Monolithic material in the form of a macrocellular aerogel

Abstract

The present invention relates to a material in the form of a monolithic silica aerogel with multi-scale porosity, characterized in that the material comprises macrocellular walls in the form of aerogel, to the use of said material and to a method of preparing a material in the form of a monolithic silica aerogel.

Description

Title of the invention: Monolithic material in the form of a macrocellular aerogel. TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a new material in the form of a monolithic silicic aerogel with multi-scale porosity, a process for preparing said material and its use.

[0002] The demand for ever lighter and higher-performance materials has been steadily increasing in recent years. Aerogels, falling into the class of ultralight materials, are therefore being studied more and more.

[0003] Aerogels, often described as "miracle materials of the 21st century," are attracting increasing interest due to their exceptional properties. In particular, silica aerogels combine the properties of various materials, such as near-glass-like transparency, thermal conductivity equivalent to that of polystyrene or polyurethane foams, and a specific surface area similar to that of activated carbon. These materials are distinguished by their low apparent density (between 0.003 and 0.5 g cm³), high porosity (80 to 99.8%), large specific surface areas (500 to 1200 m² g'), low thermal conductivity (0.005-0.1 W mK⁻¹), and low refractive index (1.05). This porosity, primarily composed of micro- and mesopores, makes them excellent thermal insulators. However, this same porosity limits their applications in other areas such as catalysis or decontamination.

[0004] To promote open porosity, polymers can be added to silica matrices, leading to the formation of organic-inorganic hybrid aerogels (Teo, N. et al., SC Open Cell Aerogel Foams via Emulsion Templating, Langmuir 2017, 33, 12729-12738). However, while interesting for certain applications, organic-inorganic hybrid aerogels have many drawbacks, particularly in terms of thermal stability and synthesis complexity.

[0005] Since the 2000s, the inventors have developed inorganic monoliths with multi-scale porosity, of particular interest in the fields of catalysis and decontamination (French patent application FR 0303774; F. Cam et al. J. Mat. Chem. 2004, 14, 1370). The synthesis of these monoliths, named Si(HIPE) (High Internal Phase Emulsion), is based on a combination of sol-gel chemistry and the physical chemistry of complex fluids, involving concentrated direct emulsions and lyotropic mesophases. Since then, Numerous inorganic monoliths with varied morphologies and properties have been developed. Among these monolithic materials, inventors have developed porous silica monoliths incorporating titanium oxide nanoparticles of the TiO2@Si(HIPE) type (FR2113537) or monolithic nanoparticulate metallo-oxides (FR2058457). All these monoliths are prepared from an emulsion comprising an oil phase and an aqueous phase (surfactant and silica precursor), then subjected to a polycondensation step for approximately one week until gels are obtained. These gels are then dried, for example in a desiccator, for about one month. Slow drying is necessary to preserve the monolith's structure. The monoliths are then heat-treated to release the mesoporosity (by producing CO2) and sinter the silica matrix.

[0006] These synthesis processes are particularly lengthy, making their implementation difficult on an industrial scale. The drying stage, considered one of the most demanding, generates significant capillary forces that weaken the mechanical stability of the monoliths. Furthermore, the heat treatment generally requires two successive calcination stages at high temperatures (above 600°C), which contributes to a high carbon footprint in the preparation of the monoliths.

[0007] Thus, there is a need to develop more efficient and faster processes, while minimizing their environmental impact.

[0008] The work of the inventors has made it possible to develop a process for synthesizing new materials in the form of monolithic silicic aerogels.

[0009] One object of the present invention is to provide a material in the form of monolithic silica aerogels, having macrocellular walls that exhibit the properties and morphological characteristics of traditional silica aerogels, and that is suitable for various applications, particularly in catalysis and decontamination. Another object of the present invention is to provide a process for preparing these materials in the form of monolithic silica aerogels that is simple, rapid, capable of reducing environmental impact, and transferable to an industrial scale. Description of the invention

[0010] The present invention thus has as its first object a material in the form of a monolithic silicic aerogel with multi-scale porosity, characterized in that the material comprises macrocellular walls in the form of aerogel.

[0011] By "aerogel" is meant an ultralight porous material, consisting mainly of gas and a very low density solid structure, formed in particular by a three-dimensional network of nanoparticles.

[0012] The term "monolith" refers to a solid object whose smallest dimension is at least 1 mm. Monoliths are easy to shape (columns, films, beads) due to the absence of dust. The monolith is self-supporting; in other words, it has sufficient mechanical strength to be used as is, i.e., without support.

[0013] By “multi-scale porosity” is meant a material with a hierarchical structure, comprising macropores, mesopores, and micropores.

[0014] The different pore size classes are determined according to the IUP AC nomenclature (in English, for International Union of Pure and Applied Chemistry).

[0015] Macropores can be identified by scanning electron microscopy (SEM) and their junction windows quantified by mercury intrusion measurements. Implementation of the mercury intrusion technique demonstrates the good mechanical strength of the resulting monoliths, which withstand the mercury pressures to which they are subjected during the measurements.

[0016] Mesoporosity can be identified by transmission electron microscopy (TEM). The vermicular texture of the mesoporosity can be identified by small-angle X-ray diffraction (SAXS), this technique also serving to quantify the pore-to-pore distance. Mesoporosity and microporosity can be quantified and segregated by a nitrogen adsorption-desorption technique, the analysis of which is done by the BET calculation method, which aggregates 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 BJH 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.

[0017] According to an advantageous embodiment of the invention, the material according to the present invention comprises open-porosity macropores, having an average diameter of 0.5 to 60 micrometers, mesopores having an average diameter of 2 to 10 nanometers and micropores having an average diameter of less than 2 nm, said pores being interconnected.

[0018] It should be noted that within the framework of the present invention, and unless otherwise stipulated, the ranges of values ​​indicated are understood to include the limits.

[0019] According to an advantageous embodiment of the invention, the material has a surface 0 1 0 1 0 1 specific between 800 mg and 2000 mg, preferably between 900 mg and 1900 mg.

[0020] According to the present invention, the specific surface area is determined by methods well known to those skilled in the art, and in particular by the BET method, preferably using dinitrogen as the gas.

[0021] Advantageously, the material has a porosity greater than 90%, preferably between 92% and 99%, on the total volume of the material.

[0022] Advantageously, the material according to the invention has a density between 0.02 g cm3 and 0.1 g cm3, preferably between 0.05 g cm3 and 0.07 g cm3.

[0023] Conventional silica aerogels, obtained by simply mixing a silica precursor, water, and an acidic or basic catalyst followed by drying with CO2 under supercritical conditions, have a high specific surface area, mainly composed of mesopores. However, this porosity is not readily accessible, as it is governed by a slow diffusive process (according to Donnan's law). Consequently, these conventional silica aerogels are primarily limited to use as thermal insulators.

[0024] The material according to the present invention, unlike conventional silica aerogels, has macrocellular walls (Bedside) in aerogel form, that is to say having the properties and characteristics of aerogels within the macrocellular walls themselves, with macropores with open porosity.

[0025] According to the present invention, the material, in the form of a monolithic silicic aerogel, is self-supporting and has an interconnected macroporosity, thus offering easy accessibility to the active sites, both for a carrier fluid and for photons.

