WO2018197573A1 - Method for preparing mesoporous silica - Google Patents

Abstract

A method for preparing mesoporous silica from a hybrid silica/alkylene polyoxide material, comprising the extraction of the organic agents used in the preparation of the hybrid silica/alkylene polyoxide material by means of a treatment with supercritical CO2.

Description

 PROCESS FOR PREPARING MESOPOROUS SILICA Technical Field

The present invention relates to a process for preparing mesoporous silica from a hybrid silica / polyalkylene oxide material, which process comprises treating the hybrid material with supercritical carbon dioxide (C0 ₂ ) to extract organic structuring agents therefrom. .

 State of the art

 Since their identification as ordered mesostructured materials, mesoporous silicas have aroused great interest in the field of materials and catalysis, marked by thousands of publications. Among the materials reported so far, the best known are MCM-41, SBA-15 and HMS.

 Organized mesoporous silicas (SMO) are of great interest because of their particular properties. The definition of mesoporosity is a pore size ranging from 2 to 50 nm, but the range of pore sizes currently attained by organized mesoporous silicas is narrower. These consist in particular of a regular arrangement of the channels, a pore size which some authors claim is adjustable from 2 to 30 nm, higher than that of zeolites, but with a size distribution actually observed generally narrower and defined. between 1.0 nm and 15 nm. Known syntheses generally lead to materials having a single pore size: 4.0 nm, 9.0 nm and 2.5 nm for MCM-41, SBA-15, HMS, respectively, MCM, SBA, and HMS being the acronyms under which these different materials are usually cataloged, according to their synthesis protocol. The means of modifying these values have been described, but they are rarely used.

Mesoporous silicas are also characterized by a high surface area usually greater than 400 m 2 / g, sometimes greater than 1000m 2.g- ^"1.

 The physicochemical characteristics of these materials make it possible to envisage their use in various and varied fields, as in the fields of catalysis, the absorption of chemical elements, the separation, for example by chromato graphy on column, in the biochemistry as a carrier for immobilization of enzymes, in the field of medicament for the controlled release of active ingredients, or as nano-containers for protecting and delivering active agents.

The porosity of these porous silicas can also be used as a mold (or impression) allowing the synthesis of an organic or inorganic material, or a precursor of carbon within their porosity, this synthesis being followed by the removal of the matrix of silica by treatment with fluoridic acid. However, few, if any, real applications have been developed by this date. The reasons for this fruitless development are manifold.

 The use of mesoporous silica powders remains in particular limited to very limited applications, because of the cost of production and the constraints, including pollution, associated therewith. The existing products marketed are rare, do not offer a large variety of possible pore sizes, and are extremely expensive (on the order of several thousand euros per kilogram), which hampers any industrial development requiring several tens or hundreds of kilos of material.

 The removal of organic structuring agents used to synthesize this silica is essential for the creation of porosity. Indeed, it results from the departure of organic agents initially incorporated within the silica network.

 These structuring agents usually constitute about 40% of the mass of the composite silica / crude block copolymer product. Their removal is usually obtained by heat treatment at high temperature, between 400 ° C and 650 ° C, for several hours. This process is effective but suffers from several handicaps that have prevented the industrial development of such materials: (i) the energy cost is high, (ii) the heat treatment leads to the permanent loss of the organic agents, (iii) the duration of the process is long, (iv) the environmental impact is negative.

 Other methods such as solvent assisted extraction, generally using a "Soxhlet" method, have been used, but such methods are also time consuming and expensive, and remain limited to laboratory use on very small quantities.

 Bagshaw, S.A. et al., Science (1995) 269, 1242-1244 describes the first synthesis of mesoporous silica with nonionic polyoxyethylene (PEO) copolymers as a structuring agent (MSU-X silica).

 Boissière, C, Prouzet, E. and ah, Chemical Communications (1999) 2047-2048 and

Prouzet, E. and Boissière, C, CR Chimie (2005) 8, 579-596 describe a two-step mesoporous silica synthesis route, with as support non-ionic surfactants or tri-block copolymers, which has numerous advantages. compared to the initial method: i) Synthesis at ambient pressure, mild acidity (pH 1-3) and moderate temperature (20-60 ^° C.).

As with other synthetic routes, the shift to a higher scale requires rapid non-destructive shrinkage of the organic carrier satisfying from an environmental point of view, and from an economic point of view since allowing reuse of the organic compound. The present invention is based on a supercritical CO ₂ extraction step. The work done by the inventors demonstrates that such extraction, when used for the synthesis of mesoporous silica from a composite with polyoxyethylene-based nonionic block copolymers, leads to several major advantages in the optimization of method, compared with the heat treatment: (i) reduced treatment time from 10-12 hours (according to the prior art) at 1 hour, (ii) recovery of the organic structuring agents to allow a new use, (iii) absence of gaseous emissions, or other atmospheric pollutants (iv) improvement of the structure of silicas obtained by this process.

 C. Aymonier et al., J. of Supercritical Fluids (2006) 38, 242-251, DOI:

10.1016 / j.supflu.2006.03.019 describes the use of supercritical fluids in the production and processing of materials. Supercritical CO ₂ was mainly used as a reactive or additive medium in the synthesis of mesoporous materials or as an impregnating agent for incorporating active molecules into mesoporous matrices.

Z. Huang et al., Chemical Engineering Journal (2011) 166, 461-467 as well as Z. Huang et al., Chemical Engineering Research and Design (2014) 92, 1371-1380 describe the manufacture of a mesoporous silica from of a hybrid silica / alkylamine structure by means of a step of extraction with a mixture of C0 ₂

supercritical and methanol. The authors indicate that C0 ₂ extraction

supercritical alone does not work. It can be seen from this last document that the extraction step practiced in these studies does not modify the structure of the mesoporous silica.

