the macroporosity makes it possible to obtain a good diffusion of the constituents, thus inducing high separation rates, the nanoporosity making it possible for it to obtain a good separation efficiency.
• the aerogels of the invention can typically be used for the constitution of membrane or filtration materials, especially for the filtration of biological media (blood for example) or for cleaning (chemical and / or bacteriological) d 'waste.
• the aerogels of the invention may also be used as liquid chromatography column fillers.
• the aerogels of the invention are also well suited to other applications.
• they can especially be used as biomaterials, and in particular as a cell growth support, in particular fibroblasts or osteoblasts.
• the particular porosity of the material ensures optimal colonization: the macroporosity ensures a diffusion of the cells, which can reach substantially all of the surface of the material, and the nanoporosity induces an irregularity of the surface of the material clean to ensure a good fixation of the cells on the airgel.
• the aerogels of the invention may in particular be used in the context of bone tissue replacement.
• the aerogels of the invention can be used in most known applications of carbon-based aerogels, insofar as they have the specific advantages of these materials, in particular a high chemical inertness, particularly with respect to reducers, a high thermal stability up to more than 2000 0 C (in non-oxidizing medium), as well as a very good thermal and electrical conductivity.
• the aerogels of the invention can thus, in particular, be used as support for catalytic species, for example for the catalysis of reactions in a reducing medium, especially at high temperature, and all the more so because they present the more often than not the advantage of being supercompressible.
• the aerogels of the invention can also be used for the storage of non-wetting liquid, in particular for the storage of energy liquids in fuel cell membranes, or as a negative electrode in lithium batteries.
• the aerogels according to the invention can also be used as acoustic or anti-vibrational insulators.
• the method of the invention has, among others, the advantage of being simple to implement and inexpensive. In addition, it is implemented in an aqueous medium and does not involve, in the general case, releases subject to specific treatments, which makes it an industrially exploitable process.
• Step (A) of the process of the invention consists in producing an aqueous dispersion of nanotubes.
• nanotube is intended to mean a tubular structure based on carbon, which is generally essentially based on carbon in the form of graphene sheets, and which has a diameter of between 1 and 200 nm.
• the nanotubes employed in the process of the invention are typically nanotubes of the type mutifills, ie hollow cylindrical structures based on graphene sheets wound on themselves, comprising a plurality of concentric cylinders based on graphene.
• nanotubes of the following type will be used having a mean diameter of between 10 and 100 nm, this diameter being most often at least 30 nm, for example between 50 and 80 nm.
• the average length of nanotubes employed in step (A) is generally between 1 and 20 microns, typically between 5 and 15 microns (for example of the order of 10 microns).
• step (A) the dispersion of the nanotubes in the water is carried out in the presence of a suitable surfactant, namely adapted to stabilize the nanotube / water interface.
• the dispersion can be carried out according to any means known per se. It is typically carried out by subjecting a water / nanotube / surfactant mixture to sufficient shear, advantageously by ultrasound (sonication).
• the surfactant employed to provide the dispersion is preferably an ionic surfactant.
• Particularly suitable for the implementation of the invention are anionic surfactants, especially sulphates, sulphonates or carboxylates, for example alkyl sulphates or alkyl sulphonates, or carboxymethyl cellulose salts (especially the sodium salt of carboxymethyl cellulose).
• the surfactant employed in step (A) is used in the dispersion of nanotubes in a concentration below its critical micelle concentration (CMC), the threshold beyond which the nanotubes tend to aggregate by depletion interaction.
• CMC critical micelle concentration
• the concentration of nanotubes in the dispersion of step (A) can vary to a large extent depending on the surfactant used.
• the dispersion prepared in step (A) comprises at least 0.5 g of nanotubes per liter, more preferably at least 1 g of nanotubes per liter, more preferably at least 5 g of nanotubes per liter. g of nanotubes per liter, or even at least 10 g of nanotubes per liter, this concentration being most often between 1 and 15 g of nanotubes per liter.
