WO2004087610A2

WO2004087610A2 - Inorganic material with a hierarchical structure and method for the preparation thereof

Info

Publication number: WO2004087610A2
Application number: PCT/FR2004/000589
Authority: WO
Inventors: Rénal BACKOV; Annie Colin
Original assignee: Centre National De La Recherche Scientifique; Universite Des Sciences Et Technologies (Bordeaux 1)
Priority date: 2003-03-27
Filing date: 2004-03-11
Publication date: 2004-10-14
Also published as: FR2852947B1; WO2004087610A3; FR2852947A1

Abstract

The invention relates to an inorganic material with a hierarchical structure and to a sol-gel method for the preparation thereof. The material is monolithic and is comprised of an inorganic matrix. The matrix consists of a metal oxide polymer and comprises macro-pores, meso-pores and micro-pores, said pores being interconnected. The preparation method consists in preparing an emulsion by introducing an oily phase into an aqueous solution consisting of a surfactant and a metal alcoxide; the reaction mixture is left to rest until condensation of the precursor occurs; the mixture is subsequently dried and calcinated.

Description

Inorganic material with hierarchical structure, and process for its preparation

The present invention relates to a material with a hierarchical structure and a method for its manufacture.

It is known to prepare inorganic polymers, in particular of silica, having one or two types of porosity. Macroporosity is obtained by the shaping process by direct emulsification and microporosity is inherent in amorphous silica.

Thus, A. Imhof, et al. (Nature, vol. 389, October 30, 1997, p. 948-951) describe the implementation of sol-gel processes starting from alkoxides dissolved in a lower alcohol and hydrolysed by addition of a small amount of water, it being recalled that most of the alkoxides are very reactive with water and do not give stable emulsions. This document further describes the preparation of monodisperse macroporous materials of titanium oxide, zirconia, or silica, with pore diameters between 50 nm and several μm, from a monodisperse oil emulsion in formamide. It also describes a process consisting in preparing an "oil in formamide" emulsion, in fractionating the emulsion to obtain a uniform droplet size, in preparing a metal oxide solution in formamide with a small amount of water to hydrolyze , that is to say dissolving the oxide, dispersing the emulsion in the solution, centrifuging to adjust the volume of the droplets, gelling by addition of ammonia to increase the pH, washing with water, dry and then calcine. This process has many steps which make it uneconomic.

BP Pinks (Adv. Mater. 2002, 14, no. 24, December 17) describes the preparation of porous silica from an emulsion stabilized by silica particles only, in the absence of surfactant. -Evaporation • at ^' Air produces ^1' solids with different degrees of porosity and inherent wettability. Gi-Ra Yi, et al. (Chem. Mater. 1999, 11, 2322-2325) describe a process which uses sodium dodecylsulfonate (SDS) as a surfactant. A silica sol is prepared by mixing tetraethoxysilane (TEOS) or tetra-methoxysilane (TMOS) and HCl with stirring. The gelation is obtained by addition of ammonium hydroxide. The final product is obtained by calcination. In another embodiment, an "isooctane in formamide" emulsion is used as a system congruent with formamide. The emulsion is stabilized by a triblock PEG-PPG-PEG copolymer.

JS Beck, et al. (J. Am. Chem. Soc. 1992, 114, 10834-10843) describe the preparation of mesoporous solids constituted by a silicate or an aluminosilicate. The solid compounds obtained have a specific surface of the order of 700 m ² / g and the pore size is between 15 and 100 Å.

The object of the present invention is to provide a simple process for the preparation of an inorganic material having a large specific surface.

The material according to the invention is a monolith constituted by an inorganic matrix. It is characterized in that said inorganic matrix is constituted by a polymer of a metal oxide, and that it comprises macropores having an average dimension d _a of 0.5 μm to 60 μm, mesopores having an average dimension d _E from 20 to 30 Å and micropores having an average dimension di from 5 to 10 Å, said pores being interconnected.

By monolith is meant a solid object having an average dimension of at least 1 mm.