JP-3524681 describes the manufacture of a mesoporous silica from a silica / alkyl ammonium chloride hybrid structure by means of a supercritical CO ₂ extraction step. The pressures used for this extraction are of the order of 400 bar (4.10 Pa), which is a very high value and binding for an application on an industrial scale. Summary of the invention

The invention relates to a process for the preparation of mesoporous silica from a hybrid silica / polyalkylene oxide material, said process comprising at least one step consisting in a treatment of the material with supercritical CO ₂ .

According to a preferred embodiment, the treatment with supercritical CO ₂ is carried out under the following conditions: • Temperature ranging from 20 ° C to 200 ° C,

• Pressure ranging from 1.10 ⁵ Pa to 300.10 ⁵ Pa,

 • Duration of treatment ranging from 5 minutes to 100 minutes.

According to a still preferred embodiment, the treatment with supercritical CO ₂ is carried out under the following conditions:

 • Temperature ranging from 35 ° C to 50 ° C,

• Pressure ranging from 50.10 ⁵ Pa to 150.10 ⁵ Pa,

 • Duration of treatment ranging from 10 minutes to 60 minutes.

According to a preferred embodiment, the process comprises at least one step consisting of a step of extracting the organic structuring agents used to form the mesoporous silica with supercritical CO 2, the extraction step being carried out at a temperature ranging from 20 ° C to 50 ° C.

According to a preferred embodiment, the treatment with supercritical CO ₂ is carried out under the following conditions:

• Pressure ranging from 1.10 ⁵ Pa to 300.10 ⁵ Pa,

 • Duration of treatment ranging from 5 minutes to 100 minutes.

According to a preferred embodiment, the treatment with supercritical CO ₂ is carried out under the following conditions:

• Pressure ranging from 50.10 ⁵ Pa to 150.10 ⁵ Pa,

 • Duration of treatment ranging from 10 minutes to 60 minutes.

According to a preferred embodiment, before the supercritical CO ₂ treatment, the hybrid silica / polyalkylene oxide material is placed in a solvent, for example ethanol, and subjected to sonication. According to a preferred embodiment, the hybrid silica / polyalkylene oxide material is in the form of a powder.

According to a preferred embodiment, the method further comprises, after supercritical CO ₂ extraction, a heat treatment step at a temperature of

temperature ranging from 80 ° C to 500 ° C.

According to a preferred embodiment, the polyalkylene oxide is chosen from ethylene polyoxides, propylene polyoxides, copolymers of ethylene oxide and propylene oxide, comprising from 2 to 500 oxide units. alkylene, esters of C10-C30 alkyl carboxylic acids and of C2-C3 alkylene oxide, C10-C30 alkyl ethers and C2-C3 alkylene polyoxides comprising from 2 to 500 oxide units; alkylene.

According to a still more preferred embodiment, the polyalkylene oxide is chosen from:

 block copolymers of polyethylene oxide and polypropylene oxide comprising from 10 to 200 alkylene oxide units;

- C10-C24 alcohol ethers and polyethylene oxide comprising 4 to 30 alkylene oxide units.

According to a preferred embodiment, the synthesis of the hybrid silica / polyalkylene oxide material comprises:

 a first step of contacting at least one silica precursor chosen from alkoxysilanes with at least one alkylene polyoxide in an aqueous medium at a pH ranging from 1 to 3,

 a second step comprising adding to the reaction medium a fluoride salt.

The combination of supercritical CO ₂ as extraction agent, with the synthesis, preferably in two steps, of the silica, offers a synergy allowing access to a quick and easy synthesis for easy scaling and a mesoporous silica synthesis that is structurally adaptable and compatible with respect for the environment and reasonable production costs.

In contrast to the processes of the prior art, the process of the invention avoids the destruction of the organic support which has allowed the synthesis of the mesoporous silica. The organic structuring agents are recoverable and reusable without modification and without the need for reprocessing, at the outlet of the supercritical C0 ₂ extraction reactor.

 This technical advantage leads to a saving of material and a reduction of the cost of production. It avoids the gaseous emissions resulting from the burning of the organic support.

The extraction time of the structuring agents is reduced: it is about ten hours in the thermal process, and less than one hour in the supercritical CO ₂ extraction. Thus the preparation time of the mesoporous silica is very significantly reduced, which also contributes to the reduction of production costs.

 In addition, there is an improvement in the structure of some of the porous silicas thus obtained with possibly an enlargement of the pore size compared to removal by heat treatment.

 It has also been found a better resistance to hydrolysis silicas mesoporous structure obtained by the method of the invention compared to silicas obtained by the methods of the prior art.

The method for synthesizing the hybrid structure uses reagents that have little or no pollutant and that present little or no risk for the manipulators: water, an alkoxide-type silica precursor (for example Si (OCH ₂ CH ₃ ) ₄ ) , surfactants nonionic, it is implemented at ambient pressure and at very moderate temperatures. Supercritical C0 ₂ extraction is rapid and applied at moderate pressures.

detailed description

 The expression "consists essentially of" followed by one or more characteristics means that, in addition to the components or steps explicitly listed, may be included in the process or material of the invention, components or steps that do not significantly modify the properties and characteristics of the invention.

 The expression "between X and Y" includes the terminals, unless explicitly stated otherwise. This expression therefore means that the target range includes X, Y values and all values from X to Y.

 For the purposes of the invention, the term "polymer" means oligomers, prepolymers, homopolymers and copolymers.

 Throughout the description, the terms "composite" and "hybrid" are used interchangeably to designate a mixed organic / inorganic material.

 By mesoporous silica is meant a material having an organized porous network, of pore size defined in the field of mesoporosity, that is to say between 2 and 50 nanometers, according to the classification of the International Union of Pure Chemistry and Applied (International Union of Pure and Applied Chemistry, IUPAC). More specifically, the mesoporous silica has a pore size generally between 1.5 and 20 nm. The mesoporous structure of the silica is characterized by X-ray diffraction allowing to qualify the organization of the silica network, and by gas absorption (nitrogen) making it possible to qualify the value of the specific surface of the porous material, its size distribution of porosity, and if necessary its surface roughness.