• this concentration of nanotubes is limited in the case of the use of certain surfactants. For example, maximum concentrations of about 1 g of nanotubes per liter are attained with surfactants such as sodium dodecyl sulfate SDS.
• (A) is a salt of carboxymethylcellulose, in particular the sodium salt, which makes it possible to obtain a nanotube concentration of up to 15 g of nanotubes per liter of dispersion, while maintaining a homogeneous dispersion.
• step (B) of the process of the invention a foam is produced from the dispersion produced in step (A) by expansion under the effect of a gas. Most often, this expansion is effected by bubbling a gas within the dispersion produced in step (A), this gas being advantageously delivered through a sintered glass.
• the foam produced in the course of step (B) is a liquid / gas type foam whose structure determines the morphology of the final airgel.
• This foam is formed in the presence of a foaming agent, namely an agent capable of stabilizing the water / gas interface in the formed foam.
• a foaming agent is generally added to the dispersion as obtained at the end of step (A), for example a surfactant suitable for this purpose, such as, for example, a nonionic ethoxylated surfactant such as Tergitol (especially Tergitol NP9 (of formula C 15 H 24 O (C 2 H 4 O) 9 )
• a surfactant suitable for this purpose such as, for example, a nonionic ethoxylated surfactant such as Tergitol (especially Tergitol NP9 (of formula C 15 H 24 O (C 2 H 4 O) 9 )
• Tergitol especially Tergitol NP9 (of formula C 15 H 24 O (C 2 H 4 O) 9
• starch, or of sugar, in particular of the polysaccharide type is particularly advantageous in the case where step (D) is Implementation
• step (B) the starch introduced in step (B) is found in the material obtained at the end of step (C).
• step (D) is carried out, at least a portion of the starch is carbonized, which generally makes it possible to bond together the constituent nanotubes of the material, which results in an increase in the cohesion of the material, and therefore an improvement in its mechanical properties, measurable in particular by the Young's modulus.
• step 5D) be conducted at a temperature of at least 600 ° C., preferably at least 800 ° C. 0 C, more preferably at least 1000 0 C, or even at least 1200 0 C.
• the surfactant in the case of SDSj, will advantageously be used in a content of greater than 1 g / l or even 2 g / l in the dispersion of step (A). Nevertheless, SDS is generally used in a concentration below its critical micellar concentration (CMC), the threshold beyond which the nanotubes would aggregate by depletion interaction.
• CMC critical micellar concentration
• the formation of the foam in this step is advantageously carried out by injecting gas bubbles into the dispersion obtained at the end of step (A). ), additivée if necessary (which is most often the case) of a foaming agent, this gas bubble injection being advantageously through a porous membrane such as a sintered glass above which the dispersion is placed.
• the solution is generally placed at the bottom in a container of sufficient size and morphology adapted to allow expansion of the foam.
• a tubular cylindrical container provided with a porous sintered glass type membrane is used at its base (its bottom), this container having a volume at least 20 times equal to the initial volume of the dispersion before foaming.
• the gas used for foaming may vary to a large extent. It is advantageously air or nitrogen, which may optionally be supplemented with a hydrophobic organic compound, for example perfluorohexane. This additional organic compound has the role of stabilizing the air / water interfaces of the foam, thereby minimizing the Ostwald ripening and coalescence phenomena which destabilize neoformed foams.
• a hydrophobic organic compound for example perfluorohexane.
• the morphology of the foam obtained in step (B) can be controlled by varying several parameters, and in particular the following:
• the formed foam tends to lose water by gravity, the water tending to flow down. This phenomenon tends to form with a relatively “dry” foam with nonspherical bubbles, polygonal and relatively unstable.
• the moisture content of the foam can be increased, in particular by counterbalancing the loss of water due to gravity by "feeding" the formation foam to the water at the same time. from its upper part.
• the top of the foam in formation in step (B) can typically be sprayed with a dispersion having the same composition as the dispersion which is foamed by expansion.
• step (B) controlling the volume fraction of water in the foam is a good way of controlling the shape of the cells made.