Macropores can be identified by scanning electron microscopy (TEM) and quantified by mercury intrusion measurements. The implementation of the mercury intrusion technique shows the good mechanical strength of the monoliths obtained, which resist the mercury pressures to which they are subjected during the measurements. Mesoporosity can be identified by transmission electron microscopy (TEM). The vermicular texture of mesoporosity can be identified by X-ray diffraction, this technique also serving to quantify the pore to pore distance. Mesoporosity and microporosity can be ¹ quantified - and ^' segregated ^' by a nitrogen adsorption-desorption technique, which is analyzed by the BET calculation method (globalizing mesoporosity and microporosity) and by the calculation method B.JH according to which the segregation between micorporosity and mesoporosity becomes effective. The metal oxide is an oxide of one or more metals, at least one of the metals being of the type capable of forming an alkoxide. By way of example of metals capable of forming an alkoxide, mention may be made of Si, Ti, Zr, Th, Nb, Ta, V, W and Al.

The metal oxide can be a simple oxide, and it is then an oxide of one of the above metals. The metal oxide can also be a mixed oxide of at least two metals, and at least one of the metals is then chosen from the above metals, the other metal (s) being able to be chosen from other metals, in particular B and Sn.

Materials whose inorganic matrix is a polymer of silica or of a mixed silica oxide are particularly preferred.

The inorganic matrix can also contain a filler, for example a carbonaceous filler such as nanofilaments, nanotubes or carbon powder. The filler can also be a polymer of a monomer which has a marked hydrophilic character, chosen for example from acrylic acid, acrylamide, vinylpyrolidone, methacrylates carrying hydrophilic functions such as in particular OH, COOH, NH, (especially hydroxymethyl methacrylate).

A material according to the present invention is obtained by a sol-gel process, characterized in that:

• an emulsion is prepared by introducing an oily phase into an aqueous solution containing a surfactant and at least one metal alkoxide precursor of the oxide forming the inorganic matrix, in amounts such that the volume fraction p ₀ oily phase / aqueous phase is lower at 0.78, then an emulsion is formed by subjecting the mixture to stirring; • the reaction mixture is left to stand until the precursor has condensed, o the oily phase is extracted by washing with a volatile solvent (for example a THF-acetone mixture), then the solid is dried residual to obtain a monolith which is then subjected to calcination.

The final calcination has the effect of releasing the mesoporosity (by calcination of the surfactant) and of sintering the material, which densifies it. Preferably, the pH of the reaction medium is adjusted to a value less than or equal to 1.2 (for example by addition of the appropriate quantity of HCl) or to a value greater than 9 (for example by addition of NaOH).

The oil phase / aqueous phase volume fraction makes it possible to control the size of the macropores. An increase in said volume fraction leads to an increase in the viscosity of the reaction medium, and consequently of the shear during stirring. The droplets in the emulsion become smaller. However, a reduction in the size of the droplets causes a reduction in the thickness of the walls of said droplets, said walls forming the structure of the porous material and ensuring its mechanical strength. Below a certain thickness, which generally results from the volume ratio oily phase / aqueous phase greater than 0.78, the mechanical strength is insufficient to obtain a monolith and the final product is in powder form.

The oxide precursor solution forming the inorganic matrix of the material of the invention contains at least one alkoxide. When the inorganic matrix consists of a mixed oxide, a mixture of precursors is used, of which at least one is an alkoxide.

As examples of alkoxides, mention may be made of the compounds R ' _n (OR) _m _ _n M in which M represents a metal having the valence m, and 0 <n <m, R represents an alkyl radical having from 1 to 5 atoms of carbon, R ′ represents an alkyl radical or an aryl radical which optionally carry one or more functional groups. We can cite for example Si (OR) ₄ , Ti (OR) ₄ , Zr (OR) ₄ , Th (OR) ₄ , Nb (OR) ₅ , Ta (OR) ₅ and Al (OR) ₃ . Mention may also be made of the compounds VU (OR) ₃ and (OR) 6. The alkoxides in which R is methyl or ethyl are particularly preferred. The precursors which can be used in combination with at least one alkoxide can be chosen, for example, from hydroxides such as Al (OH) ₃ , oxides such as B ₂ 0 ₃ or V ₂ 0 ₅ , chlorides such as SnCl ₄ , ZrCl ₄ or TiCl ₄ , or oxychlorides such as ZrOCl ₂ .2H ₂ 0, or mixed oxides such as sodium metavanadate.

For example, a silica matrix is obtained from an aqueous solution of an alkoxide R ' _n (OR) ₄ _ _n Si in which R, R' and n have the previous meaning. When the inorganic matrix consists of a mixed oxide of silicon and of a metal M, use is made of a solution of silicon alkoxide and a precursor of the metal M which can be an alkoxide of M or a precursor non-alkoxide of M.

