FR3130172A1 - method for preparing a material in the form of a porous silica monolith comprising titanium oxide nanoparticles, said material and its applications - Google Patents

Abstract

The invention relates to a method for preparing a material in the form of a porous monolith comprising a silica matrix and TiO2 nanoparticles, a material in the form of a porous monolith comprising a silica matrix and nanoparticles of TiO2, its uses in particular as a photocatalyst or in the field of the photo-purification of air, the purification of confined atmospheres and the photo-reduction of CO2 into fuels, as well as a process for decontaminating a gaseous medium implementing said material. Figure to be published: figure 1

Description

method for preparing a material in the form of a porous silica monolith comprising titanium oxide nanoparticles, said material and its applications

The present invention relates to a method for preparing a material in the form of a porous monolith comprising a silica matrix and TiO ₂ nanoparticles, to a material in the form of a porous monolith comprising a silica matrix and nanoparticles of TiO ₂ , to its uses in particular as a photocatalyst or in the field of the photo-purification of air, the purification of confined atmospheres and the photo-reduction of CO ₂ into fuels, as well as to a process for decontaminating a gaseous medium implementing said material.

Volatile organic compounds (VOCs) are among the main air pollutants. VOCs are defined as any organic compound, of natural or anthropogenic origin, which has the particularity of having a very low boiling point: consequently, it evaporates or sublimates easily from its solid or liquid form under normal temperature and pressure. More specifically, according to European Council Directive 1999/13/EC, a VOC corresponds to any compound containing at least the element carbon and one or more of the following elements: hydrogen, halogen, oxygen, sulphur, phosphorus, silicon or nitrogen, except carbon oxides and inorganic carbonates and bicarbonates, and having a vapor pressure of 0.01 kPa or more at a temperature of 293.15 K. This gives it the ability to propagate more or less far from its place of emission, resulting in direct and indirect impacts on animals and nature. The VOCs detected in the indoor environment are generated mainly by furniture, cleaning products, cosmetics, paints, wood, glues, textiles, plastics, tobacco, etc... Many VOCs are toxic, especially the most volatile and/or or the most responsive. Some are, for example, considered to cause chronic health effects during long-term exposure, such as toluene, benzene, xylene, which are solvents commonly used in industry belonging to the family of aromatic hydrocarbons. polycyclics (PAHs). Certain alcohols (methanol, propanol), halogenated compounds, or even aldehydes (formaldehyde), are also described as proven or probable carcinogens and/or endocrine disruptors in humans. Cases of cancer resulting from occupational exposure to methanol and formaldehyde have now been confirmed. Finally, the emission of VOCs can contribute to the formation of urban pollution clouds and ozone, to the thinning of the ozone layer in the stratosphere, and to the greenhouse effect. The main VOCs in ambient air are: formaldehyde, acetaldehyde, acetone, octane, alcohols including isopropanol, decane, benzene, and toluene.

To overcome the health risk associated with the emission of VOCs, many techniques have been proposed to reduce or even eliminate the presence of VOCs in the air. Pollutants such as VOCs are conventionally removed using air purifiers which employ filters to remove particulate matter, or which use adsorbent materials such as, for example, granular activated carbon. These techniques only transfer contaminants to another phase instead of destroying them. Additional processing or manual steps are therefore required. An alternative to air purifiers is the photocatalytic oxidation of VOCs in the presence of semiconductor catalysts such as TiO ₂ , ZnO, WO ₃ , ZnS, and CdS, and ultraviolet or near-ultraviolet illumination, which oxidizes organic compounds to carbon dioxide, water, and mineral acids. Powders of these catalysts were initially used. Then, in order to avoid the propagation of nanoparticles in the effluent to be treated or to avoid a tedious step of nanofiltration, many studies have focused on the design of supported photocatalytic materials. In particular, the Quartzel®PCO photocatalytic material developed and marketed by Saint Gobain Quartz is a felt of tangled quartz fibers (purity > 99.99%, diameter 8-12 µm) on which is supported a photocatalyst based on synthesized TiO ₂ by sol-gel method. Such a material has the drawbacks of being brittle, in particular due to the deposition of TiO ₂ particles on fibres, of having a surface efficiency only, and of inducing a risk of loss of particulate TiO ₂ . Moreover, such a material is difficult to miniaturize.