[0026] In addition, this macroporosity allows for easy transport of photons to the heart of the materials (the aerogel nature of the macrocellular walls accentuates this phenomenon Rayleigh scattering added (intra-wall) to a Mie scattering (within the macroporosity).

[0027] Thus, a material in the form of a monolithic silica aerogel according to the invention having macrocellular walls in the form of aerogel and an interconnected macroporosity presents an ideal structure for many applications.

[0028] The material according to the present invention can be used, for example, as a catalyst, in particular as a photo-activated catalyst (gaseous or liquid phases) or thermo-activated catalyst (gaseous or liquid phases), adsorber, passive or active, filter (active or passive if operated in continuous flow), residual water sensor, as well as acoustic or thermal insulation.

[0029] The material conforming to the first object of the invention can advantageously be obtained according to a process as defined below.

[0030] Another object of the invention is to provide a method for preparing a material in the form of a monolithic silica aerogel with multi-scale porosity, said material comprising macrocellular walls in aerogel form and said process comprising:

[0031] a) an emulsification step of an oily phase in an acidic aqueous phase, said aqueous phase comprising at least one surfactant and at least one silicic precursor, in order to obtain an emulsion;

[0032] b) a polycondensation step of the emulsion obtained in step a), preferably between 1 and 20 days, in order to obtain a gel; and

[0033] c) A step of drying the gel obtained in step b) by CO2 under pressure and temperature, in particular under supercritical conditions, at a pressure greater than or equal to 73.8 bar and at a temperature greater than 31°C.

[0034] By "acidic aqueous phase" is meant an aqueous phase having a pH below the isoelectric point of silica, i.e. below 2.1, and preferably between 0.01 (Hammet acidity) and 1. The strong acid used to adjust the pH of the aqueous surfactant solution is preferably hydrochloric acid, for example at a concentration between 30% and 40% by mass.

[0035] According to an advantageous embodiment of the invention, the oily phase consists of one or more compounds selected from linear and branched alkanes having at least 9 carbon atoms. Even more advantageously, the linear alkanes comprise between 10 and 12 carbon atoms. Decane or dodecane are examples.

[0036] In an advantageous embodiment of the invention, the silica precursor is a silicon alkoxide, preferably selected from tetraethyl orthosilicate (TEOS), (3-mercaptopropyl)trimethoxyxilane, (3-aminopropyl)triethoxysilane, N-(3-trimethoxysilylpropylpyrrole), 3-(2,4-dinitrophenylamino)propyltriethoxysilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, phenyltriethoxysilane, and methyltriethoxysilane, sodium silicate solutions, or mixtures thereof. TEOS is particularly advantageous.

[0037] A mixture of silicon alkoxide precursors in which TEOS remains predominant may also be envisaged.

[0038] The concentration of silica precursor 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.

[0039] According to an advantageous embodiment of the invention, the aqueous phase further comprises at least one inorganic precursor and / or at least one metal salt precursor of metal oxide M and / or nanoparticles of at least one metal oxide M, where M represents a metal or metalloid selected from the following metals and metalloids: Cr, Co, Mn, Ni, Ce, V, Y, W, Nb, Mo, Fe, Zn, Ta, Sn, Cd, Cu, In, Ti, Zr, Al, Nb, Pt, Pd, Au, Hg, preferably W, Ti, Al and Zr.

[0040] For the purposes of the present invention, nanoparticles of a metal oxide M are understood to mean 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.

[0041] The metal oxide nanoparticles M preferably have at least one dimension greater than 10 nm, and preferably greater than 50 nm.

[0042] Advantageously, the molar ratio M / Si is between 0.01 and 1, preferably between 0.05 and 0.5.

[0043] According to the present invention, the molar ratios refer to those present in the oil-in-water emulsion obtained at the end of step a). This molar ratio remains substantially constant during steps a), b) and c).

[0044] Advantageously, the concentration of metal oxide M within the aqueous solution varies preferably from 1 g 11 to 100 g 1', preferably from 2 g 11 to 50 g 1'.

[0045] Advantageously, the inorganic precursor is aluminium oxide, preferably with a Si / Al molar ratio between 1 and 35.

[0046] In an advantageous embodiment of the invention, the metal salts that are precursors of metal oxides (i.e., metal oxides M) are selected from chlorides and nitrides. Such metal salts that are precursors of metal oxide M are, for example, described in patent application WO2022 / 058449. These metal salts that are precursors of metal oxide M are transformed during the process into metal oxide M nanoparticles.

[0047] Thus, in association with the silica precursor, the precursor forms a matrix cooxide (mixed oxide), while the nanoparticles form a nanoparticulate cooxide. The material, in the form of a monolithic silica aerogel, is in mixed oxide form.

[0048] According to an advantageous embodiment of the invention, the surfactant is a cationic surfactant selected from quaternary ammonium compounds having at least 8 carbon atoms. Examples include tetradecyltrimethylammonium bromide (TTAB), dodecyltrimethylammonium bromide, and cetyltrimethylammonium bromide.

[0049] According to another advantageous embodiment of the invention, the surfactant is a non-ionic surfactant.

[0050] The presence of a nonionic surfactant enhances the mesoporosity of the material obtained at the end of the process. In this case, the nonionic surfactant is preferably chosen from among ethoxylated head surfactants and nonylphenols. Among such surfactants, one can particularly mention the block copolymers of ethylene glycol and propylene glycol sold, for example, under the trade names Pluronic® P123, Pluronic® P125 and Pluronic® F127 by BASF.

[0051] Advantageously, the nonionic surfactant is mixed with at least one salt such as ammonium sulfate, sodium chloride, magnesium chloride or sodium sulfate.

[0052] The concentration of surfactant used in step a) preferably varies from 20 to 40% by mass.

[0053] Step b) of polycondensation (or maturation) is preferably carried out at room temperature, in particular for a period of at least 1 day, preferably from 2 to 8 days.

[0054] At the end of step b), a gel is obtained, which can also be referred to as a solid material (solidified emulsion).

[0055] Since step b) is carried out in an acidic medium, only the silica precursor hydrolyzes and polycondenses.

[0056] The process according to the invention may further comprise after step b) of polycondensation, a step b') of removal of the oily phase, by treatment with at least one organic solvent such as tetrahydrofuran, dichloromethane, or chloroform.

[0057] By "treatment with at least one organic solvent" is meant a washing of the gel obtained in step b) with an organic solvent, preferably carried out by several successive baths, preferably three successive baths.

[0058] Advantageously, the step of removing the oily phase with an organic solvent preferably lasts about 24 hours, even more preferably about one night.

[0059] Advantageously, when mixed oxides are prepared, dichloromethane is used in step b') of removing the oily phase. Preferably, when dichloromethane is used, step b') of removing the oily phase is carried out protected from all moisture, preferably in a desiccator. This prevents excessively rapid evaporation of the dichloromethane.

[0060] Advantageously, this step b') makes it possible to eliminate the organic residues from the oily phase which are found essentially in the macropores.

[0061] The process according to the invention may further include a step of transferring the organic solvent used in step b'), with absolute ethanol, for example, by immersing the gels in successive baths, typically three baths, of absolute ethanol. This solvent is perfectly miscible with CO2 under supercritical conditions. The gels are then stored in this same solvent until drying.