• a foam with a low volume fraction in water dry foam
• a foam with a high volume fraction in water moist foam
• the control of the water volume fraction of the starting foams also makes it possible to control the width of the alveolar walls (called “Plateau edges").
• This size can be controlled by varying the size of the gas bubbles initially introduced to effect the expansion (in particular by varying the pore size of the porous membrane used), as well as the flow rate of the gas used.
• the bubble size of the foam obtained is generally higher as the size of the injected gas bubbles are large.
• the average cell size of the cell can be varied. foam obtained between 50 and 600 microns.
• step (B) a foam is obtained in which the liquid / gas volume ratio is less than or equal to 0.1, this ratio typically being between 0.01 and 0.1.
• Step (C) of the process schematically aims to freeze the structure of the foam obtained in step (B).
• step (B) The foam obtained at the end of step (B) is relatively stable, but not sufficient to allow its drying by evaporation. As a result, in step (C), the water is removed by freezing and lyophilization.
• step (C) is advantageously carried out so as to freeze the structure of the foam as quickly as possible, so as to avoid any phenomenon of coalescence.
• the freezing is advantageously carried out by placing the foam obtained in step (B) at a temperature below -50 ° C., more advantageously below -80 ° C.
• the sublimation step is most often cold lyophilization conducted according to any means known per se (especially according to the usual technique described for example on the site www.lyo-san.ca/lyophilisation.htm).
• the optional heat treatment step (D) may be carried out in particular in an oven or in an oven, generally in a non-oxidising inert atmosphere, preferably with a progressive temperature rise and cooling, with typically rising and falling temperatures. descents in the temperature of a few degrees per minute.
• the material subjected to the heat treatment may comprise carbonaceous adjuvants used as precursor of carbon.
• these carbonaceous additives are decomposed into carbon, whereby they are able to bind the nanotubes together in the formed structure, thus leading to an improvement in the cohesion and the mechanical properties of the material obtained.
• Such carbonaceous adjuvants may for example be introduced during steps (A) and / or (B).
• starch or a sugar (polysaccharide for example) is advantageously used as foaming agent in step (B).
• Other carbonaceous adjuvants may also be introduced during steps (A) and / or (B), provided that they do not prevent the formation of the desired foam, for example sucrose, melamine, or a phenolic resin
• such carbonaceous adjuvants may be introduced near step (C) prior to step (D), for example by impregnating the structure obtained at the end of step (C) before the heat treatment of the step (D).
• an airgel is generally obtained in the form of a macroscopic material generally monolithic, typically having dimensions of the order of one centimeter or ten centimeters or more.
• the nanotubes are advantageously bonded to one another, the materials preferably having a Young's modulus greater than or equal to 5 MPa, for example between 5 and 10 MPa.
• the aerogels obtained can be post-treated, for example to be impregnated with catalytic species.
• carbon nanotubes mutifeuillets (commercial Pyrographs III, type PR-24-PS) were used.
• aqueous dispersion of these nanotubes was made using the sodium salt of carboxymethylcellulose as a dispersing surfactant.
• the medium obtained was placed in a bath at 0 ° C. (liquid water + ice) and then sonicated for 30 minutes using a Branson 250 type sonicator fitted with a type 12 probe. whose end measures 3mm in diameter (pulse duration of
• the dispersion thus obtained was placed in a bubbler, constituted by a vertical column of PVC (height: 60 cm, diameter: 5 cm), provided with a sintered glass bottom having a pore diameter of 30 microns, through which Nitrogen is injected at a constant rate of 0.2 mL / s.
• the dispersion was introduced through a pipe above the column, fed by a peristaltic pump.
• the beaker was immediately placed at -80 ° C. and was kept at this temperature for 5 hours.
• the frozen foam obtained was lyophilized by using a freeze-dryer (vacuum chamber). The water was then removed by sublimation, whereby an airgel was obtained.
• the airgel obtained in the previous step was heat-treated under the following conditions:

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