Other materials based on TiO ₂ have been proposed in patent application FR2975309, such as a self-supporting macrocellular monolith composed of a material comprising a titanium oxide matrix comprising at least 80% by mass of titanium dioxide in anatase form, and at most 20% by mass of titanium dioxide in rutile form, said monolith being free of micropores and comprising macropores having an average dimension dA of 100 nm to 3 µm and interparticle mesopores having an average dimension dE of 2 to 20 nm, said pores being interconnected. The process for preparing the monolith uses a precursor of titanium oxide Ti(OiPr ₄ ), and the preparation of an oil-in-water emulsion. The monolith is used in the decontamination of a gaseous medium likely to contain one or more volatile organic compounds. However, said monolith has low levels of VOC adsorption. Furthermore, when the titanium oxide precursor is replaced by a mixture of a titanium oxide precursor and a silicon oxide precursor (TEOS), the monolith obtained no longer exhibits photocatalytic activity.

Therefore, there is a need for catalysts with improved photocatalytic performance capable of reducing or even eliminating VOCs from ambient air. There is also a need for a process for manufacturing a photocatalyst which is industrializable, simple and economical.

Thus, the object of the present invention is to overcome the drawbacks of the aforementioned prior art and to provide a catalyst having improved photocatalytic properties for the purification of air, and which can be easily handled and disposed of after use. Another object of the present invention is to provide a process for preparing a photocatalyst which is simple, economical and industrializable.

The object of the invention is achieved by the process for preparing a material in the form of a porous monolith which will be described below.

The first subject of the present invention is thus a method for preparing a material in the form of a porous monolith comprising a matrix of silicon oxide, and nanoparticles of TiO ₂ , said method comprising:
i) a step for preparing an acidic aqueous phase comprising at least one cationic surfactant and TiO ₂ nanoparticles,
ii) a step of adding a silica precursor to the acidic aqueous phase of step i),
iii) a step of adding an oily phase to the acidic aqueous phase of step ii) to form an emulsion,
iv) a polycondensation step,
v) a step for removing the oily phase, and
vi) a calcining step.

Thanks to the process of the invention, and in particular thanks to the use of TiO ₂ nanoparticles in association with the polycondensation of the silica precursor, a porous monolith with multi-scale porosity is obtained in a few steps and in a simple manner. comprising a silica matrix and TiO ₂ nanoparticles. The method makes it possible to guarantee the preservation of the native photocatalytic activity of the initial TiO ₂ nanoparticles which are implemented in step i). Moreover, it makes it possible to dispense with the use of TiO ₂ precursors such as for example Ti(OiPr ₄ ) which are difficult to eliminate at the end of the synthesis and are therefore not suitable for industrialization of the process. Finally, the method makes it possible to obtain a porous material having improved performance as a photocatalyst, comprising a distribution by volume of the TiO ₂ nanoparticles, thus avoiding a loss of particulate TiO ₂ and making it possible to miniaturize air purifiers.

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

In the material obtained according to the process of the invention, the matrix of silicon oxide is a continuous matrix.

Within the meaning of the present invention, the term “TiO ₂ nanoparticles” means particles having a size less than or equal to approximately 500 nm, preferably less than or equal to 200 nm, and in a particularly preferred manner less than or equal to 100 nm.

The TiO ₂ nanoparticles preferably have at least one size greater than 1 nm, and preferably greater than 3 nm.

According to a particularly preferred embodiment of the invention, the TiO ₂ nanoparticles have a size ranging from approximately 5 to 40 nm.