[0062] Next, step c) of drying the gel obtained in step b) is carried out by CO2 under pressure and temperature, in particular under supercritical conditions, at a pressure greater than or equal to 73.8 bar and at a temperature greater than 31°C, preferably at a pressure between 73.8 bar and 250 bar, even more preferably between 73.8 and 200 bar and at a temperature between 31°C and 100°C, even more preferably between 31°C and 60°C.

[0063] Advantageously, step c) is carried out over a period of between 10 minutes and 5 hours, and includes a depressurization rate of between 1 bar min 1 and 50 bar min *, preferably from 2 bar min 1 to 10 bar min *.

[0064] According to the present invention, the drying step (c) surprisingly allows for the leaching of more than 90% of the surfactant, without heat treatment, while simultaneously extracting the solvent (ethanol) under the temperature and pressure conditions used. The extraction of the surfactant during drying releases the mesoporous material, making it accessible prior to heat treatment.

[0065] Thus, this drying step c) is characterized by its speed, its reproducibility, and its ability to eliminate the capillary forces that generally occur in traditional drying methods, such as the use of a desiccator for a minimum period of one month.

[0066] In addition, the depressurization kinetics that can be implemented during this step make it possible to open the junction windows between the intermacrocells in a controlled manner, while preserving the microporosity and mesoporosity of the material.

[0067] According to an advantageous embodiment of the invention, the process does not include step b') of removing the oily phase. In this case, step c) of drying can simultaneously wash the gel obtained in step b) of polycondensation, while removing the oily phase and the surfactant. This removal is carried out using a CO2-ethanol mixture.

[0068] Advantageously, the oil and / or surfactant thus removed can be recovered and / or recycled for further use, thereby contributing to the sustainability and reduction of the environmental impact of the synthesis process.

[0069] The process may further include calcination after step c) of drying, comprising:

[0070] i) A first stage at a temperature between 160°C and 200°C, with a heating rate between 0.5 and 3°C / min, the platform being maintained for a period of 2 to 6 hours, preferably for 3 hours; and

[0071] ii) A second stage at a temperature between 300°C and 600°C, preferably between 400°C and 500°C, with a heating rate between 0.5 and 2°C / min, the platform being maintained for 3 to 8 hours, preferably for 7 hours.

[0072] The first platform can allow the physisorbed water to be eliminated first, then the chemisorbed water in a second step.

[0073] The second platform can, for its part, allow the elimination of residual surfactant molecules, but also sinter the silica framework to densify the materials, and thus give the material better mechanical strength, thanks to the sintering of the silica.

[0074] The present invention also relates to a material in the form of a monolithic silicic aerogel with multi-scale porosity that can be obtained or obtained by a synthesis process as described above. BRIEF DESCRIPTION OF THE FIGURE

[0075] Other features and advantages of the invention will become apparent from the following illustrative examples, with reference to:

[0076] [Fig-1] Fig. 1 represents a CO2 drying setup under conditions supercritical implemented by the process according to the present invention.

[0077] [Fig. 2] [Fig. 2] shows a photograph of a monolithic silica aerogel of the SiO2 type according to the present invention (obtained in Example 1.1) (left) and a silica xerogel (right), dried under atmospheric conditions, shaped in hemolysis tubes. The aerogel and the xerogel are arranged on a textured fabric background, with a ruler graduated in centimeters visible on the right to provide a scale.

[0078] [Fig.3] Fig.3 represents scanning electron microscopy images of the following materials: a), b): silica xerogel, c), d): monolithic silica aerogel of the SiO2 type (depressurization rate: 5 bar min 1 and drying time: 1 hour), e), f): monolithic silica aerogel of the SiO2 type (depressurization rate: 2 bar min 1 (from 160 to 80 bar) then 5 bar min 1 (from 80 bar at atmospheric pressure) and drying time: 1 hour; g), h): monolithic silica aerogel of the SiO2 type (depressurization rate: 5 bar min 1 (from 160 to 80 bar) then 10 bar min 1 (from 80 bar at atmospheric pressure) and drying time: 1 hour).

[0079] [Fig. 4] [Fig. 4] shows the pore size distribution obtained by mercury intrusion porosimetry of monolithic silica aerogels of the SiO2 type according to the present invention (obtained in Example 1.1) at different depressurization rates (SiO2_Aero_2: monolithic silica aerogel of the SiO2 type at a depressurization rate of 2 bar min⁻¹, SiO2_Aero_5: depressurization rate of 5 bar min⁻¹, SiO2_Aero_2+5: depressurization rate of 2 bar min⁻¹ (from 160 to 80 bar) then 5 bar min⁻¹ (from 80 bar to atmospheric pressure), SiO2_Aero_2+10: depressurization rate of 2 bar min⁻¹ (from 160 to 80 bar) then 10 bar min⁻¹ (from 80 bar to atmospheric pressure) atmospheric pressure), SiO2_Aero_5+10: depressurization rate: 5 bar min 1 (from 160 to 80 bar) then 10 bar min 1 (from 80 bar to atmospheric pressure), SiO2_Aero_10: rate depressurization: 10 bar min 1 and SiO2_Xero: silica xerogel). In all cases, the drying time is 1 hour.

[0080] [Fig. 5] [Fig. 5] shows the nitrogen adsorption / desorption isotherms of monolithic silica aerogels of the SiO2 type according to the present invention (obtained in Example 1.1) as a function of the depressurization rate (from bottom to top) (Aero_5: depressurization rate: 5 bar min*, Aero_2+5: depressurization rate: 2 bar min⁻¹ (from 160 to 80 bar) then 5 bar min⁻¹ (from 80 bar to atmospheric pressure), Aero_10: monolithic silica aerogel of the SiO2 type at a depressurization rate of 10 bar min*, Aero_2+10: monolithic silica aerogel of the SiO2 type at a depressurization rate of 2 bar min⁻¹ (from 160 to 80 bar) then 10 bar min⁻¹ (from 80 bar at atmospheric pressure), Aero_2: monolithic silica aerogel of the SiO2 type at a depressurization rate: 2 bar min 1, Aero_5+10: depressurization rate: 5 bar min 1 (from 160 to 80 bar) then 10 bar min 1 (from 80 bar to atmospheric pressure)) and for a silica xerogel (Xero).In all cases, the drying time is 1 hour.

[0081] [Fig.6] Fig.6 represents the pore size distribution (mesopores) for monolithic silicic aerogels according to the present invention (obtained in Example 1.1) (SiO2_Aero_2_lh (monolithic silicic aerogel of the SiO2 type at a depressurization rate: 2 bar min 1 and drying time: 1 hour), SiO2_Aero_5_lh (depressurization rate: 5 bar min 1 and drying time: 1 hour) and SiO2_Aero_10_lh (depressurization rate: 10 bar min 1 and drying time: 1 hour).

[0082] [Fig.7] Fig.7 represents two graphs of a small-angle X-ray scattering (SAXS) analysis of a monolithic silica aerogel according to the present invention (obtained in Example 1.1) (SiO2_Aero_2_lh (depressurization rate: 2 bar min*, drying time: 1 hour); a) Small-angle scattered intensity and linear regression (dotted line) and b) Kratky plot of the same data I(q)xq2 as a function of q.

