WO2022058449A1

WO2022058449A1 - Monolithic nanoparticle metal oxides with multiscale porosity

Info

Publication number: WO2022058449A1
Application number: PCT/EP2021/075529
Authority: WO
Inventors: Rénal BACKOV; VANG (épouse LY), Isabelle
Original assignee: Centre National De La Recherche Scientifique; Universite de Bordeaux
Priority date: 2020-09-18
Filing date: 2021-09-16
Publication date: 2022-03-24
Also published as: FR3114317B1; EP4214177A1; FR3114317A1

Abstract

Disclosed is a method for preparing a monolith with multiscale porosity, the monolith comprising a silicon oxide matrix, and nanoparticles of a metal oxide M, M representing a metal or a metalloid chosen from the following metals and metalloids: Cr, Co, Mn, Ni, Ce, V, Y, W, Nb, Mo, Fe, Zn, Ta, Sn, Cd, Cu and In, the method comprising a step of emulsifying an oily phase in a homogeneous acidic aqueous phase, the aqueous phase comprising a cationic surfactant, at least one metal oxide precursor metal salt and one silica precursor, a step of polycondensation, a step of eliminating the oily phase by treatment with dichloromethane, and calcination taking place in two steps.

Description

Monolithic nanoparticle metallo-oxides with multi-scale porosity

The present invention relates to a process for the synthesis of monoliths of mixed oxides with multi-scale porosity, the materials obtained and their use.

State of the art

Metal oxides are specific catalysts which are implemented in very many applications depending on the type of metal oxide. Examples include the oxidation of CO to CO2 for cobalt oxide, the oxidation of cyclohexane and n-hexane for manganese oxide, hydro-deoxygenation for molybdenum oxide , photocatalysis for tungsten, zinc or tantalum oxide. These metal oxides are all the more active as they are divided (small) and this at the nanometric scale. The problem is that these nano-objects are then very powdery, so it is advantageous to stabilize/anchor them in a host matrix.

This is how the inventors proposed monoliths of mixed oxides with multi-scale porosity. The synthesis of these materials called Si(HIPE), HIPE being the acronym for High Internal Phase Emulsion, which is based on the coupling between sol-gel chemistry and complex fluids (oil-in-water emulsions, and lyotropic mesophases ) was the subject of numerous works by the inventors in the early 2000s (patent application FR. 0303774; F. Carn et al. J. Mat. Chem. 2004, 14, 1370). Since then, a whole range of silicic inorganic materials with various morphologies have been developed (A. Roucher et al. J. Sol-Gel Sci. Tech., 2019, 90, 95) and these materials have been hybridized by ormosils, enzymes , bacteria to establish a specific catalytic property (A. Roucher et al. The Chemical Record, 2018, 18, 776).

Among these monolithic materials, the inventors have prepared mixed oxides of the TiO2@Si(HIPE) type which are intended to be used in the volume photoreduction of CO2 for the continuous formation of “solar” fuels (S. Bernadet et al. , Adv.Funct Mat., 2019, 29, 1807767). These materials are synthesized in two steps: a synthesis of an Si(HIPE) ceramic foam followed by infiltration (by pulling under vacuum) of an acid sol of TiOz precursors based on titanium isopropoxide (FR.18-53644, FR.17-53757, FR. -1753758 and FR.17-53759).

However, these two-step synthesis methods are long and therefore difficult to implement on an industrial scale. In addition, for high titanium isopropoxide contents, the viscosity of the soils of TiOz precursors does not allow homogeneous infiltration of the soils into the silica matrix, and therefore the Ti/Si molar ratio must be between 0.095 and 0. .15 to maintain a homogeneous distribution of titanium in the monolithic material.

However, the inventors have discovered a means of synthesizing metal oxides with high levels of metal salts, while establishing a nanometric character of the newly formed oxides within the silicic matrix as well as a homogeneous distribution of said oxides within the silicic matrix .