The size of the TiO ₂ nanoparticles can be determined by methods well known to those skilled in the art, and in particular by TEM (transmission electron microscopy).

The TiO ₂ nanoparticles are preferably monodisperse. In other words, their size has a standard deviation (width of the size distribution around the mean) ranging from 3 to 15 nanometers.

Step i)

In an advantageous embodiment of the invention, the cationic surfactant is chosen from quaternary ammoniums having at least 8 carbon atoms. Examples include tetradecyltrimethylammonium bromide (TTAB), dodecyltrimethylammonium bromide, and cetyltrimethylammonium bromide.

The cationic surfactant preferably represents at least 15% by mass approximately, and particularly preferably from 20 to 35% by mass approximately, relative to the total mass of acidic aqueous phase of step i).

Such a mass concentration makes it possible to obtain an initial viscous reaction medium and to promote the formation of a stable emulsion during the subsequent stages.

The titanium dioxide (TiO ₂ ) of the nanoparticles of step i) preferably comprises at least 60% by mass approximately of titanium dioxide in anatase form, and in a particularly preferred manner at least 80% by mass approximately of titanium dioxide in anatase form, relative to the total mass of titanium dioxide. This thus makes it possible to improve the photocatalytic performance of the material obtained.

The titanium dioxide (TiO ₂ ) of the nanoparticles of step i) preferably comprises at most 40% by mass approximately of titanium dioxide in rutile form, and in a particularly preferred manner at most 20% by mass approximately of titanium dioxide in rutile form, relative to the total mass of titanium dioxide. This thus makes it possible to improve the photocatalytic performance of the material obtained.

The TiO ₂ nanoparticles preferably represent at least 0.5% by mass approximately, and in a particularly preferred manner from 0.8 to 8.0% by mass approximately, relative to the total mass of acidic aqueous phase of step i). Such a minimum mass concentration makes it possible to guarantee sufficient photocatalytic activity. Furthermore, such a maximum mass concentration makes it possible to limit the viscosity of the acidic aqueous phase and to guarantee good dispersion of the TiO ₂ nanoparticles without risk of phase separations and/or loss of the monolithic nature of the final material.

The aqueous phase of step i) is an acidic aqueous phase. Its pH is in particular lower than the isoelectric point of silica, i.e. lower than 2.1, and preferably ranges from 0.05 (Hammet's acidity) to 1.

According to a particularly preferred embodiment of the invention, step i) is carried out according to the following sub-steps:
i-1) a sub-step of preparing an aqueous phase comprising water and the cationic surfactant,
i-2) a sub-step of adding TiO ₂ nanoparticles to the aqueous phase, and
i-3) a sub-step of acidification of the aqueous phase.

The cationic surfactant preferably represents at least 10% by mass approximately, and particularly preferably from 20 to 45% by mass approximately, relative to the total mass of water.

The TiO ₂ nanoparticles preferably represent at least 0.5% by mass approximately, and particularly preferably from 1 to 10% by mass approximately, relative to the total mass of aqueous phase.

Sub-step i-3) is carried out by adding a strong acid solution such as a hydrochloric acid solution at approximately 37% by mass, to the aqueous phase of sub-step i-2).

Step i) [respectively sub-steps i-1), i-2) and i-3)] are generally carried out at room temperature (i.e. 18-25°C).

Step ii)

Stage ii) makes it possible to add a silica precursor to the acidic aqueous phase of stage i).

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

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

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

The silica precursor preferably represents at least 5% by mass approximately, and particularly preferably from 10 to 30% by mass approximately, relative to the total mass of the acidic aqueous phase. This mass concentration makes it possible to obtain total mineralization of the material and good mechanical strength.

Step ii) is preferably carried out at room temperature (i.e. 18-25°C).

Step ii) is preferably carried out with stirring, for example for a period of approximately 5 to 20 min. This thus makes it possible to promote the hydrolysis of the silica precursor, as well as the at least partial evaporation of the solvents resulting from the silica precursor.