[0083] [Fig.8] Fig.8 represents scanning electron microscopy (SEM) images of monolithic silica aerogels according to the present invention: a,b) WO3@SiO2_Aero_5_lh (monolithic silica aerogel of type WO3@SiO2 at a depressurization rate: 5 bar min*, drying time: 1 hour), c,d) P25-TiO2@SiO2_Aero_5_lh (monolithic silica aerogel of type P25-TiO2@SiO2 at a depressurization rate: 5 bar min*, drying time: 1 hour) and e,f) A-TiO2@SiO2_Aero_5_lh (monolithic silica aerogel of type A-TiO2@SiO2 at a depressurization rate: 5 bar min*, drying time: 1 hour).

[0084] [Fig.9] Fig.9 represents the nitrogen adsorption / desorption isotherms monolithic silicic aerogels of type A-TiO2@SiO2_Aero_5_lh and WO3@SiO2_Aero_5_lh.

[0085] [Fig. 10] The [Fig. 10] represents an adsorption / desorption isotherm of nitrogen of a monolithic silicic aerogel obtained in example 1.5.

[0086] [Fig. 11] The [Fig. 11] represents two thermogravimetric analysis curves of a silica xerogel (SiO2_Xero) and a monolithic silicic aerogel according to the present invention (SiO2_Aerio_5_lh: depressurization rate: 5 bar min 1 and drying time: 1 hour).

[0087] [Fig. 12] The [Fig. 12] represents a photograph of a monolithic silicic aerogel SiO2_Aero_5_lh after heat treatment (calcination) at 450°C.

[0088] [Fig. 13] The [Fig. 13] represents the pore size distribution obtained by mercury intrusion porosimetry of a monolithic silicic aerogel of type SiO2SiO2_Aero_5_lh uncalcined (example 1.1), calcined at 450°C and calcined at 700°C.

[0089] [Fig. 14] The [Fig. 14] represents the nitrogen adsorption / desorption isotherms; a) Nitrogen adsorption / desorption isotherms of a non-calcined (example 1.1), calcined at 450 °C, and calcined at 700 °C monolithic silicic aerogel SiO2_Aero_5_lh and b) Developed areas calculated from the BET and BJH models as a function of calcination temperature.

[0090] [Fig. 15] The [Fig. 15] represents the X-ray diffractograms of an aerogel calcined at 450 °C (light grey) and uncalcined (dark grey). EXAMPLES

[0091] The raw materials used in the examples are listed below: - Dodecane, purity > 99%, Sigma-Aldrich, - Hydrochloric acid (HCl), 37% by mass, Sigma-Aldrich, - Myristyltrimethylammonium bromide, 35% by mass, Alfa-Aesar, - Tetraethyl orthosilicate (TEOS), purity > 99%, Sigma-Aldrich, - Absolute ethanol, purity = 99.9%, AnalaR NORMAPUR®, VWR Chemical, - Tetrahydrofuran (THF), Sigma-Aldrich, - Dichloromethane (CH2Ci2), Sigma-Aldrich, - Materials of the type P25-TiO2@SiO2 were obtained from commercial TiO2 Aeroxide® P25 (purity > 99.5%, purchased from Sigma-Aldrich). - Deionized water, obtained using a Milli-Q type water purification system.

[0092] Unless otherwise indicated, all materials were used as received from the manufacturers.

[0093] Example 1: A process for preparing monolithic silica aerogels according to the present invention

[0094] Example 1.1: Process for preparing a monolithic silica aerogel of the SiO2 type - Emulsification stage

[0095] The emulsification step includes an aqueous phase composed of a surfactant solution (myristyltrimethylammonium bromide) concentrated at 35% by mass (16 g). This concentration, significantly higher than the critical micelle concentration of the surfactant, was chosen to ensure the presence of a sufficient number of micelles to contribute to the formation of the mesostructure. The concentrated surfactant solution is acidified by adding 5 g of hydrochloric acid concentrated at 37% by mass. The solution is placed under magnetic stirring for a few minutes. The pH of the reaction medium is then close to 0. Next, 5 g of a silicic precursor (tetraethyl orthosilicate (TEOS)) are added to the aqueous phase. The aqueous phase is then placed under magnetic stirring for about ten minutes to initiate the hydrolysis of the precursor.This aqueous phase is then transferred to a mortar where 37 g of dodecane, constituting the oily phase, are added drop by drop to the aqueous phase and incorporated using a pestle. - Polycondensation stage (or maturation)

[0096] The emulsion thus obtained is poured into suitable containers (here, hemolysis tubes) and left to mature for one week to obtain gels. The quantities used in this synthesis allow for the production of approximately ten gels. - Oil phase removal stage

[0097] The gels obtained after seven days are washed in successive baths (3 baths) of tetrahydrofuran (THF) for 24 hours. - Washing stage (solvent transfer)

[0098] A solvent transfer from tetrahydrofuran (THF) to absolute ethanol is then carried out by immersing the gels in three successive baths of absolute ethanol. This solvent was chosen because it is perfectly miscible with CO2 under supercritical conditions. The gels are stored in this same solvent until drying time. - Drying stage

[0099] The gel to be dried is placed in a stainless steel reactor with an internal diameter of approximately 1.2 cm. The gel is covered with absolute ethanol (approximately 15 mL) and the reactor is closed and then connected to the circuit as illustrated in [Fig. 1].

[0100] The temperature of the cryostat is 3°C, that of the preheater is 70°C and that of the water bath is 50°C. The pressure is regulated by a back pressure regulator (BPR, Back Pressure Regulator (in English). The pressure is increased in 20 bar increments until the desired pressure of 160 bar is reached. These conditions allow CO2 to be obtained in its supercritical state and a high density to be achieved.

[0101] The drying time implemented is 1 hour.

[0102] Different depressurization conditions were also implemented. Initially, three constant depressurization rates were applied: 2 bar min₁ (slow rate), 5 bar min₁ (intermediate rate), and 10 bar min₁ (fast rate). Subsequently, a change was made to the depressurization ramp at 80 bar. The mixed depressurization ramps studied are as follows: 2 bar min₁ (from 160 bar to 80 bar) then 5 bar min₁ (from 80 bar to atmospheric pressure), 2 bar min₁ then 10 bar min₁*, and finally 5 bar min₁ then 10 bar min₁*.

[0103] Monolithic silicic aerogels of the SiO2 type were obtained.

[0104] Example L2: process for preparing a monolithic silica aerogel of the P25-TiO2 @SiO2 type

[0105] The emulsification step is identical to that described in Example 1.1, except that TiO2 nanoparticles (300 mg) are added to the aqueous phase comprising a surfactant (myristyltrimethylammonium bromide) concentrated at 35% by mass (16 g) and hydrochloric acid (5 g, 37% by mass). The suspension is then placed in an ultrasonic bath for about ten minutes. Next, the silica precursor (tetraethyl orthosilicate (5 g)) is added, and the aqueous phase is placed under magnetic stirring for about ten minutes to initiate the hydrolysis of the precursor. The subsequent steps of emulsification, as well as polycondensation (maturation), removal of the oily phase (with dichloromethane), washing and drying, are identical to those described in Example 1.1, leading to the formation of a monolithic silicic aerogel of the P25-TiO2@SiO2 type.