The oily phase consists of one or more compounds chosen from linear or branched alkanes having at least 12 carbon atoms. By way of example, mention may be made of dodecane and hexadecane. The oily phase can also consist of a silicone oil of low viscosity, that is to say less than 400 centipoise.

The viscosity of the reaction medium increases exponentially when the volume fraction p ₀ increases. For the lower values of p ₀ , the oily phase is advantageously introduced all at once into the aqueous solution of surfactant and precursor, and the emulsification of the reaction mixture is carried out using energetic means, such as for example an Ultraturax® device. For higher values of p ₀ , it is preferable to introduce the oily phase drop by drop or in the form of a continuous stream into the aqueous solution containing the surfactant and the. precursor, by carrying out emulsification using low energy means, such as for example a colloid mill.

The surfactant can be a cationic surfactant chosen in particular from tetradecyltrimethyl bromide. ammonium (TTAB) or dodecyltrimethylammonium bromide. In this case, the reaction medium is brought to a pH less than or equal to 1.2, preferably less than 1.

The surfactant can be an anionic surfactant chosen from sodium dodecylsulfate, sodium dodecylsulfonate and sodium dioctylsulfosuccinate (AOT).

In this case, the reaction medium is brought to a pH greater than 10.

The surfactant can be a nonionic surfactant chosen from surfactants with an ethoxylated head, and nonylphenols. In this case, the reaction medium is brought to a pH greater than 10 or less than or equal to 1.2, preferably less than 1.

When the monolith contains a carbonaceous filler such as nanofilaments, nanotubes or carbon powder, this filler is introduced into the reaction medium by dispersion in the solution of surfactant. When the filler is a polymer of a monomer which has a hydrophilic character, it is introduced by dispersing nanofilaments or powder of said polymer in the surfactant solution.

The concentration of surfactant is preferably greater than 10% by mass relative to the total amount of water, in order to obtain an initial viscous reaction medium and to promote emulsification.

The concentration of precursor of the inorganic matrix is preferably greater than 10% by mass relative to the aqueous phase, in order to obtain total mineralization of the material and good mechanical strength. During the condensation step, when the reaction medium is left to stand after mixing the inorganic precursor solution and the surfactant solution, the alkoxide (s) undergo a hydrolysis which gives a hydroxylated compound, said hydroxylated compound then condensing to form the inorganic matrix of the material. The duration of the hydrolysis is typically a few minutes. The duration of the condensation depends on the pH of the medium. When the pH of the reaction medium is less than 1, or even negative, or when the pH of the medium is very strongly basic (close to 14), the duration of the condensation is of the order of 2 to 3 hours. When the pH tends to 1.2, (which represents the isoelectric point of silica), the condensation is slower and typically requires at least a fortnight. In the latter case, the final material obtained is a very hard inorganic polymer.

The hydrolysis / condensation step of precursors of the alkoxide type is advantageously carried out at a temperature close to ambient temperature.

When the aqueous precursor solution also contains at least one precursor other than an alkoxide, the condensation can be carried out in two stages, the condensation of the alkoxide (s) being catalyzed by the pH of the medium, the subsequent condensation non-alkoxide precursor (s) being optionally catalyzed by heating.

Drying can be carried out in air, or by freeze-drying.

The monolith obtained after drying has the three degrees of porosity (microporosity, mesoporosity and macroporosity). However, the pores can be blocked by organic residues. Organic residues from the oily phase are mainly found in macropores and can be removed by simple washing before drying, for example using a volatile organic solvent such as acetone, THF or their mixtures. The organic residues originating from the surfactant are mainly found in the mesopores and can be removed by calcination, carried out for example at a temperature between 600 ° C and 700 ° C. When the monolith contains a carbonaceous filler, after drying it forms a composite material consisting of the inorganic polymer matrix having the three degrees of porosity, within which the carbonaceous filler is distributed. The calcination of this composite material must be carried out in a reducing medium to avoid the elimination of the carbonaceous charge in the form of CO or C0 ₂ . The proposed material can be used in a variety of applications, including as a synthetic bone substitute.

The present invention is described in more detail by the following examples, to which it is not however limited.

The materials obtained were characterized on different size scales.