An object of the invention is therefore to propose a process for the synthesis of nanoparticulate monolithic metallo-oxides with multi-scale porosity which is rapid, in a single step and makes it possible to prepare materials with Ti/Si molar ratios greater than 0, 15 while keeping the nanometric character of the neoformed metallo-oxides as well as their homogeneous distribution.

Presentation of the invention

The invention therefore firstly relates to a method for preparing a monolith with multi-scale porosity, said monolith comprising a matrix of silicon oxide, and nanoparticles of a metal oxide M, M representing a metal or a metalloid selected from the following metals and metalloids: Cr, Co, Mn, Ni, Ce, V, Y, W, Nb, Mo, Fe, Zn, Ta, Sn, Cd, Cu and In, said method comprising: a) a step of emulsifying an oily phase in a homogeneous acidic aqueous phase, said aqueous phase comprising a cationic surfactant, at least one metallic salt precursor of metal oxide M, and a precursor of silica, such that the molar ratio M /Si (referred to below as ratio x) is a number between 0.05 and 0.57 (limits included), b) a polycondensation stage, c) a stage of elimination of the oily phase by treatment with dichloromethane for a period of between 10 and 15 hours (limits included), advantageously for a period of 12 hours, d) a first calcining step at a temperature of between 160 and 200°C (limits included), advantageously 180°C, with a heating rate of between 0.5 and 2°C/min (limits included), advantageously equal to l ° C / min, the plateau being maintained for 3 to 6 hours (limits included), advantageously for 3 hours and e) a second calcination step at a temperature between 600 and 800 ° C (limits included), advantageously 700 ° C , with a heating rate of between 0.5 and 2° C./min (limits included), advantageously equal to 1° C./min, the plateau being maintained for 3 to 6 hours (limits included), advantageously for 5 hours.

Within the meaning of the present invention, monolith is understood to mean a solid object whose smallest dimension is at least 1 mm. The monoliths are easy to shape (columns, films, balls) due to the absence of dustiness.

In the monolith obtained according to the method of the invention, the silicon oxide matrix is a continuous matrix.

Said monolith is a monolith of mixed oxides in that it comprises several oxides, i.e. at least one silicon oxide and at least one metal oxide M.

Within the meaning of the present invention, by nanoparticle metal oxide M or nanoparticles of a metal M oxide is meant an oxide having at least one dimension less than or equal to approximately 500 nm, preferably less than or equal to 200 nm, and particularly preferably less than or equal to 100 nm.

The metal oxide M preferably has at least one dimension greater than 10 nm, and preferably greater than 50 nm. The dimension(s) of a metal oxide M can be determined by methods well known to those skilled in the art and in particular by TEM (transmission electron microscopy).

Within the meaning of the present invention, the acidic aqueous phase has a pH below the isoelectric point of silica, i.e. below 2.1, and preferably between 0.05 (Hammet's acidity) and 1 (limits included).

The M/Si molar ratio or ratio x is a number between 0.05 and 0.57 (limits included) and designates the molar ratio in the oil-in-water emulsion obtained at the end of step a), x remains substantially constant during steps a), b) and c).

The material obtained at the end of step a) can be represented by the following formula (I):

(MO _y )xSiO ₂ (I) in which: x represents the M/Si molar ratio and is a number between 0.05 and 0.57 (limits included), and y represents a number ranging from 1 to 5.

In an advantageous embodiment of the invention, x is between 0.1 and 0.57, preferably between 0.2 and 0.57, and even more advantageously between 0.3 and 0.57 (limits included) . Thus x can for example be equal to 0.05; 0.1; 0.15; 0.2; 0.25; 0.3; 0.4; 0.5; 0.55 or 0.57.

Beyond 0.57, the material obtained according to the method of the invention may lose its monolithic character. x is preferably determined with the aid of the respective quantities of silica precursor and metal salt precursor of metal oxide (metal oxide M) used during step a).