[0106] Monolithic silicic aerogels of the P25-TiO2@SiO2 type were obtained.

[0107] Example L3: process for preparing a monolithic silica aerogel of the A-TiO2 @SiO2 type

[0108] The anatase TiO2 nanoparticles (hereafter referred to as A-TiO2) are synthesized according to a protocol already described in the literature (Tebby, Z. et al. Low-Temperature UV-Processing of Nanocrystalline Nanoporous Thin TiO2 Films: An Original Route toward Plastic Electrochromic Systems. Chem. Mater. 2008, 20, 7260-7267).

[0109] More specifically, a solution of titanium isopropoxide (19 g) in propan-2-ol (20 mL) is added dropwise to a mixture of glacial acetic acid (91 mg) in deionized water (26 mL) (pH = 3). A white precipitate appears immediately. The mixture is placed under vigorous stirring at room temperature for 15 hours. The volatile compounds (acetic acid and propan-2-ol) are evaporated. Using a rotary evaporator (30 minutes), the resulting white powder is mixed with 20 mL of deionized water and treated at 250°C for 13 hours in an autoclave. The powder is then dried at 300°C for 30 minutes and subsequently treated under UV irradiation for 3 hours. Titanium dioxide (~5 g) is obtained as a white powder (yield: 99%).

[0110] The quantity of TiO2 used for this synthesis is 300 mg. The steps of emulsification, polycondensation (maturation), removal of the oily phase (with dichloromethane), washing and drying are identical to those described in Example 1.2.

[0111] Monolithic silicic aerogels of the A-TiO2@SiO2 type were obtained.

[0112] Example L4: process for preparing a monolithic silica aerogel of the WO3 @SiO2 type

[0113] Monolithic silica aerogels of the WO3@SiO2 type are prepared as follows:

[0114] - Emulsification step:

[0115] The emulsification step comprises an aqueous phase consisting of a 35% (16 g) concentrated surfactant solution (myristyltrimethylammonium bromide), followed by the addition of 0.952 g of tungsten hexachloride (WC16), i.e., metal oxide precursors, until dissolution or homogeneous dispersion (by magnetic stirring and ultrasonic bath). Next, 5 g of 37% (12 M) hydrochloric acid are added, and 5 g of silica precursor (TEOS tetraethyl orthosilicate) are introduced into the acidic aqueous phase (pH = 0.05) and hydrolyze to Si(OH)4. This acidic aqueous phase is stirred for 10 minutes to homogenize, and then 37 g of dodecane are added dropwise for emulsification (direct oil-in-water emulsion).

[0116] Next, the polycondensation (maturation), oil phase removal (with dichloromethane), washing and drying steps, as described in examples 1.2 and 1.3, are reproduced.

[0117] Monolithic silicic aerogels of the WO3@SiO2 type were obtained.

[0118] Example L5: process for preparing a monolithic silicic aerogel of the Al2O3-SiO2 type (HIPE)

[0119] Monolithic silica aerogels of the Al2O3-SiO2 (HIPE) type are prepared by following the same emulsification, polycondensation (maturation), oil phase removal (with dichloromethane), and washing steps as described in Examples 1.2 to 1.4. During the emulsification step, 0.8 g of AlCl3 is incorporated into the aqueous phase of the emulsion. The Si / Al ratio is 4.

[0120] As described in Example 1.1, the gel is also dried by covering it with absolute ethanol (approximately 15 mL). The reactor is then closed and connected to the circuit at a temperature of 50°C (maintained with a water bath, preheater set to 70°C). The pressure is gradually increased to 160 bar. The drying time is 1 hour in CO2 under supercritical conditions. The depressurization time is then 20 minutes (at a rate of 8 bar / min).

[0121] Monolithic silicic aerogels of the Al2O3-SiO2(HIPE) type have been obtained.

[0122] Example 2: Characterization of monolithic silica aerogels according to the present invention - Characterization techniques The silica aerogels obtained in the examples were characterized at all scales (macro-, micro-, and mesoscopic) after drying (with CO2 under supercritical conditions). Properties such as specific surface area and pore size distribution of the aerogels will be compared to those of a silica xerogel (hereafter referred to as SiO₂Xero). A xerogel is a material dried by slow drying under atmospheric conditions.

[0123] Macroporosity was qualitatively characterized by scanning electron microscopy (SEM) using a Hitachi TM-1000 scanning electron microscope operating at 15kV. The samples were coated under a gold / palladium plasma prior to characterization.

[0124] Macroporosity was also quantified by mercury intrusion measurements at room temperature, using a device marketed under the name Autopore V from Micromeretics, to achieve the characteristics of the macroscopic cells composing the skeleton of the monolith.

[0125] Mesoporosity and microporosity were characterized by small-angle X-ray scattering (SAXS). This analysis allows the study of the structure and organization of materials at the nanoscale (1-100 nm). The instrument used is a XEUSS 2.0 (XENOCS, Grenoble, France) equipped with a GeniX3D system (XENOCS, microfocal copper anodic source coupled to a FOX3D single-reflecting mirror). The instrument generates a monochromatic beam of 8 keV (Cu Ka radiation, X = 1.5418 Å). The beam is collimated by a set of two motorized slits (4 blades). The samples are analyzed in powder form in a 1.5 mm thick glass capillary and are exposed for 2 hours. The data are collected by a 2D DECTRIS PILATUS300k detector (Baden-Dattwill, Switzerland). The sample-to-detector distance is 1634 mm, calibrated with a silver behenate standard.This distance allows probing a range of wave vectors q from 0.007 to 0.23 Â-1. 1D diffractograms. (I as a function of q) are obtained by analyzing the detector images with the FOXTROT software.

[0126] Mesoscopic-scale characterizations were performed using nitrogen adsorption / desorption techniques to obtain information on porosity size and specific surface area. The instrument used was a 3Flex from Micromeretics.

[0127] X-ray diffraction (XRD) has also been used to obtain information on the crystalline phases in a material. The instrument used is a Bruker AXS diffractometer (D2 PHASER A26-X1+A2B0D3A) having a copper anode (Kα radiation).

[0128] Example 2.1, Characterization of monolithic silica aerogels obtained in Example 1.1

[0129] Example 2.L1: Macroscopic scale characterization

[0130] On a macroscopic scale, the silica aerogels obtained in Example 1.1 retain properties similar to those of silica xerogels. This is demonstrated in [Fig. 2], which shows a silica aerogel of the SiO2 type (left) according to the present invention and a silica xerogel (right). This image shows that the silica aerogel according to the present invention retains its monolithic structure. Furthermore, the aerogel obtained is white, and not semi-transparent or turbid like a conventional aerogel. This characteristic is due to the macroporosity, which, by scattering light, induces a white coloration. Moreover, the shrinkage observed during drying is less significant under supercritical conditions (aerogel according to the present invention) than under atmospheric conditions (xerogel), approximately 10% for aerogels and around 20% for the silica xerogel.The shrinkage observed during drying under atmospheric conditions is primarily linked to capillary forces existing at the liquid / solid interfaces. These capillary forces are reduced under supercritical conditions, which limits the shrinkage of the aerogel.