 Preparation of the precursor silica / alkylene oxide

In a known manner, the preparation of the mesoporous silica is carried out by the controlled polycondensation of an inorganic precursor of the silane type in the presence of one or more structuring agents in a solvent medium, in particular in an aqueous medium. The synthesis is based on a cooperative self-assembly mechanism, CTM (for "cooperative templating mechanism"). The structuring agents, nonionic surfactants, are dispersed in the medium so as to form polymolecular assemblies called micelles. The concentration of the structuring agents is chosen so as to reach at least one critical micelle concentration (CMC) at which the surfactants organize themselves into micelles, which may or may not be Aggregate into a network of micelles or liquid crystals, depending on the structuring agents and their concentration, around which the polymeric silica network will form.

 Advantageously, the inorganic precursor of the silica is chosen from alkoxysilanes, preferably from the group consisting of:

 tetraalkoxysilanes, for example tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS);

 trialkoxysilanes, for example (3-mercaptopropyl) trimethoxysilane, (3-aminopropyl) triethoxysilane, N- (3-trimethoxysilylpropyl) pyrrole, 3- (2,4-dinitrophenylamino) -propyltriethoxysilane, N- ( 2-aminoethyl) -3-aminopropyltrimethoxysilane, phenyltriethoxysilane and methyltriethoxysilane;

 dialkoxysilanes, for example dimethyldiethoxysilane (DMDES);

 mixtures of these compounds.

 Preferably, the precursor of silicon oxide may be chosen from the group consisting of tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), (3-mercaptopropyl) trimethoxysilane, (3-aminopropyl) triethoxysilane, N- ( 3-trimethoxysilylpropyl) pyrrole, 3- (2,4-dinitro-phenylamino) -propyltriethoxysilane, N- (2-aminoethyl) -3-aminopropyl-trimethoxysilane, phenyltriethoxysilane, methyltriethoxysilane, dimethyldiethoxysilane (DMDES), and mixtures thereof .

 According to Boissière et al., Chemistry of Materials (2000) 12, 1937-1940, the inorganic silica precursor used in the synthesis may also be advantageously sodium silicate.

 According to a preferred embodiment, the silicon oxide precursor is TEOS.

 In known manner, the nature of the structuring agent conditions the structure of the silica obtained.

 According to the invention, the structuring agent is chosen from nonionic compounds of polyalkylene oxide type, denoted by the generic term:

"Polyalkylene oxide", which include alkylene oxide polymers and their derivatives.

Alkylene oxide polymers are understood to mean oligomers, polymers and copolymers of C 2 -C 6, preferably C 2 -C 4, alkylene oxides, even more preferentially ethylene polyoxides and propylene polyoxides. For example, the alkylene polyoxide compounds, especially ethylene polyoxides, propylene polyoxides and copolymers of ethylene oxide and propylene oxide, may comprise from 2 to 500 alkylene oxide units. Compounds derived from alkylene oxide polymers are esters of alkyl carboxylic acids and polyalkylene oxide, alkyl ethers and alkylene polyoxides. For example, polyalkylene oxide derivatives include C10-C30 alkyl carboxylic acid and polyalkylene oxide esters, C10-C30 alkyl ethers and alkylene polyoxides. For example, compounds derived from alkylene polyoxides, especially compounds derived from ethylene polyoxides and propylene polyoxides, may comprise from 2 to 500 alkylene oxide units.

 Preferably, compounds selected from:

 copolymers of ethylene oxide and of propylene oxide comprising from 2 to 500 alkylene oxide units;

 esters of C10-C30 alkyl carboxylic acids and of polyethylene oxide comprising from 2 to 500 ethylene oxide units;

 C10-C30 alkyl ethers and ethylene polyoxides comprising from 2 to 500 ethylene oxide units.

 Even more preferentially, the structuring agent is chosen from:

 block copolymers of polyethylene oxide and polypropylene oxide comprising from 10 to 200 alkylene oxide units;

 C10-C24 alcohol ethers and polyethylene oxide having 4 to 30 alkylene oxide units.

 Preferably, the synthesis of the silica / polyalkylene oxide precursor is carried out in an aqueous medium. By aqueous medium is meant water, and mixtures of water with an organic solvent miscible with water. Among the organic solvents that may be used, mention may be made of alcohols or polyols, for example methanol, ethanol, glycerol or ketones, such as propanone or 2-butanone. Advantageously, the solvent in which the synthesis of the silica precursor / polyalkylene oxide is carried out is water.

Preferably, the synthesis of the precursor silica / polyalkylene oxide is carried out in aqueous medium at pH 1-3, because this pH allows the hydrolysis of the silicon alkoxide, but not the condensation of the silica. As a result, the silica oligomers create hydrophilic interactions preferentially by hydrogen bonding with the oxygen atoms maintained by the polyalkylene oxide chains of the structuring agent, resulting in stable hybrid micelles, as described by Boissière, C. et al., Chemistry of Materials (2001) 13, 3580-3586. In a second step, the formation of silica is induced by the addition of a fluoride salt, for example NaF. The ion F ^"is well known as a catalyst for condensation of the silica.

 The dynamics of nonionic micelles is preserved in hybrid micelles. In particular their capacity to adapt their structure, especially their size, to solvent temperature changes between 5 and 100 ° C, as described in A. Kacheff et al, C.R. Chimie, (2017)

(Http://dx.doi.org.proxy.lib.uwaterloo.ca/10.1016/j.crci.2016.11.001)

 In the second step, the choice of temperature can easily help to control the final pore size in a range of 2.0 to 50.0 nm, preferably 2.0 to 30.0 nm, even more preferably 2 to 30.0 nm. 0 to 15.0 nm. Preferably, the temperature of the reaction medium in the second step is 5 to 60 ° C, preferably 10 to 55 ° C, more preferably 10 to 50 ° C.

 Advantageously, at the end of the synthesis, the hybrid material silica / polyalkylene oxide is recovered in powder form, for example by filtration of the reaction medium, and dried. The drying can be partial or complete.