In an advantageous embodiment of the invention, the oily phase consists of one or more compounds chosen from linear and branched alkanes having at least 9 carbon atoms. Even more advantageously, the linear alkanes comprise between 10 and 12 carbon atoms. By way of example, mention may be made of decane or dodecane. In an advantageous embodiment of the invention, the cationic surfactant is chosen from quaternary ammonium compounds having at least 8 carbon atoms. Mention may be made, by way of examples, of tetradecyltrimethylammonium bromide (TTAB), dodecyltrimethylammonium bromide and cetyltrimethylammonium bromide.

The concentration of cationic surfactant is preferably greater than 10% by mass relative to the total amount of water, to obtain a viscous initial reaction medium and promote good stability of the emulsion despite the release of ethanol induced by the hydrolysis of Si(0Et)4 (TEOS).

In an advantageous embodiment of the invention, the metal salts precursors of metal oxide (i.e. of oxides of metal M) are chosen from chlorides and nitrides.

In an advantageous embodiment of the invention, the silica precursor is a silicon alkoxide.

Mention may be made, by way of example, of tetraethoxy-orthosilicate (TEOS), (3-mercaptopropyl)trimethoxysilane, (3-aminopropyl)triethoxysilane, N-(3-trimethoxysilylpropyl)pyrrole, 3-(2,4- dinitrophenylamino)propyltriethoxysilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, phenyltriethoxysilane and methyltriethoxysilane. The TEOS is particularly advantageous.

Sodium silicate solutions can also be considered as a silica precursor.

The concentration of precursor of the inorganic matrix is preferably greater than 10% by mass relative to the total mass of the aqueous phase, to obtain total mineralization of the material and good mechanical strength.

The method may further comprise, before step a) of emulsification, a step of adding the metal salt to an aqueous phase comprising the cationic surfactant, followed by the addition of an acid (eg HCl) to form a phase aqueous acid comprising said cationic surfactant and said metal salt, followed by the addition of the silica precursor the time necessary to promote the hydrolysis of the silica precursor before step a).

Stage b) of polycondensation is preferably carried out at ambient temperature, in particular for a period of at least 15 days.

At the end of step b), a solid material is obtained (solidified emulsion).

Stage b) being carried out in an acid medium, only the silica precursor hydrolyses and polycondenses. In other words, the metal salt precursor of metal oxide (i.e. precursor of the metal oxide M) remains intact and does not co-condense with the silicon (it is a spectator of the silicic polycondensation).

The stage c) of elimination of the oily phase with dichloromethane preferably lasts about one night.

The removal step c) is preferably followed by a drying period or step c′), for example 2 to 3 days.

The elimination step c) makes it possible to eliminate the organic residues originating from the oily phase which are essentially found in the macropores. The substitution of THF (tetrahydrofuran) by dichloromethane is an essential difference with the processes known from the prior state of the art, in particular that described in WO 2004087610. Indeed, THF has a good affinity with water and this causes rapid drying of the aqueous continuous medium, which generates a great deal of capillary force during drying, and induces cracking of the monoliths during drying if this drying is not carried out slowly over at least a week. In addition, the affinity of THF with water causes the oxide precursor salts to diffuse out of the monoliths and therefore the THF washes not only the oily phase but also partially washes the salts from the aqueous phase, which avoid. The use of dichloromethane thus avoids any diffusion of the metal salt precursor of the metal oxide M outside the monolith and guarantees its homogeneous dispersion within the silica matrix. According to a preferred embodiment of the invention, the elimination stage c) is carried out by placing the material resulting from the preceding stage b) in a container containing dichloromethane (CH2Cl2), itself being enclosed in the protected from all humidity, preferably in a desiccator. This thus makes it possible to avoid too rapid evaporation of the dichloromethane.

Then, the material obtained in step c) can be dried in the air (step c′), for example under a hood.

During the drying step c′), the metal salts precursors of the metal oxide M come to precipitate (ie recrystallize) within the silica matrix, producing a heterogeneous and dense nucleation of said metal salts within the silica matrix (on the surface of the silica walls), while guaranteeing a homogeneous distribution of the nanoparticles of metallic salts within the silica matrix.