[0131] By "shrinkage of an aerogel" is meant the reduction in its volume or structural deformations which may occur during the drying process.

[0132] Figure 3 shows scanning electron microscopy (SEM) images of The following materials are shown: a), b): silica xerogel; c), d): monolithic silica aerogels of the SiO2 type (depressurization rate: 5 bar min⁻¹ and drying time: 1 hour); e), f): monolithic silica aerogels of the SiO2 type (depressurization rate: 2 bar min⁻¹ (from 160 to 80 bar) then 5 bar min⁻¹ (from 80 bar to atmospheric pressure); g), h): monolithic silica aerogels of the SiO2 type (depressurization rate: 5 bar min⁻¹ (from 160 to 80 bar) then 10 bar min⁻¹ (from 80 bar to atmospheric pressure) and drying time: 1 hour). This [Fig. 3] represents the morphology of the aerogels. and xerogels at the macroscopic scale. At this scale, observations on aerogels are similar to those made on xerogels, regardless of the experimental conditions studied. The macroporosity induced by the oil droplets of the initial direct emulsion resembles aggregated hollow spheres, typically observed in this type of material (Cam, F. et al. Inorganic Monoliths Hierarchically Textured via Concentrated Direct Emulsion and Micellar Templates. J. Mater. Chem. 2004, 14, 1370-1376).

[0133] Mercury intrusion porosimetry was used to evaluate the size distribution of the junction windows connecting two spheres, as shown in [Fig. 4]. This [Fig. 4] shows that the size distributions change as a function of the depressurization rate. Indeed, for the slowest depressurization rates (2 and 5 bar min⁻¹), the distributions exhibit a majority population (1 peak) with an average size centered around 10 pm. This size distribution is close to that observed for a silica xerogel, named SiO2_Xero, in [Fig. 4]. As the depressurization rate increases, a bimodal trend appears, with a second population located between 1 pm and 10 pm. This change is observed between the sample named SiO2_Aero_2+5_lh and the sample named SiO2_Aero_2+10_lh. The reasons behind this trend would stem from a decrease in the mechanical relaxations of gas expansions.This effect is all the more pronounced when the depressurization is rapid.

[0134] Mercury intrusion porosimetry also provides information on the percentage of porosity in the material as well as its apparent density. The values ​​found are similar for all samples. The average values ​​of porosity and density are 92.7 ± 0.7% and 0.068 ± 0.013 g mL⁻¹, respectively.

[0135] Example 2.1.2: Characterization at the microscopic and mesoscopic scales

[0136] The microscopic and mesoscopic scales were probed by nitrogen adsorption / desorption. Aerogels are known to have very high specific surface areas, reaching 1000 m² g⁻¹. These surface areas are generally greater than those of xerogels, in which a collapse of porosity is frequently observed during drying due to capillary forces. In order to evaluate the nature of the porosity within the aerogels synthesized in Example 1.1, nitrogen adsorption / desorption analyses at 77 Kelvin (-196°C) were performed. The isotherms obtained for the different materials, as well as the specific surface areas, were determined from the BET (Brunauer-Emmet-Teller) and BJH (Barrett, Joyner, and Halenda) models.The BJH surface area was determined from the adsorption isotherm and characterizes the contribution of the mesopores to the total surface area calculated by the BET model. The BET (SBet) and BJH (SBjh) surfaces, as well as the average mesopore diameters, are presented in the following Table 1: [Tables 1] Material S bet (mg 1 ) S bjh (m2 g 1 ) D avg mesopores (nm) SiO2_Aero_2_lh 1360+70 1340+140 3.9+0.2 SiO2_Aero_5_lh 1470+30 880+155 2.4+0.3 SiO2_Aero_10_lh 1360+100 940+300 3.0+0.7 SiO2_Xero* (Comp arative) 526+20 36+1 - SiO2** (Comparative) 930+30 500+30 - Table 1: Average values ​​(triplicates) of developed areas (BET and BJH) (m2

[0137] g1) and mean diameters (Dmoy) of mesopores (nm).

[0138] * A SiO2-type material not conforming to the present invention. This material is dried under atmospheric conditions without heat treatment (calcination).

[0139] ** Material of the SiO2 type not in accordance with the present invention. This material has was obtained according to the classically applied protocol: slow drying under atmospheric conditions followed by heat treatment up to 700°C

[0140] Figure 5 shows the nitrogen adsorption / desorption isotherms of aerogels obtained as a function of the depressurization rate and the silica xerogel. All isotherms show significant gas adsorption at low relative pressure, characteristic of microporous materials. The developed microporosity arises from the organization of the silica tetrahedra. The isotherm of the SiO2_Xero sample is limited to this initial rapid adsorption, highlighting an essentially microporous material. This observation is confirmed by the calculation of the surface areas developed by the BET and BJH models (Table 1 above). Indeed, the BET surface area (SBET) of the (uncalcined) xerogel is approximately 530 m² g⁻¹ and the BJH surface area (SBJH) is 40 m² g⁻¹. The small BJH surface area for the SiO2_Xero sample is linked to the presence of the surfactant, which serves as an imprint on the mesopores.Furthermore, the specific surface areas BET and BJH of the monolithic silica aerogel according to the invention are significantly greater, regardless of the depressurization rate, than those obtained with a silica monolith produced by heat treatment (calcination) at 700°C. The isotherms of the aerogels exhibit a second phase of adsorption in the relative pressure range of 0.2 to 0.4. This adsorption, and the inflection observed at this point, is characteristic of mesoporous materials possessing mesopores of . small sizes (between 2 and 4 nm). In addition, a narrow hysteresis loop is observed between the adsorption and desorption branches for aerogels, around p / pO = 0.5 (the hysteresis loops are those described according to the IUPAC nomenclature, and illustrated in the article Thommes, M et al. Physisorption of Gases, with Special Reference to the Evaluation of Surface Area and Pore Size Distribution (IUPAC Technical Report). Pure Appl. Chem. 2015, 87, 1051-1069). This hysteresis is of the H4 type, characteristic of mesoporous materials with a non-rigid structure (Rouquerol, F. et al., Texture of divided materials - Pore size of nanoporous materials by nitrogen adsorption. Techniques de l'ingénieur 2017 and Rouquerol, F. et al., Texture of divided materials - Specific area of ​​powdery or nanoporous materials, Techniques de l'ingénieur 2017, 29).

[0141] Figure 6 shows the pore size distributions determined by the BJH model and confirms the values ​​mentioned above, with pore sizes centered around 3 nm (Fig. 6). Furthermore, the surface areas developed by the aerogels are significantly larger than those of the xerogels. The calculated BET and BJH surface areas are indeed on the order of 1400 m² g⁻¹ and 900 m² g⁻¹, respectively. The contribution of mesoporosity is very pronounced in these samples, where the BJH surface area sometimes even approaches the entire BET surface area.

[0142] Fig. 7 represents two graphs of a small-angle ray scattering analysis of a monolithic silicic aerogel obtained in Example 1.1.