 The material consists essentially of a mesoporous silica whose pores are occupied, partially or totally, by the organic support, in this case the polyalkylene oxide or the derivative compound.

Treatment of the precursor silica / polyalkylene oxide by supercritical CO 2 The removal of the structuring agent (s) after formation of the silica makes it possible to remove the organic structuring agents and to open the porosity of the silica

mesoporous.

According to the invention, the hybrid silica / polyalkylene oxide material is subjected to a supercritical CO ₂ treatment. This treatment allows the elimination of the structuring agent (s).

Advantageously, before the treatment with supercritical CO ₂ , the silica / polyalkylene oxide precursor powder is placed in a solvent, such as, for example, ethanol, and subjected to treatment with ultrasound. This treatment does not in any way remove the organic agents, but prepares the powder for the subsequent steps. The powder is then filtered, the solvent having been used for this operation can be recovered and reused many times.

C0 ₂ is under supercritical conditions when it is heated above its critical temperature (31 ° C) and when it is compressed above its critical pressure (73.8.10 ⁵ Pa).

The treatment with supercritical CO ₂ is advantageously carried out under the following conditions:

 Temperature ranging from 20 ° C to 200 ° C, preferably from 35 ° C to 50 ° C;

Pressure ranging from 1.10 ⁵ Pa to 300.10 ⁵ Pa, preferably from 50.10 ⁵ Pa to 150.10 ⁵ Pa;

 Duration of treatment ranging from 5 minutes to 100 minutes, preferably from 10 minutes to 60 minutes.

At the end of this treatment, the mesoporous silica is recovered on the one hand from the nacelle of the sample, on the other hand the polyalkylene oxide in a supercritical C0 ₂ collection chamber having passed through the material, while C0 ₂ is removed as a gas or can be stored for further use.

 The advantages of this method are:

(i) the polyalkylene oxide can be reused because it can easily be recovered at the outlet of the reactor, independently of the evolution of CO ₂ as a gas, the CO ₂ can also be recycled.

 (ii) the time required for this extraction of surfactants is considerably reduced

 (iii) the energy cost required for the furnaces is canceled,

(iv) there is no effluent discharge from the combustion waste, and

 (v) it is possible to reuse both the ethanol used to prewash the powder, and the surfactants involved in this process.

In addition to these very positive characteristics, it has been observed that the structure of some of the materials obtained via supercritical C0 ₂ extraction in two stages is improved and has larger pores because the absence of heat treatment has not been achieved. induces the usual shrinkage of the silica network resulting from this heat treatment. The silica obtained by this method is substantially devoid of structuring agent, even if it can remain in the pores in the state of traces depending on the duration and conditions (P, T) of the treatment.

Optionally, the process may comprise a subsequent treatment such as, for example, a heat treatment. This subsequent treatment may consist in subjecting the mesoporous silica resulting from the supercritical CO ₂ treatment to a heat treatment at a temperature ranging from 80 to 500 ° C. Such subsequent treatment can serve to enhance the mechanical and chemical resistance of the silica, and to remove residual traces of organic compound. It is shorter than the

heat treatment used to burn organic compounds, and does not cause polluting emissions because it applies to a material that is totally or almost completely free of these organic compounds

 Advantageously, the heat treatment is carried out at a temperature ranging from 300 ° C. to 500 ° C. for a period ranging from 15 minutes to 6 hours.

uses

 The mesoporous silicas obtained by the process of the invention can be used in the following applications, in a non-limiting way:

 (i) As a vehicle for the vectorization of active ingredients or "Drug

 In fact, most of the molecules of biological or pharmaceutical interest have nanometer-sized sizes, compatible with the size of the porosity (2.0 to 20.0 nm) of the mesostructured silicas. as a host for vectorization, the relative instability of these materials with respect to hydrolysis, which are therefore easily

 biodegradable, makes them good candidates for biomedical applications.

 (ii) As a catalyst support, because of the large surface area of these materials, in particular for producing catalysts of high added value products.

(iii) As a carrier for diagnostic agents, because of the chemical stability, the lack of toxicity of the mesoporous silicas, and also because of the possibility of incorporating in the mesoporous structure contrast, such as fluorescent markers or dyes used to trace the location of therapeutic agents.

 (iv) As an adsorbent structure, taking advantage of the large surface of mesoporous silicas, which makes them good candidates for the adsorption of gases, liquids, or heavy metals. Their adsorption properties can be adjusted or optimized by appropriate grafting of selective molecules.

(v) In chromato graphy as a stationary phase. Indeed, the large pore volume, the large specific surface, as well as the tight and controllable pore size distribution, make them relevant candidates in size exclusion chromatography, in porous chromatography, proteomic separation, and HPLC chromatography. (high performance liquid chromatography) in normal or reverse phase.

 (vi) As water retaining agents and fertilizer distributors, in the field of plant treatment.

 (vii) As a micro- or nanocontainer containing, within the pore volume, biological, organic or inorganic agents which can be protected from the outside and distributed progressively by interaction with the outside. The various embodiments, variants, preferences and advantages described above for each of the objects of the invention apply to all the objects of the invention and can be taken separately or in combination.

Figures:

 Figure 1 :

 Figure 1 shows the small angle X-ray scattering (DPAX) curves for samples prepared with Tergitol ™ 15S 12 (Figures a & b) or

Pluronic ™ P123 (Figures c & d), and treated either by calcination at 600 ° C under air (Figures a & c) or by supercritical CO2 extraction (Figures b & d).

 (101) is the diffusion vector q = (4π.8ΐη (θ) / λ), where Θ is the diffraction angle, and λ is the wavelength of the X-rays used for the experiment.

 (102) is the intensity of the radiation collected on the detector, normalized in counts per second (cps).