The calcination or heat treatment step (steps d) and e)) is important because it calcines the micellar phases by creating a mesoporosity, it sinters the ceramic, and it induces the transformations of the nanoparticle metal salts into metal oxide nanoparticles Mr.

The first calcination step (step d)) is implemented by heating the material from the previous step with a heating rate of between 0.5 and 2°C/min, advantageously equal to 1°C/min, d an initial temperature at a first calcination temperature of between 160 and 200° C., advantageously 180° C., then this temperature (also called plateau temperature) is maintained for 3 to 6 hours), advantageously for 3 hours.

The second calcination step (step e)) is implemented by heating the material from the previous step with a heating rate of between 0.5 and 2°C/min, advantageously equal to 1°C/min, d an initial temperature at a second calcination temperature of between 600 and 800° C., advantageously 700° C., then this temperature (also called plateau temperature) is maintained for 3 to 6 hours, advantageously for 5 hours. The heat treatment (steps d) and e)) is preferably carried out in air, for example with a first temperature rise ramp from 25° C. to 180° C. (at 1° C. per minute), remaining at 180° C. C for 3 hours. Then, for example, a second temperature rise ramp at 1° C./min is applied from 180° C. to 700° C. and the materials are maintained at 700° C. for 5 hours. The temperature drop to room temperature is uncontrolled and induced by the inherent thermal inertia of the oven. This step makes it possible to eliminate the organic residues originating from the surfactant which are essentially found in the mesopores.

According to an advantageous embodiment, M represents a metal or a metalloid chosen from the following metals and metalloids: Cr, Co, Mn, Ni, Ce, V, W, Fe, Nb, and Y.

At the end of the method of the invention, a porous, self-supporting monolith with multi-scale porosity is obtained, thus comprising macropores, mesopores and micropores, said pores being interconnected.

Said monolith obtained according to said method of the invention comprises (preferably consists of) a matrix of silicon oxide, and nanoparticles of metal oxide M, said nanoparticles of metal oxide M being homogeneously distributed within said silicon oxide matrix.

In particular, the nanoparticles are found on the surface of the macropores as well as on the surface of the walls forming the interconnected porous network.

According to a preferred embodiment of the invention, the monolith obtained according to the process in accordance with the first object of the invention (ie at the end of step e)) has a final molar ratio M/Si ranging from 0, 0001 to 0.57, particularly preferably 0.001 to 0.4, and more particularly preferably 0.05 to 0.2. This final molar ratio depends in particular on the ratio x as defined in the invention.

This final molar ratio can be determined by ICP (Inductively Coupled Plasma Analytical Technique) elemental analysis. The material obtained according to the process in accordance with the first object of the invention can be as defined in the second object of the invention described below.

The second subject of the invention is a material in the form of a monolith, characterized in that it comprises a matrix of silicon oxide, and nanoparticles of a metal oxide M, and in that:

- M represents a metal or a metalloid chosen from the following metals and metalloids: Cr, Co, Mn, Ni, Ce, V, Y, W, Nb, Mo, Fe, Zn, Ta, Sn, Cd, Cu and In,

- said material has a multi-scale porosity, and

- said material is obtained from an oil-in-water emulsion comprising a silica precursor and at least one metallic salt precursor of metal oxide M, such that the M/Si molar ratio (called ratio x in the present invention ) in said emulsion is a number between 0.05 and 0.57 (limits included).

In the monolith of the invention, the matrix of silicon oxide is a continuous matrix.

In an advantageous embodiment of the invention, x is between 0.1 and 0.57, preferably between 0.2 and 0.57, and even more advantageously between 0.3 and 0.57 (limits included) . Thus, x can for example be equal to 0.05; 0.1; 0.15; 0.2; 0.25; 0.3; 0.4; 0.5; 0.55 or 0.57.