Step iii)

Stage iii) is a stage of emulsification of an oily phase in the acidic aqueous phase of stage ii).

In an advantageous embodiment of the invention, the oily phase comprises one or more compounds chosen from linear and branched alkanes having at least 9 carbon atoms. Even more advantageously, the linear alkanes comprise between 10 and 12 carbon atoms. By way of example, we can cite decane or dodecane.

According to one embodiment, the oily phase represents approximately 50 to 80% by volume, relative to the total volume of the emulsion [i.e. oily phase + acid aqueous phase from step ii)]. This emulsification step is essential to obtain a macroporous network within the material.

In accordance with the invention, the oily phase/acid aqueous phase volume fraction of step ii) makes it possible to control the size of the macropores. An increase in said volume fraction leads to an increase in the viscosity of the reaction medium, and consequently in shear during stirring. The droplets in the emulsion become smaller and thus develop more surface area to be covered by mineralization. Thus, at a constant silica precursor concentration, the increase in specific surface to be covered by increasing the volume fraction of oil, will induce only partial mineralization of the edges of the Plateau, which can cause a loss of the monolithic appearance. Thus, for an oily phase/aqueous phase volume fraction greater than 0.78, the mechanical strength could be insufficient to obtain a self-supporting monolith and the final product can then be in the form of a powder. Thus, the oily phase/aqueous phase volume ratio is preferably less than 0.78, and advantageously ranges from 0.6 and 0.7.

Stage iii) is preferably carried out by adding to the acidic aqueous phase of stage ii) with continuous stirring, the oily phase drop by drop.

Step iv)

During step iv) polycondensation, only the silica precursor hydrolyzes and polycondensates. In other words, the titanium oxide nanoparticles remain intact and the titanium does not co-condense with the silicon (the titanium is a spectator of the silicic polycondensation).

Step iv) is preferably carried out at room temperature (i.e. 18-25°C).

Stage iv) can last from 1 to 15 days, and preferably from 4 to 10 days.

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

The method may further comprise between step iii) and step iv) a step iii′) during which the emulsion of step iii) is poured into one or more containers, and the containers are closed, in particular in order to promote the following step iv) (caking).

Step v)

The elimination step v) makes it possible to eliminate the organic residues originating from the oily phase which are essentially found in the macropores.

The stage of elimination of the oily phase is in particular carried out by washing the solid obtained in stage iv) with an organic solvent.

The organic solvent can be tetrahydrofuran, optionally mixed with acetone (e.g. >90% by volume of THF), or dichloromethane, and preferably dichloromethane.

Washing can last from 10 to 15 hours, and advantageously 12 hours.

According to a preferred embodiment of the invention, the washing with an organic solvent is carried out by placing the solid obtained in one or more containers containing the organic solvent, said container being itself enclosed away from any humidity, preferably in a desiccator.

The removal step v) preferably comprises the aforementioned washing step, followed by a drying step, for example for a period of 1 to 20 days. This avoids too rapid evaporation of the organic solvent. Too rapid evaporation of the organic solvent can alter the mechanical properties of the material because the capillary forces induce random and uncontrolled fractures of the monoliths during drying.

During this drying step, the excess organic solvent is extracted from the containers. To do this, the washed materials are preferably placed in a desiccator for a period ranging from 1 day to 1 week. The lid of the desiccator can then be replaced by a material that allows the passage of (or favors the extraction of) the organic solvent, such as paper.

The drying step may further comprise an additional drying step during which the materials are removed from the desiccator and are left to air dry, for example under a fume hood, without special precautions, preferably for a period of 1 to 5 days. .

Stage v) promotes the homogeneous distribution of the TiO ₂ nanoparticles within the silica matrix.

Step v) is preferably carried out at ambient temperature (i.e. 18-25°C).