Figure la:

•: silica prepared with Tergitol ™ 15S 12, at 6 ° C and calcined at 600 ° C □: silica prepared with Tergitol ™ 15S12, at 21 ° C and calcined at 600 ° C

 ■: silica prepared with Tergitol ™ 15S12, at 35 ° C and calcined at 600 ° C

o: silica prepared with Tergitol ™ 15S12, at 45 ° C and calcined at 600 ° C Figure lb:

•: silica prepared with Tergitol ™ 15S12, at 6 ° C and treated with C0 ₂

supercritical

□: silica prepared with Tergitol ™ 15S12, at 21 ° C and treated with supercritical C0 ₂

■: silica prepared with Tergitol ™ 15S 12 at 35 ° C and treated with C0 ₂

supercritical

o: silica prepared with Tergitol ™ 15S12, at 45 ° C and treated with C0 ₂

supercritical Figure the (y-axis in log scale):

 □: silica prepared with Pluronic ™ P123 at 21 ° C and calcined at 600 ° C

 ■: silica prepared with Pluronic ™ P123 at 35 ° C and calcined at 600 ° C

o: silica prepared with Pluronic ™ P123, at 45 ° C and calcined at 600 ° C Figure ld (ordinate axis in log scale):

□: silica prepared with Pluronic ™ PI 23, at 21 ° C and treated with C0 ₂

supercritical

■: silica prepared with Pluronic ™ P123, at 35 ° C and treated with C0 ₂

supercritical

o: silica prepared with Pluronic ™ PI 23, at 45 ° C and treated with supercritical C0 ₂

Figure 2: Figure 2 shows the calculated pore size distribution by applying the DFT model to the nitrogen adsorption curves, for samples prepared at 45 ° C with Tergitol ™ 15S12 (Figures a & b) or Pluronic ™ P123 (Figures c & d), and treated either by calcination at 600 ° C under air (Figures a & c) or by extraction with supercritical C0 ₂ (Figures b & d).

 (201) is the size of the pore diameter in nanometers.

 (202) is the relative importance of the pore size distribution, in arbitrary units. Figure 2a:

 •: silica prepared with Tergitol ™ 15S12, at 6 ° C and calcined at 600 ° C

□: silica prepared with Tergitol ™ 15S12, at 21 ° C and calcined at 600 ° C ■: silica prepared with Tergitol ™ 15S12, at 35 ° C and calcined at 600 ° C

o: silica prepared with Tergitol ™ 15S12, at 45 ° C and calcined at 600 ° C

Figure 2b:

·: Silica prepared with Tergitol ™ 15S12, at 6 ° C and treated with C0 ₂

supercritical

□: silica prepared with Tergitol ™ 15S12, at 21 ° C and treated with supercritical C0 ₂

■: silica prepared with Tergitol ™ 15S12, at 35 ° C and treated with supercritical C0 ₂

o: silica prepared with Tergitol ™ 15S12, at 45 ° C and treated with C0 ₂

supercritical

Figure 2c (ordinate axis in log scale):

□: silica prepared with Pluronic ™ P123 at 21 ° C and calcined at 600 ° C

 ■: silica prepared with Pluronic ™ P123, at 35 ° C and calcined at 600 ° C

o: silica prepared with Pluronic ™ P123, at 45 ° C and calcined at 600 ° C

Figure 2d (y-axis in log scale):

□: silica prepared with Pluronic ™ PI 23, at 21 ° C and treated with supercritical C0 ₂

■: silica prepared with Pluronic ™ P123, at 35 ° C and treated with C0 ₂

supercritical

o: silica prepared with Pluronic ™ PI 23, at 45 ° C and treated with C0 ₂

supercritical

Figure 3 shows the calculated porosity size distribution by applying the DFT model to the nitrogen adsorption curves, for samples prepared with Tergitol ™ 15S12 at 45 ° C, and then processed in different ways to test their strength. to the action of water at 90 ° C for 3 hours. Peak areas have been standardized for ease of comparison. Figure 3a compares a calcined sample at 600 ° C before and after immersion in water at 90 ° C for 3 hours. Figure 3b compares an extracted sample with supercritical CO ₂ before and after immersion in water at 90 ° C for 3 hours. Figure 3c compares a sample

extracted with supercritical CO ₂ before and after heat treatment at 400 ° C. in air for 2 hours. Figure 3d compares a sample extracted with C0 ₂ supercritical and heat treatment at 400 ° C in air, for 2 hours, before and after immersion in water at 90 ° C for 3 hours.

 (301) is the size of the pore diameter in nanometers.

 (302) is the relative importance of the pore size distribution in arbitrary units.

Figure 3 a:

 •: silica prepared with Tergitol ™ 15S12, at 45 ° C and calcined at 600 ° C

 □: silica prepared with Tergitol ™ 15S12, at 45 ° C, calcined at 600 ° C, and then immersed in hot water at 90 ° C for 3 hours.

Figure 3b:

·: Silica prepared with Tergitol ™ 15S12, at 45 ° C and treated with C0 ₂

supercritical

□: silica prepared with Tergitol ™ 15S12, at 45 ° C, treated with C0 ₂

supercritical, then immersed in hot water at 90 ° C for 3 hours.

Figure 3c:

·: Silica prepared with Tergitol ™ 15S12, at 45 ° C and treated with C0 ₂

supercritical

□: silica prepared with Tergitol ™ 15S12, at 45 ° C, treated with C0 ₂

supercritical, then heat-treated at 400 ° C for 2 hours.

Figure 3d:

·: Silica prepared with Tergitol ™ 15S12, at 45 ° C and treated with C0 ₂

supercritical

□: silica prepared with Tergitol ™ 15S12, at 45 ° C, treated with supercritical C0 ₂ , then heat-treated at 400 ° C for 2 hours, then immersed in hot water at 90 ° C for 3 hours. Figure 4 shows the calculated pore size distribution by applying the DFT model to the nitrogen adsorption curves, for samples prepared with Pluronic ™ P123 at 45 ° C, and then processed in different ways to test their strength. to the action of water at 90 ° C for 3 hours. Peak areas have been standardized for ease of comparison. Figure 4a compares a calcined sample at 600 ° C before and after immersion in water at 90 ° C for 3 hours. Figure 4b compares an extracted sample with supercritical CO ₂ before and after after immersion in water at 90 ° C for 3 hours. Figure 4c compares an extracted sample with Supercritical C0 ₂ before and after heat treatment at 400 ° C in air for 2 hours. Figure 4d compares an extracted sample with supercritical CO ₂ and heat treatment at 400 ° C in air, for 2 hours, before and after immersion in water at 90 ° C for 3 hours.