The macroporosity was characterized qualitatively by a scanning electron microscopy (SEM) technique using a Jeol JSM-840A scanning microscope which operates at 10 kV.

The mesoporosity was characterized qualitatively by a transmission electron microscopy (TEM) technique using a Jeol 2000 FX microscope with an acceleration voltage of 200 kV. The samples were prepared by depositing powdered silica skeletons on a copper grid coated with a Formvar® carbon membrane.

The mesoporosity and the microporosity were quantified and segregated by a nitrogen adsorption-desorption technique using a device sold under the name Micromeritics ASAP 2010, the analysis being carried out by the BET and BJH calculation methods.

The macroporosity was quantified by mercury intrusion measurements, using an apparatus marketed under the name Micromeritics Autopore IV, to achieve the characteristics of the macroscopic mineral cells composing the inorganic skeleton.

In order to verify the mesoporous structure, the samples were subjected to X-ray diffraction analysis using an 18 kW rotating anode X-ray source (Rigaku-200) using a crystal (111) of Ge as a monochromator. The scattered radiation was collected on a two-dimensional collector (Imaging Plate system, marketed by Mar Research, Hamburg). The distance from the detector to the sample was 500 mm. Figures 1a to 1f represent SEM micrographs of various samples of monoliths, on a scale making it possible to determine the macroporosity.

FIGS. 2a to 2f represent the TEM micrographs of various samples of monoliths, on a scale allowing the determination of the mesoporosity.

Each of Figures 3a to 3f shows the X-ray diffractograms at small angles of a sample of monolith, before and after drying. N represents the number of strokes in arbitrary units, q represents the wave vector, which makes it possible to calculate a distance d in real space, by the formula d = 2π / q. Figures 4a to 4h represent the SEM micrographs of various samples of monoliths, at two scales used to determine macroporosity. EXAMPLE 10 3.5 g of tetraethoxysilane (TEOS) were added to 16.5 g of a 35% aqueous solution of tetradecyltrimethylammonium bromide (TTAB) which was brought to pH 0.07. The solution was deposited in a mortar to which 30 g of dodecane was additionally added and it was emulsified using an ultraturax sold under the brand Polytron®. The concentrated emulsion obtained was deposited in a mold in which the sol-gel reaction (that is to say the solidification of the inorganic polymer) took place. This condensation stage took place over a period of one week. The material obtained, designated by A, was washed in an acetone / tetrahydrofuran (THF) mixture (3 times for 24 hours) and air dried. The organic part, which produces the imprint of the mesostructure, was extracted by calcination at 650 ° C. The mineral material with hierarchical porosity thus obtained is designated by AC.

Figure la represents a micrograph of material A, showing the macroporosity, obtained by SEM. The scale bar represents 1.9 μm. The size of the macropores has an average distribution around 0.5 μm. FIGS. 2a and 2b represent a micrograph of the materials A and AC respectively, showing the mesoporosity, obtained by TEM. The scale bars represent 100 nm and 50 nm respectively. They also show that the sopores are organized and have a vermicular texture. FIG. 3a represents X-ray diffractograms at small angles of the material A (curve defined by solid squares), and of the material AC (curve defined by triangles). It confirms the vermicular type mesoporosity and makes it possible to associate a characteristic pore-to-pore distance of approximately 40 Å.

Example 2 4 g of TEOS was added to 10 g of a 35% aqueous TTAB solution which was brought to pH -0.2. The solution was deposited in a mortar to which 30 g of dodecane were added in addition and emulsified using an ultra-turax. The concentrated emulsion obtained was deposited in a mold in which the sol-gel reaction took place. This condensation stage took place over a 24-hour period. The material obtained, designated by B, was washed in an acetone / THF mixture (3 times for 24 hours) and air dried. The organic part was extracted by calcination at 650 ° C. The mineral material with hierarchical porosity thus obtained is designated by BC.

Figure 1b represents a SEM micrograph of the material B, showing the macroporosity. The scale bar represents 1.9 μm. It shows that the diameter of macropores is generally less than 1 μm. FIG. 3b represents X-ray diffractograms at small angles of the material B (curve defined by solid squares), and of the material BC (curve defined by triangles).