Said monolith of the invention comprises (preferably consists of) a silicon oxide matrix, and metal M oxide nanoparticles, said metal M oxide nanoparticles being distributed homogeneously within said silicon oxide matrix.

The metal oxide nanoparticles M are homogeneously distributed or dispersed at the macroscopic scale. The monolith of the invention is a self-supporting monolith.

In accordance with the invention, material with multi-scale porosity is understood to mean a material comprising macropores (these originating from the oily phase and the elimination thereof by treatment with dichloromethane), mesopores (those ci from micellar or lyotropic systems), and micropores (these come from the statistical distribution of SiCU tetrahedra in geometric space).

Macropores can be identified by scanning electron microscopy (TEM) and their junction windows 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 the mesoporosity can be identified by small angle X-ray diffraction, this technique also being used to quantify the pore-to-pore distance. Mesoporosity and microporosity can be quantified and segregated by a nitrogen adsorption-desorption technique whose counting is done by the BET calculation method globalizing mesoporosity and microporosity (Brunauer, Emmett and Teller model or BET method (S. Brunauer, PH Emmet, E. Teller, Journal of the American Chemical Society, vol 60(2), pages 309-319 (1938)) and by the BJ H calculation method (Barrett, Joyner and Halenda (1951) Journal of the American Chemical Society, 73, 373-380), according to which the segregation between microporosity and mesoporosity becomes effective, the BJH method only considering pores greater than 1.5 Angstroms.

As explained above, the material of the invention is a porous monolith with multi-scale porosity, thus comprising macropores, mesopores and micropores, said pores being interconnected.

In particular, the metal oxide nanoparticles M are found on the surface of the macropores as well as on the surface and in the volume of the walls forming the interconnected porous network.

In an advantageous embodiment of the invention, the material comprises macropores having an average dimension dA of 0.5 to 60 micrometers, mesopores having an average dimension dE of 20 to 30 Å, and micropores having an average dimension dl of 5 to 10 Å, said pores being interconnected.

In accordance with the invention, the oily 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 in 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 oily phase/aqueous phase volume ratio greater than 0.78, the mechanical strength may be insufficient to obtain a monolith and the final product is in the form of a powder. Also, the oily phase/aqueous phase volume ratio is preferably less than 0.78, advantageously between 0.6 and 0.7 (limits included).

In an advantageous embodiment of the invention, the material has a specific surface of between 400 and 1000 m ² /g, preferably between 400 and 800 m ² /g, and particularly preferably between 400 and 600 m ² /g (terminals included). This thus makes it possible to obtain a material having optimal adsorption properties.

In the present invention, the specific surface is determined by methods well known to those skilled in the art, and in particular by the BET method, preferably using dinitrogen as gas.

According to a preferred embodiment of the invention, the monolith of the invention has a final M/Si molar ratio ranging from 0.0001 to 0.57, particularly preferably from 0.001 to 0.4, and more particularly preferred from 0.05 to 0.2. This molar ratio depends in particular on the ratio x as defined in the invention.

The material in accordance with the second object of the invention can advantageously be obtained according to a method as defined in the first object of the invention. The materials according to the invention can be used in processes for the adsorption and specific storage of toxic gases such as in particular CO, NOx and CO2.

They can more particularly be used in the catalytic oxidation of CO to CO2.

Brief description of the drawings

The invention is illustrated by FIGS. 1 to 4 and the example which follow.

FIG. 1 represents images of monoliths obtained by the process according to the invention, (a) monoliths of cerium oxide with from right to left Si(HIPE); Ceo,osSi(HIPE); Ceo,iSi(HIPE) and Ceo,5?Si(HIPE). b-g Scanning electron microscopy (SEM) images showing macroporosity interconnected with b-c Ceo,osSi(HIPE); d-e Ceo,iSi(HIPE) and f-g Ceo,5?Si(HIPE). h-j investigation by energy dispersive X-ray spectroscopy with focus on silica and cerium atoms with h Ceo,osSi(HIPE); i Ceo,iSi(HIPE) and j Ceo, 57 i(HIPE).