(401) is the size of the pore diameter in nanometers.

 (402) is the relative importance of the pore size distribution in arbitrary units.

Figure 4a:

 •: silica prepared with Pluronic ™ P123, at 45 ° C and calcined at 600 ° C

□: silica prepared with Pluronic ™ P123, at 45 ° C, calcined at 600 ° C, and then immersed in hot water at 90 ° C for 3 hours. Figure 4b:

•: silica prepared with Pluronic ™ PI 23, at 45 ° C and treated with C0 ₂

supercritical

□: silica prepared with Pluronic ™ PI 23, at 45 ° C, treated with C0 ₂

supercritical, then immersed in hot water at 90 ° C for 3 hours. Figure 4c:

·: Silica prepared with Pluronic ™ PI 23, at 45 ° C and treated with C0 ₂

supercritical

□: silica prepared with Pluronic ™ PI 23, at 45 ° C, treated with C0 ₂

supercritical, then heat-treated at 400 ° C for 2 hours. Figure 4d:

•: silica prepared with Pluronic ™ PI 23, at 45 ° C and treated with C0 ₂

supercritical

□: silica prepared with Pluronic ™ PI 23, at 45 ° C, treated with C0 ₂

supercritical, then heat treated at 400 ° C for 2 hours, and then immersed in hot water at 90 ° C for 3 hours.

The invention is illustrated by the following examples given without limitation.

Experimental part :

In these Examples, parts and percentages are by weight unless otherwise indicated. I- Materials and equipment

 A- Raw materials:

Tetraethylorthosilicate Si (OCH ₂ CH ₃ ) 4, supplier Sigma Aldrich Chemicals, M (molar) = 208g

Alkyl polyethylene oxide: Tergitol 15S12 ([CH ₃ (CH ₂ ) ₄ (EO) i ₂ ], supplier

Dow Chemicals, M (molar) = 756g EO = ethylene oxide

 Block Copolymer: Pluronic P123 ([(EO) 20 (PO) 70 (EO) 20], supplier: Sigma Aldrich Chemicals, M (molar) ~ 7.750 g, EO = ethylene oxide, PO: propylene oxide

 B -Equipment:

The tests were carried out with a 20 cm ³ reactor capable of operating up to 400 ° C. and 300 bar.

 II- Synthesis:

 A- Preparation of the organic / inorganic hybrid material

 Principle of the reaction:

 The TEOS (tetraethylorthosilicate) is dispersed in a 0.02 M aqueous acid solution of Tergitol 15S12 until reaching a TEOS / Tergitol 15S12 molar ratio of 10. When organic molecules of much higher molecular weight, such as Pluronic P123, are used, the molar ratio TEOS / organic is then lower. TEOS is hydrolysed by the acidic medium, but the pH value corresponds to the minimum of the condensation kinetics of the silica, which prevents silicate oligomers resulting from the hydrolysis from reacting and precipitating. These silicate oligomers interact with the micelles of copolymers to form silicate / organic hybrid micelles, as described in Boissière, C. et al. Chemistry of Materials (2001) 13, 3580-3586. The stabilization of the medium is visually controlled when the initially turbid solution upon addition of TEOS became transparent once all the TEOS has been hydrolysed to silicate. Once the medium is stabilized, this solution is left for about 12 hours at a temperature of about 5 ° C. It is then brought to the desired synthesis temperature, and a salt

fluoride, for example sodium fluoride (NaF), is added as a reaction catalyst to induce condensation of the silica. A usual value of the NaF / TEOS molar ratio is 1%.

 B- Elimination of the organic support:

Elimination of the organic support by supercritical CO ₂ (according to the invention): The dried powders are left in ethanol for 5 minutes under sonication treatment, to exchange as much as possible the impregnating water of the powder with water. alcohol. The ethanol used for this process can be reused several times because it is mainly used as a washing solvent and the presence of residual water does not interfere with the process.

The extraction is then carried out with supercritical C0 ₂ in a cylindrical autoclave, under C0 ₂ circulation, at 35 ° C. and 100 bar (1.10 ⁵ Pa), for 30 minutes. The entire procedure, including the increase in pressure and temperature, then the descent in pressure and temperature, lasted 60 minutes.

 Elimination of the organic support by calcination (comparative):

The powders were calcined in air according to the following procedure: ramp to 3 ⁰ C.min ^_1, with a plateau of 4 hours at 220 ° C, a plateau of 2 hours at 450 ° C and a plate 4 hours at 600 ° C. The entire heat treatment lasted 13 hours.

 Examples:

 Example 1

First step: 7.5 g of Tergitol 15S 12 are dissolved at room temperature in 500 ml of deionized water, previously acidified to pH 2 with a hydrochloric acid solution ( _aq HCl). After complete dissolution, 20.8 g of TEOS are added with controlled mechanical stirring to allow the TEOS droplets to be sufficiently dispersed in the aqueous solution. The solution, initially cloudy due to oily droplets of TEOS, becomes progressively transparent in 10 minutes, once all of TEOS has been completely hydrolysed to silicate oligomers, following the mechanism described in Boissière, C. et al. Chemistry of Materials (2001) 13, 3580-3586. The solution thus obtained is left to stand in a refrigerator (6 ⁰ C) overnight to obtain the complete formation of micelles hybrid silicate / organic. This stock solution is left at 60 ^° C. and samples are taken if necessary. This is the first step (the assembly step) of this synthesis in two steps.