Example 3 0.5 g of Ti (OEt) _{4 was added} , then 5 g of TEOS to 16.5 g of a 35% aqueous TTAB solution which was brought to pH -0.05. The solution was placed in a mortar to which 30 g of hexadecane was further added and emulsified using an ultraturax. The concentrated emulsion obtained was deposited in a mold in which the sol-gel reaction took place, resulting in solidification of the inorganic polymer. ganic Si0 ₂ . This condensation stage took place over a 24-hour period. Then, the material obtained was brought to 75 ° C to catalyze the condensation reaction of the Ti0 ₂ system. This second condensation stage lasted 2 hours.

The material designated by C obtained was washed in an acetone / THF mixture (3 times for 24 hours) and air dried. The organic part was extracted by calcination at 650 ° C. The mineral material with hierarchical porosity thus obtained is designated by CC.

The figure represents a SEM micrograph of the material C, showing the macroporosity. The scale bar represents 8 μm. The distribution of macropore dimensions is polydisperse between 1 and 4 μm. Figures 2c and 2d represent a TEM micrograph of materials C and CC respectively, showing the mesoporosity. The scale bars each represent 20 nm. Here too, the mesopores are organized and have a vermicular sturcture. FIG. 3c represents X-ray diffractograms at small angles of the material C (curve defined by solid squares), and of the material CC (curve defined by triangles).

The specific surface of the CC material, measured by the BET method, is 1248 m ² / g.

Example 4

5 g of TEOS were added to 15 g of a 35% aqueous TTAB solution which was brought to pH -0.1. The solution was placed in a mortar to which 30 g of dodecane was added further and emulsified using an ultraturax. The concentrated emulsion obtained was deposited in a mold in which the sol-gel reaction took place. This condensation stage took place over a period of 48 hours. The resulting material, designated as D, was washed ^• "in a" mixed foreign acetone / THF (3 times during 24 hours) and air dried. The organic part was extracted by calcination at 650 ° C. The mineral material with hierarchical porosity thus obtained is designated by .DC: _ '^'

FIG. 1d represents a SEM micrograph of the material D, showing the macroporosity. The scale bar represents 1.9 μm. The distribution of macropore dimensions is polydisperse and between 1 and 8 μm.

Figure 3d represents X-ray diffractograms at small angles of the material D (curve defined by solid squares), and of the material DC (curve defined by triangles).

Example 5

3. g of TEOS was added to 16.5 g of a 35% aqueous TTAB solution which was brought to pH +0.05. The solution was placed in a mortar to which 30 g of hexadecane was further added and emulsified using an ultraturax. The concentrated emulsion obtained was deposited in a mold in which the sol-gel reaction took place. This condensation stage took place for a week. The material obtained, designated by E, was washed in an acetone / THF mixture (3 times for 24 hours) and air dried. The organic part was extracted by calcination at 650 ° C. The mineral material with hierarchical porosity thus obtained is designated by EC.

The figure represents a SEM micrograph of the material C, showing the macroporosity. The scale bar represents 8 μm. The distribution of macropore dimensions is polydisperse and between 1 and 8 μm.

Figures 2e and 2f represent a TEM micrograph of materials E and EC respectively, showing the mesoporosity. The scale bars represent 100 nm and 20 nm respectively. The mesopores are organized and have a vermicular sturcture.

FIG. 3e represents X-ray diffractograms at small angles of the material e (curve defined by "solid squares), and of the material EC (curve defined by triangles). Example 6

4 g of TEOS were added to 16 g of a 35% aqueous TTAB solution which was brought to pH +0.5. The solution was placed in a mortar to which 35 g of dodecane was further added and emulsified using an ultra-turax. The concentrated emulsion obtained was deposited in a mold in which the sol-gel reaction took place. This condensation stage took place over a period of fifteen days. The material obtained, designated by F, was washed in an acetone / THF mixture (3 times for 24 hours) and air dried. The organic part was extracted by calcination at 650 ° C. The mineral material with hierarchical porosity thus obtained is designated by FC.

FIG. 1f represents a SEM micrograph of the material F, showing the macroporosity. The scale bar represents 4 μm. The compound has a polydisperse distribution of pore sizes between 0.5 μm and 5 μm.

FIG. 3f represents X-ray diffractograms at small angles of the material F (curve defined by solid squares), and of the material FC (curve defined by triangles). The diffraction curve of the material F presents a broad peak centered on 0.13 Å ^-1 which moves to 0.17 Å ^-1 for the material FC (after calcination). This peak is in agreement with the mesoscopic structure of vermicular type. The FC material was subjected to the various analyzes mentioned above. The characteristics obtained are collated in Table 1 below.