FIG. 2 represents the images obtained by transmission electron microscopy (TEM) for Ce-Si(HIPE) a-b Ceo,osSi(HIPE) oxides; c-d Ceo,iSi(HIPE) and e-f Ceo,5?Si(HIPE). gh are images obtained by transmission electron diffraction which show the aggregation of Ceo,5?Si(HIPE) nanoparticles which correspond to the diffraction diagrams h,k, and I of the cubic phase of CeC in accordance with figure 3 .

FIG. 3 represents the diagrams obtained by wide-angle X-ray scattering on various Si-oxides (HIPE) in accordance with the invention. a) Nickel oxides Si (HIPE) lower curve: Nio,os-Si (HIPE), middle curve: Nio,i-Si (HIPE), upper curve: Nio,57-Si (HIPE) representing the main diagrams of diffraction h, k, I (111) and (200) of cubic phase NiO (JCPDS 47-1049), b) Chromium oxides Si (HIPE) lower curve: Cro,o5-Si (HIPE), middle curve: Cro ,i-Si (HIPE), upper curve: Cro,57-Si (HIPE) representing the main diffraction patterns h, k, I (012), (104), (110), (006), (113), (202) and (024) of the rhombohedral phase CrzCh (JCPDS 38-1479), c) Iron oxides Si (HIPE) lower curve: Feo,os-Si (HIPE ), middle curve: Feo,i-Si (HIPE), upper curve: Feo,57-Si (HIPE) representing the main h, k, I diffraction patterns (012), (110), (113), ( 202) and (024) of the rhombohedral hematite phase alpha-FezOs (JCPDS 33-0664), d) Manganese oxides Si (HIPE), lower curve: Mno, os-Si (HIPE), middle curve: Mno,i -Si (HIPE), upper curve: Mno.57-Si (HIPE); one can observe the main diffraction diagrams of Mn3Û4 (JCPDS 24-0734) and MnzOs (JCPDS 41-1442) biphasic system, e) Oxides of cerium Si (HIPE) lower curve: Ceo,os-Si (HIPE), curve of the middle: Ceo,i-Si (HIPE), upper curve: Ceo,57-Si (HIPE) we can observe the main (111), (200) and (220) diffraction peaks of the monophasic cubic phase CeOz (JCPDS 34 -0394), f) Vanadium oxides Si (HIPE), lower curve: Vo,os-Si (HIPE), middle curve: Vo,i-Si (HIPE), upper curve: Vo,57-Si (HIPE) one can observe the main diffraction peaks (020), (001), (011), (110), (010) and (130) of the cubic phase V2O5 (JCPDS 41-1426), g) Cobalt oxides Si ( HIPE), lower curve: Coo,os-Si (HIPE), middle curve: Coo,i-Si (HIPE), upper curve: Coo,57-Si (HIPE) we can observe the main diffraction peaks (111) , (220), (311), (222) and (400) of the cubic phase CO3O4 (JCPDS 74-167) with minor diffraction peaks (*) c orresponding to C02SiO4 phase (JCPDS- 76-1501), h) Tungsten oxides Si (HIPE), lower curve: Wo,os-Si (HIPE), middle curve: Wo,i-Si (HIPE), upper curve : Wo,2-Si (HIPE) we can observe the main (002), (020), (200), (120), (112), (112), (022), (202: 220) diffraction peaks cubic phase WO3 (JCPDS 43-1035).