- Second step: 100 ml samples of the stock solution are taken, allowed to come back (if necessary) to room temperature and then adjusted to the appropriate temperature: 6 ° C (refrigerator), 21 ° C (room temperature) ), 35 ° C or 45 ° C (thermostatic bath). Each sample contains 4.16 g of TEOS (0.02 mol) and 1.5 g of Tergitol 15S12. 0.8 ml (2.10 ^-4 mol) of a 0.25 M NaF aqueous solution are added to obtain a NaF / TEOS molar ratio at 1%, and the NaF fluoride ion is used as a condensation catalyst. are left at the required temperature for the duration of the reaction: 72h for the synthesis at 6 ° C, 48h for the synthesis at 21 ° C and 24h for the syntheses at 35 ° C and 45 ° C. These reaction times have chosen to be sure that the reaction was complete, but most of it is usually completed in the first two hours with the exception of the reaction at 6 ° C). This constitutes the second step (the silica condensation step) of this synthesis in two steps.

Drying: The powders as synthesized are filtered and dried at 60 ^° C. overnight.

Removal of the organic support by supercritical CO ₂ according to the protocol described above.

 Example 2

The procedure is as in Example 1 with Pluronic P123 (block copolymer of ethylene oxide and propylene oxide) at temperatures of 21 ^° C. (room temperature), 35 ^° C. or 45 ^° C. (thermostatic bath). . The procedures are identical to those used in Example 1, with the exception of the amount of reagents: 7.5 g of Pluronic P123 in 500 ml of deionized water at pH 2. 33.3 g of TEOS were added .

- Elimination of the organic support by supercritical C0 ₂ according to the protocol described above

 Example 3 (comparative):

 The procedure is as in Example 1 but the organic support is removed by calcination.

 Example 4 (comparative):

 The procedure is as in Example 2 but the organic support is removed by calcination.

III- Results:

 A-Test Protocols:

 - X-ray scattering at small angles: DPAX diffractograms are obtained with a device developed in the CRPP laboratory (CNRS Research Unit UPR 8641)

N ₂ adsorption isotherms: Gaseous adsorption isotherms are obtained with a commercial device of the Autosorb iQ ™ type marketed by the company Quantachrome Inc.

- Pore size distribution: The N ₂ adsorption isotherms are transformed into a pore size distribution (DTP) by calculation, using the Density Functional Theory (DFT)

 - Fractal surface dimension Ds:

The fractal surface dimension Ds, is calculated according to Prouzet, E. et al., Microporous and Mesoporous Materials, (2009) 119, 9-17, from the part of the adsorption curve of the isotherm N ₂ , Ds is a good gauge of the surface roughness, and it varies between 2 (flat) and 3 (very rough).

 - Resistance to hydrothermal conditions:

 Some samples are subjected to an aqueous hydrolysis treatment to determine their resistance to hydrothermal conditions:

(i) samples after supercritical CO ₂ extraction or calcination at 600 ° C are immersed in hot water (90 ° C) for 3 hours.

(ii) Some samples extracted with supercritical C0 ₂ are then heat-treated at 400 ^° C. for 2 hours and then immersed in hot water (90 ° C.) for 3 hours.

B-Test results:

Efficiency of supercritical C0 ₂ extraction:

SAXS diagrams of the samples prepared with the Tergitol T15S12: Figure la, example 3 and figure lb, example 1. These diagrams present the expected single diffusion peak created by the porous 3D structure of these materials. As the synthesis temperature increases, the diffraction peaks shift to lower q values as a result of the increase in pore size. Compared to thermal treatment, supercritical C0 ₂ extraction brings two major improvements in the structure of the material: the peak diffusion of

correlation is more intense, resulting in better crystallinity, and it corresponds to a smaller value of q, which proves that the pore-pore distance is greater. The same observation was made when comparing the samples from the synthesis before removal of the support and the calcined samples, and it is found that the heat treatment induces a withdrawal of the silica network, which is not observed with the supercritical C0 ₂ extraction.

 Samples prepared with Pluronic PI 23 (Example 4, Figure 1a and Example 2, Figure 1d) show a wider variation in their SAXS structure. Several diffraction peaks are observed, in the ratio 1: Λ / 3: 2: Λ / 7. They

correspond respectively to the diffraction planes (10), (11), (20) and (21) of a honeycomb silica hexagonal frame (table SI). No significant difference in crystallinity is observed because the materials after heat treatment also display a very high order of structure. However, the same shift to a smaller q between heat treated and supercritical CO ₂ samples was observed, as was the case for Tergitol T15S 12.

As the contrast in X-ray scattering depends on the electron density contrast between the silica walls and the pores, the existence of peaks of Intense diffraction with supercritical C0 ₂ samples demonstrates actual extraction of organic matrices.

 Pore size distribution:

 The actual extraction of the organic supports has been confirmed by nitrogen adsorption isotherms. Figure 2 shows the pore size distribution deduced from the nitrogen adsorption isotherm.

These analyzes confirm that the pore size distribution increases with the synthesis temperature, and that it is also higher for samples extracted with supercritical C0 ₂ than for thermally treated samples.

The thickness of the silica wall was deduced by subtracting the pore diameter from the d (or d (10) spacing for the hexagonal phases). We compared the differences between four physicochemical parameters: the BET surface area, the pore size distribution, the pore volume and the wall thickness. We compared these parameters for samples extracted with supercritical CO ₂ to heat-treated samples at 600 ° C. For samples extracted with supercritical C0 ₂ , we observed that on average:

 - The BET surface is lower,

 - The pore size distribution is slightly higher,

 - The pore volume is quite in the same range,

 - The wall thickness of the silica is significantly higher.

 Resistance to hydrothermal conditions:

 The ability of the materials produced to withstand water has been tested because their very low resistance to wet environments, particularly at high temperatures, is a major disadvantage of many mesoporous materials.

The samples tested according to the protocol set out above were analyzed by N ₂ adsorption. For samples treated with supercritical CO ₂ , another route was tested, with thermal annealing at 400 ^° C. for 2 hours before being exposed to water. The PSD is illustrated in Figures 3 and 4.