Table 1

The intrusion volume, porosity, bulk density and skeleton density were determined from the mercury intrusion measurements.

B.E.T and B.J.H. were determined by a nitrogen adsorption-desorption technique.

It thus appears, that all other things being equal, an increase in the volume fraction of the oily phase decreases the macroporosity and t increases the density.

EXAMPLE 7 3.5 g of TEOS were added to 16 g of a 35% aqueous TTAB solution in which 0.3 g of carbon nanofilaments were dispersed beforehand and which was brought to pH +0 , 04. The solution was placed in a mortar to which 30 g of dodecane was added in addition and emulsified using an ultraturax. The concentrated emulsion obtained was deposited in a mold in which the sol-gel reaction took place. This condensation stage took place over a period of fifteen days. The material obtained, designated by G, was washed in an acetone / THF mixture (3 times for 24 hours) and air dried. The mineral material thus obtained is a composite material constituted by a matrix of Si0 ₂ in which the carbon nanofilaments are distributed. It is calcined under a reducing atmosphere to obtain a GC material in which the mineral charge is kept.

Example 8

3.5 g of TEOS were added to 16 g of a 35% aqueous TTAB solution in which 0.03 g of carbon nanofilaments were dispersed beforehand and which was then brought to pH +0, 04. The solution was placed in a mortar to which 30 g of dodecane was added in addition and emulsified using an ultraturax. The concentrated emulsion obtained was deposited in a mold in which the sol-gel reaction a _. occurred. .This stage of condensation. took place over a period of fifteen days. The material obtained, designated by H, was washed in an acetone / THF mixture (3 times for 24 hours) and air dried. The organizational part that was extracted by calcination at 1700 ° C. The mineral material with hierarchical porosity thus obtained, ^' designated by ^' HC is, as in the previous example, a composite material constituted by a matrix of Si0 ₂ in which the carbon nanofilaments are distributed. The increase in the content of carbon nanofilaments makes it possible to optimize the conduction properties of the material.

EXAMPLE 9 3.5 g of TEOS were added to 16 g of a 35% aqueous TTAB solution in which 0.3 g of sodium metavanadate was previously dispersed and which was then brought to pH +0 , 04. The solution was placed in a mortar to which 30 g of dodecane was added in addition and emulsified using an ultraturax. The concentrated emulsion obtained was deposited in a mold in which the sol-gel reaction took place. This condensation stage took place over a period of fifteen days. The material obtained, designated by I, was washed in an acetone / THF mixture (3 times for 24 hours) and air dried. The organic part was extracted by calcination at 650 ° C. The newly formed mineral material with hierarchical porosity is designated by IC.

Example 10

5 g of tetraethoxysilane (TEOS) were added to 16.5 g of a 35% aqueous solution of tetradecyltrimethylammonium bromide (TTAB) which was brought to pH 0.035 by addition of 5.84 ml of HCl at 37%. To the solution thus obtained, xg of dodecane was added. The emulsification was carried out by pouring the dodecane drop by drop into the solution and carrying out manual stirring in a mortar. The concentrated emulsion obtained was deposited in a mold in which the sol-gel reaction (that is to say the solidification of the inorganic polymer) took place. This condensation stage took place over a period of one week. The material obtained was washed in an acetone / tetrahydrofuran (THF) mixture (3 times for 24 hours) and dried at the air. The organic part, which produces the imprint of the mesostructure, was extracted by calcination at 65O ° C; ^'

Four tests were carried out with different amounts of dodecane, that is to say with different volume fractions oily phase / aqueous phase p ₀ .

The materials obtained after heat treatment were subjected to the various analyzes mentioned in Example 6. The characteristics obtained are collated in Table 2 below.

Table 2

B.E.T and B.J.H. were determined by a nitrogen adsorption-desorption technique. It thus appears, that all other things being equal, an increase in the volume fraction of the oily phase decreases the porosity and the specific surface, and increases the density.

FIG. 4 represents the SEM micrographs of the various samples at two different scales. They show that the materials are formed by an aggregation of hollow spheres and the diameter can go up to more than 100 μm. The correspondence between the curves and the samples is given in table 3 below. Table 3

Claims

Re endications

1. Material in the form of a monolith 'consisting of an inorganic matrix, characterized in that said inorganic matrix consists of a polymer of a metal oxide and in that it comprises macropores having an average dimension d _R of 0 5 .mu.m to 60.μm, mesopores having an average dimension _E from 20 to 30 Å and micropores having an average di size of 5 to 10 Å, said pores being interconnected.