[0143] The initial intensity decay is described by an exponent of -3.9, which closely approximates the Porod regime. The silica / air interfaces are therefore smooth, which is expected behavior for SiO2 matrices of this type. Several structural peaks can be identified at different wave vectors: q1 = 0.12 Å', q* = 0.14 Å', q* = 0.17 Å', q* = 0.18 Å', q = 0.21 Å' and q* = 0.22 Å'. The associated distances are respectively: 5.2 ('); 4.5 (*); 3.7 (*); 3.5 (*); 3.0 (,&) and 2.9 (4) nm. These structural peaks are linked to the organization of the mesostructure and, in particular, to the imprint left by the concentrated micellar phase. The presence of additional peaks in this sample is related to the fact that the mesostructure is highly developed in this material.

[0144] Aerogel isotherms can thus be described as a mixture of type I and IV isotherms (Thommes, M. et al. Physisorption of Gases, with Special Reference to the Evaluation of Surface Area and Pore Size Distribution (IUPAC Technical Report). Pure Appl. Chem. 2015, 87, 1051-1069), characteristic of microporous and mesoporous materials with very small pores (2-4 nm). The surface areas are significantly larger than those of xerogels, with a very important contribution from mesoporosity. The mesostructure of aerogels is particularly well defined, as identified by small-angle X-ray scattering ([Fig.7]).

[0145] Example 2.2: Characterization of monolithic silica aerogels obtained in examples 1.2 to 1.5

[0146] Example 2,2,1: Macroscopic characterization

[0147] On a macroscopic scale, the monolithic silica aerogels obtained in Examples 1.2 to 1.4 exhibit characteristics very similar to those obtained in Example 1.1 ([Fig. 8]). As with the silica matrices (of Example 1.1), shrinkage after drying is reduced (Figure not shown).

[0148] Example 2,2,2: Characterization at the microscopic and mesoscopic scales

[0149] Microscopic and mesoscopic scales were also probed by nitrogen adsorption / desorption. Intergranular mesoporosity is also observed, as evidenced by the hysteresis between the adsorption and desorption branches on the isotherms shown in [Fig. 9].

[0150] The isotherms obtained for the different materials and the specific surfaces were determined from the BET and BJH models. The BJH surface was determined from the adsorption isotherm. The BET (SBet) and BJH (SBjh) surfaces are presented in Table 2 below: [Tables2] Material 'S* AERO 'S* WO3@SiO2_Aero_5_lh 1110+30 600+20 780 + 20 390 + 10 Al2O3-SiO2(HIPE) 1820+50 1820+50 1050+ 20 1050+ 20 Table 2: Specific surfaces calculated from the BET and BJH models, Særo for monolithic silicic aerogels of type A-TiO2@SiO2,WO3@SiO2, with a depressurization rate of 5 bar min 1 and a drying time of 1 hour, and Al2O3-SiO2(HIPE) obtained in example 1.5, all are uncalcined, and Sxero ?oo c for materials (xerogels) obtained via conventional drying and calcined at 700°C.

[0151] The surface areas measured for the xerogel (Table 2) are lower than those of the silica matrices (Særo). The BET surface areas are on the order of 1100 m² g⁻¹ for samples A-TiO₂@SiO₂_Aero_5_lh and WO₃@SiO₂_Aero_5_lh and on the order of 1800 m² g⁻¹ for the aerogel obtained in Example 1.5. The specific surface areas of the aerogels are therefore much higher than those obtained for the same materials dried under atmospheric conditions and calcined at 700°C (Table 2). Despite the fact that Calcination of the silica xerogel releases the mesoporosity and sinters the silica matrix. The resulting xerogel exhibits significantly lower specific surface areas than monolithic silica aerogels obtained according to the invention. Consequently, the process implemented in the invention makes it possible to produce new materials in the form of monolithic silica aerogels, offering, in particular, optimized and accessible specific surface areas.

[0152] Example 2.3: Characterization of the monolithic silica aerogel obtained in Example 1.5

[0153] The microscopic and mesoscopic scales were also probed by nitrogen adsorption / desorption as described above. The adsorption / desorption isotherm is shown in [Fig. 10]. The specific surface area (BET), total pore volume, and mean pore size are shown in the following Table 3: [Tables3] Specific surface area (BET) (m² g⁻¹) 1820 + 50 Total pore volume (cm³ g⁻¹) (for pores with a diameter less than 193.46 nm) 1.387 Average pore size 3.04 nm Table 3: Specific surface area (BET), total pore volume and average pore size of the monolithic silica aerogel obtained in Example 1.5.

[0154] Example 3: Study of the temperature behavior of monolithic silica aerogels obtained in example 1.1

[0155] The temperature behavior of aerogels and xerogels was studied by thermogravimetric analysis coupled with a mass spectrometer. Here, a vacuum cycle was applied before the temperature rise.

[0156] Thermogravimetric analysis is a characterization technique that allows for the direct monitoring of mass loss in a sample as a function of temperature. The instrument used is a NETZSCH STA 449 F5 Jupiter coupled to a QMS 03D Aeolos Quadro analyzer. Data were recorded over a temperature range of 30°C to 700°C, with a temperature rise of 5°C per minute under airflow. The samples (4-5 mg) are ground into powder for analysis.

[0157] The thermal profiles of the materials as a function of mass loss are presented in [Fig. 11].

[0158] Two thermal events are observed in [Fig.1 1], the first before 100°C, reflecting the desorption of physisorbed molecules, such as water in particular. This initial mass loss, on the order of 5 to 7% by mass, is similar for both materials. The second thermal event occurs around 300°C. For In the SiO2_Xero sample, this loss of silica is on the order of 15% by mass and is accompanied by the production of CO2 and fragments of CxHy alkyl chains, identified by mass spectrometry. This observation reflects the degradation of an organic species, in this case the surfactant present in the mesopores of the silica matrix. For the SiO2_Aero_5_lh sample, this second loss is lower, around 5% by mass. Similar behavior has already been observed in mesoporous silicas (powders) dried by supercritical CO2 (Prouzet, E. et al. Toward a Sustainable Preparation of Tunable Mesoporous Silica. J. Supercrit. Fluids 2019, 143, 139-145). In this study, the authors demonstrate the extraction of nonionic surfactants, sometimes up to 90% by mass, during CO2 drying under supercritical conditions. The extraction of these species is possible due to the absence of electrostatic interactions between the organic fingerprint and the silica.The same phenomenon occurs here, in the case of a cationic surfactant and a monolithic macro-mesoporous silica matrix. It is therefore possible to wash gels (oil and surfactant) with a CO2-ethanol mixture directly after synthesis. This eliminates the need for washing steps in various solvents and allows for the recovery / recycling of the oil and surfactant thus obtained.

[0159] Supercritical CO2 drying thus allows the extraction of most of the surfactant before heat treatment (calcination). The mesoporosity of aerogels is therefore accessible before the calcination step, unlike traditional silica xerogels or aerogels which require heat treatment.