The method may further comprise between step iv) of polycondensation and step v) of elimination of the oily phase, a step iv′) during which the solid material resulting from step iv) is unmolded from said containers.

Step vi)

Step vi) is a calcination step.

Step vi) makes it possible to calcine the micellar phases (cationic surfactant) by creating a mesoporosity, and to sinter the ceramic.

Step vi) preferably comprises:
- a first calcination sub-step vi-1) comprising heating from an initial temperature T _i to a temperature T _{c 1} ranging from 160 to 200° C., advantageously 180° C., with a heating rate ranging from 0 5 to 3°C/min, advantageously equal to 2°C/min, then maintaining the temperature T _{c 1} for 3 to 10 hours, advantageously for 6 hours, and
- a second calcination sub-step vi-2) comprising heating from the temperature T _c1 to a temperature T _c2 ranging from 600 to 800°C, advantageously from 700°C, with a heating rate ranging from 0.5 to 2° C./min, advantageously equal to 1° C./min, then maintaining the temperature T _c2 for 3 to 10 hours, advantageously for 6 hours.

The sub-steps vi-1) and vi-2) are preferably carried out in air, for example with a first temperature rise ramp from 25° C. to 180° C. (at 2° C. per minute), remaining at 180°C for 6 hours. Then, for example, a second temperature rise ramp at 1° C./min is applied from 180° C. to 700° C. and the materials are maintained at 700° C. for 6 hours.

The temperature drop from temperature T _c2 to room temperature is uncontrolled and induced by the inherent thermal inertia of the oven. This step vi) makes it possible to eliminate the organic residues originating from the cationic surfactant which are essentially found in the mesopores.

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

The process of the invention, and in particular the use of TiO ₂ nanoparticles, makes it possible to dispense with the use of titanium dioxide precursors, and thus to avoid the control of the polycondensation kinetics of TiO ₂ and SiO ₂ . This also helps to promote the formation of a self-supporting monolith.

The material obtained according to the process in accordance with the first object of the invention may be as defined in the second object of the invention described below.

A second subject of the invention is a material in the form of a self-supporting porous monolith, characterized in that it comprises a matrix of silicon oxide, and nanoparticles of TiO ₂ , and in that:
- said material comprises macropores, mesopores, micropores, and walls forming an interconnected porous network, and
- the TiO ₂ nanoparticles are distributed statistically both on the surface of the macropores, and on the surface and in the volume of said walls (plateau edges).

Titanium dioxide nanoparticles are randomly distributed or dispersed on a macroscopic scale. In particular, the TiO ₂ nanoparticles are found on the surface of the macropores as well as on the surface and in the volume of the walls (edges of the Plateau) forming the interconnected porous network.

The monolith of the invention is a self-supporting monolith. In other words, it has sufficient mechanical strength to be able to be used as is, i.e. without support.

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

In an advantageous embodiment of the invention, said material comprises from 5% to 30% by mass approximately of TiO ₂ , and preferably from 7% to 25% by mass approximately of TiO ₂ , relative to the total mass of material (ie SiO ₂ + TiO ₂ ).

The titanium dioxide (TiO ₂ ) of the nanoparticles of the material of the invention preferably comprises at least 60% by mass approximately of titanium dioxide in anatase form, and in a particularly preferred manner at least 80% by mass approximately of titanium dioxide in anatase form, relative to the total mass of titanium dioxide. This thus makes it possible to improve the photocatalytic performance of the material obtained.

The titanium dioxide (TiO ₂ ) of the nanoparticles of the material of the invention preferably comprises at most 40% by mass approximately of titanium dioxide in rutile form, and in a particularly preferred manner at most 20% by mass approximately of titanium dioxide in rutile form, relative to the total mass of titanium dioxide.

Said material preferably consists of a matrix of silicon oxide, and nanoparticles of TiO ₂

According to a preferred embodiment of the invention, the material has a Ti/Si molar ratio ranging from 0.08 to 0.8) approximately, and in a particularly preferred manner from 0.09 to 0.65 approximately.