2. Material according to claim 1, characterized in that the metal oxide is an oxide of one or more metals, at least one of the metals being of the type capable of forming an alkoxide.

3. Material according to claim 2, characterized in that at least one of the metals is chosen from Si, Ti, Zr,

Th, Nb, Ta, V, W and Al.

4. Material according to one of claims 2 or 3, characterized in that the oxide is a mixed oxide further containing B and Sn.

5. Material according to one of claims 1 to 4, characterized in that the inorganic polymer is a polymer of silicon oxide or of a mixed oxide of silicon.

6. Material according to claim 1, characterized in that the inorganic matrix also contains a filler.

7. Material according to claim 6, characterized in that the filler is a carbonaceous filler.

8. Material according to claim 6, characterized in that the filler is a polymer of a monomer which has a hydrophilic character.

9. A method of preparing a material according to claim 1 by a sol-gel method, characterized in that: o an emulsion is prepared by introducing an oily phase into an aqueous solution containing a surfactant and at least one precursor metal alkoxide oxide forming the inorganic matrix, in amounts such as the volume fraction p ₀ oily phase / phase aqueous is less than 0.78, then an emulsion is formed by subjecting the mixture to stirring; o the reaction mixture is left to stand until the precursor has condensed, o the oily phase is extracted by washing with a volatile solvent, then the residual solid is dried to obtain a monolith which is then subjected to calcination.

10. Method according to claim 9, characterized in that the volatile organic solvent is a THF-acetone mixture.

11. Method according to claim 9, characterized in that the solution of precursors contains at least one alkoxide R ' _n (OR) _m _ _n M in which M represents a metal having the valence m, and 0 <n <m, R represents an alkyl radical having from 1 to 5 carbon atoms, R ′ represents an alkyl radical or an aryl radical which optionally carry one or more functional groups.

12. Method according to claim 11, characterized in that the alkoxide is chosen from Si (OR) ₄ , Ti (OR) ₄ , Zr (OR) ₄ ,

Th (OR) ₄ , Nb (OR) ₅ , Ta (OR) 5 and Al (OR) 3.

13. Method according to claim 9, characterized in that the alkoxide is VO (OR) ₃ or W (OR) ₆ , R representing an alkyl radical having from 1 to 5 carbon atoms.

14. Method according to claim 9, characterized in that a mixture of precursors is used comprising at least one alkoxide, the other precursor (s) being chosen from oxides, mixed oxides, hydroxides, chlorides and oxychlorides.

15. The method of claim 9, characterized in that the oily phase consists of one or more compounds chosen from linear or branched alkanes having at least 12 carbon atoms, or by a silicone oil having a viscosity of less than 400 centipoises .

16. Method according to claim 9, characterized in that the oily phase is introduced dropwise or in the form of a trickle into the solution of surfactant and precursor, and the emulsification is carried out using low-energy means.

17. The method of claim 10, characterized in that one uses to make the mixture of constituents, a device which generates intensive stirring.

18. The method of claim 9, characterized in that the pH of the reaction medium is adjusted to a value less than or equal to 1.2, or to a value greater than 9.

19. The method of claim 14, characterized in that the condensation is carried out in two stages, the condensation of the alkoxide (s) being catalyzed by the pH of the medium, the subsequent condensation of the precursor (s) not alkoxide (s) being catalyzed by heating.

20. Method according to claim 9, characterized in that one introduces a carbonaceous charge into the reaction medium by dispersion in the surfactant solution.

21. The method of claim 9, characterized in that a polymer of a monomer which has a hydrophilic nature is introduced as a filler, by dispersion of nanofilaments or powder of said monomer in the surfactant solution.

22. The method of claim 9, characterized in that the concentration of surfactant is greater than 10% by mass relative to the total amount of water.

23. The method of claim 9, characterized in that the concentration of precursor of the inorganic matrix is greater than 10% by mass relative to the aqueous phase.

24. The method of claim 9, characterized in that the calcination step is carried out at a temperature between 600 and 700 ° C.

25. The method of claim 24, characterized in that the calcination step is carried out under a reducing atmosphere.