FIG. 4 represents the catalytic activity of an oxide-Si (HIPE) in accordance with the invention. Description of embodiments

Example 1 of synthesis in accordance with the invention

All reagents and solvents used are from Sigma-Aldrich and have not been purified prior to use. The aqueous phase is a TTAB solution (tetradecyl-trimethyl ammonium bromide) at 35% by weight. To 16 g of this aqueous solution are added the appropriate quantities of metal oxide precursors, until dissolution or homogeneous dispersion (see Table 1). Then 5 g of HCl at 37% by mass (12M) are added. 5 g of silica precursor (TEOS tetraethyl-orthosilicate) are then introduced into the acidic aqueous phase (pH=0.05) and hydrolyse there, becoming Si(OH)4. This acidic aqueous phase is left stirring for 10 minutes to homogenize, then 37 g of dodecane are added drop by drop for emulsification (direct oil-in-water emulsion). This resulting emulsion is then poured into small hemolysis tubes where solidification takes place (polycondensation of the silica network: the Si(OH)4 polycondensate to give SiC and form the continuous matrix of silicon oxide). These tubes are then closed and placed in polypropylene beakers with a bottom of water (to avoid possible drying of the silica network during the polycondensation), these beakers are covered with parafilm. Maturation/aging associated with polycondensation takes place for 15 days.

Then the monoliths are unmolded and placed in beakers containing dichloromethane (CH2Cl2), and these beakers locked up in desiccators. This washing of the oily phase with CH2Cl2 is carried out overnight. Then, the monoliths are removed and left to air dry under a hood without any special precautions for 2 to 3 days. During this drying step, the metal oxide salts M crystallize by heterogeneous nucleation at the surface and in the volume of the continuous silicic network. In a last step a heat treatment is applied. The heat treatment is carried out in air with a first temperature rise ramp from 25° C. to 180° C. (at 1° C. per minute), remaining at 180° C. for 3 hours. Then a second temperature rise ramp at 1°C per minute is applied from 180°C to 700°C; the materials are maintained at 700°C for 5 hours. The temperature drop to room temperature is uncontrolled, induced by the inherent thermal inertia of the furnace. The monoliths are then removed from the oven and analyzed.

For writing convenience, the mixed oxides (MO _y )x-SiO2(HIPE) synthesized here will be named: Mx-Si(HIPE) in table 1; M represents the metal in question, “x” here represents the M/Si molar ratio used during the synthesis, and in particular during step a). The masses of precursors added to the aqueous solution of TTAB are also indicated:

TABLE 1

The figures show the monolithic character of the ceramics obtained. Figure 1 a-g illustrates interconnected macroporosity as revealed by scanning electron microscopy (SEM). Energy dispersive X-ray spectroscopy (EDX) demonstrates the homogeneous distribution of transition or metalloid elements within the silicic matrix (Figure 1 h-j).

FIG. 2 represents the images obtained by transmission electron microscopy (TEM) imaging which highlights the mesoporosity of the silica and demonstrates the nanometric character of the nucleated nanooxides within the silica matrix. This TEM is associated with DET (electronic transmission diffraction) which reveals the microstructure of these nano-oxides, characterization technique split by wide-angle X-ray scattering (WAXS: Wide Angle X-ray Scattering) with figure 3 .

Example 2 of application according to the invention

A monolith in accordance with the invention in the form of a mixed oxide CO(o.57)-Si(HIPE) (x=0.57) was prepared according to a method identical to that as described in Example 1 above. The mass of precursor C0Cl2. 6H2O added to the aqueous solution of TTAB is 3.255 g. This thus makes it possible to obtain a final molar ratio M/Si of 0.162.

The catalytic activity of the monolith was studied in the oxidation reaction of carbon monoxide to carbon dioxide. The reaction was carried out in a tubular fixed-bed reactor containing 50 mg of the previously ground cobalt-based monolith. The monolith is activated beforehand for two hours by supplying the reactor comprising said monolith with a flow of gas consisting of 29 ml/min of oxygen and 59.4 ml/min of nitrogen for 2 hours, at 500° C. (heating temperature of 10°C/min). The reactor is then fed continuously with a gas flow consisting of 11.6 ml/min of carbon monoxide, 29 ml/min of oxygen, and the remainder in nitrogen (total flow of 100 ml/min), which corresponds at a CO/O2 molar ratio of 0.4 and a weighted hourly space velocity (well known as “weight hourly space velocity” or WHSV) of 120,000 I.h ¹ . kg ¹ . The reactor is then placed in a tube furnace and subjected to a temperature increase with a heating rate of 5° C./min. The effluents from the reactor are analyzed with a gas chromatography device (Varian GC-3800) equipped with a thermal conductivity detector (the temperatures applied for the injector, the oven, and the detector are respectively 150°C, 50 °C, and 120°C).