 These preliminary tests show that generally the mesoporous silica has good resistance to hydrolysis and does not sag quickly.

For the samples initially calcined at 600 ^° C. (Examples 1 and 3, comparative), the analysis is quite simple. During the hydrolysis with water, we can see:

 - A reduction of 30% of the specific surface,

 - The pore size does not vary significantly,

 - There is no obvious correlation with the pore volume,

- The fractal surface dimension Ds, that is to say the surface roughness, remains unchanged. With the exception of the BET surface, no noticeable change is observed. This is confirmed by the N ₂ isotherms which retain the same shape, with only one shift for the sample prepared with Pluronic PI 23, due to a lower surface.

For samples extracted with C0 ₂ supercritical:

 A reduction of 30% of the specific surface area is observed for example 2 and none for example 4, but it was already quite weak,

 - The pore size does not vary significantly,

 - There is no obvious correlation with the pore volume,

 - The value of Ds is unchanged for Example 1, compared to calcined samples, and lower for Example 2, which means a smoother surface. Both are reduced after treatment with water.

The water treatment has a smoothing effect on the surface of the materials extracted with supercritical CO ₂ , with Ds from 2.41 to 2.34 and from 2.76 to 2.57, respectively for Example 1 and Example 2

However, the resistance to hydrolysis is very good. The samples extracted with supercritical CO ₂ and annealed at 400 ^° C. show surprising evolutions when exposed to water:

The BET surfaces are considerably reduced for the two samples: Example 1 (from 946 to 313 m ² · g ^-1 ) and Example 2 (from 434 to 129 m ² · g ^-1 ),

 - The pore size does not vary significantly for Example 2 whereas it is totally shifted towards larger values (between 5 and 30 nm) for the example

1 (see below)

 - There is no obvious correlation with the pore volume,

 - The fractal surface dimension Ds is reduced for Example 2 and unchanged for Example 1.

 The most drastic change is observed with Tergitol T15S12 (Example 1).

During hydrolysis, the BET drops significantly (from 946-313 m 2.g - ^"1), but without significant change in pore volume (of 0.54 to 0.60 cm 3 g ^-" 1) or Ds (from 2.45 to 2.52). This transformation can not be explained by a collapse of the material, but rather by a major reconstruction of the silica structure, leading to a much larger pore size distribution (between 5 and 30 nm), but without loss of pore volume or change of surface roughness. We conclude that annealing at 400 ° C leads to a more active silica surface capable of restructuring.

Table 1 below reports the characteristics of the mesoporous silica obtained by the various processes (ND: not determined).

 Table 1

* Deduces half-height values of the DTP peaks calculated by DFT from the isothermal adsorption branch. ** the wall thickness is calculated by subtracting the pore diameter (the middle of the DTP range) from the

Conclusions:

These observations make it possible to conclude that supercritical C0 ₂ extraction is an efficient process for rapidly removing the organic support from the mesoporous silica, without burning it, more rapidly and more environmentally friendly.

The pore size distribution is higher for samples extracted with supercritical CO ₂ because the absence of heat treatment prevents the reduction of the silica structure. It has been found that in the prior art publications (Z. Huang et al Chemical Engineering Research and Design 92 (2014) 1371-1380) that organic substrates of different nature are used which extract with C0 ₂ supercritical mixed with methanol, no improvement in the structure of the silica is observed. In addition, the condensation of silica is less advanced, and the silica walls remain thicker. These values (larger pores, greater thickness of the silica wall, same pore volume) physically result in a smaller BET surface area.

Claims

A process for the preparation of mesoporous silica from a hybrid silica / polyalkylene oxide material, said process comprising at least one step consisting of a step of extracting the organic structuring agents used to form the mesoporous silica with the C0 2. ₂ supercritical, the extraction step being carried out at a temperature ranging from 20 ° C to 50 ° C.

2. Process according to claim 1, in which the treatment with supercritical CO ₂ is carried out under the following conditions:

• Pressure ranging from 1.10 ⁵ Pa to 300.10 ⁵ Pa,

 • Duration of treatment ranging from 5 minutes to 100 minutes.

3. Process according to claim 2, in which the treatment with supercritical CO ₂ is carried out under the following conditions:

• Pressure ranging from 50.10 ⁵ Pa to 150.10 ⁵ Pa,

 • Duration of treatment ranging from 10 minutes to 60 minutes.

A process according to any one of the preceding claims wherein, prior to supercritical CO ₂ treatment, the hybrid silica / polyalkylene oxide material is placed in a solvent, for example ethanol, and subjected to a treatment with ultrasound.

5. Process according to any one of the preceding claims, in which the hybrid silica / polyalkylene oxide material is in the form of a powder.

6. A process according to any one of the preceding claims which further comprises, after supercritical CO ₂ extraction, a heat treatment step at a temperature ranging from 80 ° C to 500 ° C.

7. Process according to any one of the preceding claims, in which the polyalkylene oxide is chosen from ethylene polyoxides, propylene polyoxides and copolymers of ethylene oxide and propylene oxide, comprising from 2 to at 500 alkylene oxide units, the alkyl acid esters C10-C30 carboxylic acid and C2-C3 alkylene oxide, C10-C30 alkyl ethers and C2-C3 alkylene polyoxides comprising from 2 to 500 alkylene oxide units.

The process of claim 7, wherein the polyalkylene oxide is selected from:

 block copolymers of polyethylene oxide and polypropylene oxide comprising from 10 to 200 alkylene oxide units;

 - C10-C24 alcohol ethers and polyethylene oxide comprising 4 to 30 alkylene oxide units.

9. Process according to any one of the preceding claims, in which the synthesis of the hybrid silica / polyalkylene oxide material comprises:

 a first step of contacting at least one silica precursor chosen from alkoxysilanes with at least one alkylene polyoxide in an aqueous medium at a pH ranging from 1 to 3,

 a second step comprising adding to the reaction medium a fluoride salt.