[0160] Example 4: A process for preparing monolithic silica aerogels comprising an additional heat treatment step according to the present invention

[0161] In the conventional synthesis protocol for monoliths, where drying is carried out under atmospheric conditions, heat treatment is used both to release mesoporosity and to sinter the silica matrix. In contrast, in the process according to the present invention, heat treatment can be used to sinter the material, the surfactant having already been largely eliminated during drying under supercritical conditions with CO2 (Example 3). In this example, after drying step c), heat treatment (calcination) was carried out at two different temperatures: 700°C or 450°C, as follows:

[0162] A first heat treatment is carried out with an initial temperature increase at a rate of 2°C min 1 up to 180°C, with the plate held for 3 hours. Then a second heat treatment is applied with a second, slower temperature increase, with a ramp of 1°C min 1 up to 450°C or 700°C, with the plate held for 7 hours.

[0163] Example 5: Characterization of monolithic silica aerogels obtained in Example 3

[0164] Example 5.1: Macroscopic characterization

[0165] Initially, the monolithic character is preserved after heat treatment as shown in [Fig. 12].

[0166] At the macroscopic scale, the morphology identified by scanning electron microscopy is the same as that presented in the previous examples. The size distributions of the junction windows as shown in [Fig. 13] exhibit a single population centered around 10 µm, regardless of the heat treatment applied. A very slight decrease appears to be observed in the mean pore diameter as the calcination temperature increases. This observation could be related to the shrinkage of the silica matrix during the heat treatment (sintering).

[0167] Example 5.2: Characterization at the microscopic and mesoscopic scales

[0168] The microscopic and mesoscopic scales were also probed by nitrogen adsorption / desorption. The adsorption isotherms are shown in [Fig. 14]. Nitrogen adsorption / desorption analyses of the calcined samples show a slightly lower specific surface area than those of the uncalcined samples. Indeed, during heat treatment, porosity can collapse, totally or partially, particularly at high temperatures. Calcination at 700°C results in a significant loss of mesoporosity due to exacerbated sintering. The BET (SBet) and B JH (SBjh) surface areas are presented in the following Table 4: [Tables 4] Material S BET (m2 g •') s bjh (m2 g O SiO2_ Aero_5_lh (Uncalcined) 1465 + 45 910 + 30 SiO2_Aero_5_lh_450°C 1470 + 45 640 + 20 Si02_Aero_10_lh_700°C 1290 + 40 380+ 10 Table 4: Numerical values ​​of the developed surfaces in relation to the BET and BJH models, SiO2_ Aero_5_lh (monolithic silicic aerogel obtained in example 1.1, with a depressurization rate of 5 bar min 1 and drying time: 1 hour) SiO2_Aero_5_lh_450°C (monolithic silicic aerogel with a depressurization rate: 5 bar min *, drying time: 1 hour and calcination temperature: 450°C), SiO2_Aero_10_lh_700°C (monolithic silicic aerogel with a depressurization rate: 5 bar min *, drying time: 1 hour and calcination temperature: 700°C).

[0169] The decrease in specific surface area and the partial loss of mesoporosity represent major drawbacks for the subsequent use of the materials in catalysis. However, for heat treatment at a lower temperature (450°C), the specific surface areas are very close to those of unheat-treated aerogels (SiO2_Aero_5_lh) ([Fig. 12]). The BJH surface area is slightly lower than that of the uncalcined sample, but this heat treatment at 450°C is an acceptable temperature. Calcination can be useful to give the material better mechanical strength, thanks to the sintering of the silica.

[0170] Calcined and uncalcined monolithic silica aerogel were then analyzed by X-ray diffraction ([Fig. 15]). The materials were reduced to a powder before analysis. The broad peak of amorphous silica, between 20 and 30° (20), is observable for both samples.

[0171] The applied heat treatment therefore has no impact on the silica microstructure, where the amorphous character remains clearly defined. However, the calcination temperature can affect the mesostructure through exacerbated sintering. The temperature must therefore be chosen carefully to avoid the collapse of this mesostructure, and a temperature below 700°C is preferred.

Claims

Demands

1. Material in the form of a monolithic silica aerogel with multi-scale porosity, characterized in that the material comprises macrocellular walls in aerogel form.

2. Material according to claim 1, characterized in that it comprises open-porosity macropores, having an average diameter of 0.5 to 60 micrometers, mesopores having an average diameter of 2 to 10 nanometers and micropores having an average diameter of less than 2 nm, said pores being interconnected.

3. Material according to any one of claims 1 or 2, characterized by a specific surface area between 800 m2 g 1 and 2000 m2 g ', preferably between 900 m2 g 1 and 1900 m2 g '.

4. A method for preparing a material in the form of a monolithic silica aerogel with multi-scale porosity, said material comprising macrocellular walls in the form of aerogel and said method comprising: a) a step of emulsifying an oily phase in an acidic aqueous phase, said aqueous phase comprising at least one surfactant and at least one silica precursor, in order to obtain an emulsion; b) a step of polycondensing the emulsion obtained in step a), preferably between 1 day and 20 days, in order to obtain a gel; and c) A step of drying the gel obtained in step b) with CO2 under pressure and temperature, in particular under supercritical conditions at a pressure greater than or equal to 73.8 bar and at a temperature greater than 31 °C.

5. A process according to claim 4, characterized in that the aqueous phase comprises at least one inorganic precursor and / or at least one metal salt precursor of metal oxide M and / or nanoparticles of at least one metal oxide M, where M represents a metal or metalloid selected from the following metals and metalloids: Cr, Co, Mn, Ni, Ce, V, Y, W, Nb, Mo, Fe, Zn, Ta, Sn, Cd, Cu, In, Ti, Zr, Al, Nb, Pt, Pd, Au and Hg, preferably W, Ti, Al and Zr.

6. A process according to any one of claims 4 or 5, characterized in that a step of removing the oily phase is carried out after step b), by treatment with an organic solvent such as tetrahydrofuran, dichloromethane or chloroform.

7. A method according to any one of claims 4 to 6, characterized in that step c) is carried out over a period of between 10 minutes and 5 hours, and includes a depressurization rate of between 1 bar min-1 and 50 bar min-1, preferably between 2 bar min-1 and 10 bar min-1.

8. A process according to any one of claims 4 to 7, characterized in that after step c), a calcination is carried out comprising: i. a first step at a temperature between 160°C and 200°C with a heating rate between 0.5 and 3°C min*, the tray being maintained for 2 to 6 hours, preferably for 3 hours; and ii. a second step at a temperature between 300°C and 600°C, preferably between 400°C and 500°C, with a heating rate between 0.5 and 2°C / min, the tray being maintained for 3 to 8 hours, preferably for 7 hours.

9. A process according to any one of claims 4 to 8, characterized in that the silicic precursor is a silicon alkoxide, such as tetraethyl orthosilicate (TEOS), (3-mercaptopropyl)trimethoxyxilane, (3-aminopropyl)triethoxysilane, N-(3-trimethoxysilylpropyl)pyrrole, 3 (2,4-dinitrophenylamino)propytriethoxysilane, N- (2-aminoethyl 1)-3-aminopropyltrimethoxysilane, phenyltriethoxysilane and methyltriethoxysilane, sodium silicate solutions or mixtures thereof.

10. Use of a material according to any one of claims 1 to 3, as a catalyst, thermo-activated catalyst, photo-activated catalyst, adsorber, filter, residual water sensor, acoustic insulator or thermal insulator.

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