FIG. 4 shows the conversion of carbon monoxide (in %) as a function of the temperature (in °C), and thus translates the catalytic performances of the monolith of the invention during the oxidation. The monolith in accordance with the invention makes it possible to convert 50% of the carbon monoxide at a temperature of 145°C, and 100% at a temperature of 200°C.

The results indirectly show the adsorption capacity of carbon monoxide by the monolith of the invention. Indeed, the oxidation reaction requires the adsorption of carbon monoxide on the active sites of the catalyst.

Claims

1. Process for preparing a monolith with multi-scale porosity, said monolith comprising a matrix of silicon oxide, and nanoparticles of a metal oxide M,

M representing a metal or a metalloid chosen from the following metals and metalloids: Cr, Co, Mn, Ni, Ce, V, Y, W, Nb, Mo, Fe, Zn, Ta, Sn, Cd, Cu and In, said process comprising: a) a step of emulsifying an oily phase in a homogeneous acidic aqueous phase, said aqueous phase comprising a cationic surfactant, at least one metal salt precursor of metal oxide M, and a precursor of silica, of so that the M/Si molar ratio is a number between 0.05 and 0.57, b) a polycondensation stage, c) a stage of elimination of the oily phase by treatment with dichloromethane for a period of between 10 and 15 hours, d) a first calcining step at a temperature of between 160 and 200°C with a heating rate of between 0.5 and 2°C/min, the plateau being maintained for 3 to 6 hours, and e) a second calcination step at a temperature of between 600 and 800°C, with a heating rate of between 0.5 and 2°C/ min, the plateau being maintained for 3 to 6 hours.

2. Method according to claim 1, characterized in that the oily phase consists of one or more compounds chosen from linear and branched alkanes having at least 9 carbon atoms.

3. Method according to any one of claims 1 or 2, characterized in that the cationic surfactant is chosen from quaternary ammoniums having at least 8 carbon atoms.

4. Method according to any one of claims 1 to 3, characterized in that the metallic salts precursors of metal oxide M are chosen from chlorides and nitrides.

5. Method according to any one of claims 1 to 4, characterized in that the silica precursor is a silicon alkoxide.

6. Method according to claim 5, characterized in that the silicon alkoxide is chosen from tetraethoxy-orthosilicate (TEOS), (3-mercaptopropyl)trimethoxyxilane, (3-aminopropyl)triethoxysilane, N-(3- trimethoxysilylpropyl)pyrrole, 3 (2,4-dinitrophenylamino)propyltriethoxysilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, phenyltriethoxysilane and methyltriethoxysilane

7. Material in the form of a monolith, characterized in that it comprises a matrix of silicon oxide, and nanoparticles of a metal oxide M, and in that:

- said material has a multi-scale porosity, and

- said material is obtained from an oil-in-water emulsion comprising a silica precursor and at least one metal salt precursor of metal oxide M, such that the M/Si molar ratio in said emulsion is a number between 0.05 and 0.57.

8. Material according to claim 7, characterized in that it comprises macropores having an average dimension dA of 0.5 to 60 micrometers, mesopores having an average dimension dE of 20 to 30 Å and micropores having an average dimension dl from 5 to 10 Å, said pores being interconnected.

9. Material according to any one of claims 8 or 9, characterized in that its specific surface is between 400 and 600 m ² /g.

10. Use of a material according to any one of claims 7 to 9, in processes for the adsorption and specific storage of toxic gases such as in particular CO, NO and CO2.