This molar ratio can be determined by ICP elemental analysis (inductively coupled plasma analytical technique).

In accordance with the invention, the material is a porous monolith with multi-scale porosity, ie comprising macropores (these coming from the oily phase and the elimination thereof), mesopores (these coming from micellar or lyotropic systems), and micropores (these coming from the statistical distribution of the SiO ₄ tetrahedra in the geometric space), said pores being interconnected.

Macropores (larger than 50 nm) can be identified by scanning electron microscopy (TEM) and their junction windows quantified by mercury intrusion measurements. The implementation of the mercury intrusion technique shows the good mechanical strength of the materials obtained, which resist the mercury pressures to which they are subjected during the measurements.

Mesoporosity (size ranging from 2 to 50 nm) can be identified by transmission electron microscopy (TEM). The vermicular texture of the mesoporosity can be identified by small angle X-ray diffraction, this technique also being used to quantify the pore-to-pore (or wall-to-wall) distance. Mesoporosity and microporosity can be quantified and segregated by a nitrogen adsorption-desorption technique whose counting is done by the BET calculation method (Brunauer, Emmett and Teller model or BET method) globalizing mesoporosity and microporosity (S Brunauer, P.H. Emmet, E. Teller, Journal of the American Chemical Society, 1938, 60, 2, 309-319); and by the B.J.H calculation method (Barrett, Joyner and Halenda, Journal of the American Chemical Society, 1951, 73, 373-380), according to which the segregation between microporosity and mesoporosity becomes effective, the BJH method only considering the upper pores at 1.5 angstroms.

In an advantageous embodiment of the invention, the material comprises macropores having an average dimension dA of approximately 4 to 100 μm, mesopores having an average dimension dE of approximately 30 to 60 Å, and micropores having an average dimension dI less than about 20 Å, said pores being interconnected.

In an advantageous embodiment of the invention, the material has a specific surface area ranging from 500 to 1200 m ² /g, preferably ranging from 550 to 1100 m ² /g approximately, and in a particularly preferred manner ranging from 600 to 1040 m ² /g approx. This thus makes it possible to guarantee a certain mesoporosity and to promote a sufficient retention time of the gas to be decontaminated within the material during the photocatalysis.

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

The material preferably has a backbone density of at least 1 g/cm ³ approximately, particularly preferably ranging from 1.5 to 4.0 g/cm ³ approximately, and more particularly preferably ranging from 2.0 at approximately 3.5 g/cm ³ . This thus makes it possible to promote the propagation of photons within the material and thus its photonic properties during catalysis.

In the present invention, the density of the skeleton is determined by methods well known to those skilled in the art, and in particular by helium pycnometry, for example using a device sold under the trade name Micromeritics AccuPy 1330.

The material preferably has a bulk density of at least 0.06 g/cm ³ approximately, particularly preferably ranging from 0.07 to 0.20 g/cm ³ approximately, and more particularly preferably ranging from 0. 07 to 0.15 g/cm ³ approximately. This thus makes it possible to guarantee a certain macroporosity and to limit the pressure drop during catalysis.

In the present invention, the apparent density is determined by methods well known to those skilled in the art, and in particular by mercury intrusion measurements at room temperature, for example using a device marketed under the name Micromeritics Autopore IV.

The material preferably has a porosity of at least 80%, particularly preferably ranging from 85 to 98% approximately, and more particularly preferably ranging from 90 to 98% approximately.

The material in accordance with the second object of the invention can advantageously be obtained according to a method as defined in the first object of the invention.

A third object of the invention is the use of a material as defined in the second object of the invention or as obtained according to a process as defined in the first object of the invention, as a photocatalyst.

According to an advantageous embodiment, the material is used to catalyze the degradation of volatile organic compounds in a gaseous medium under the influence of UV irradiation.

The fourth object of the invention is the use of a material as defined in the second object of the invention or as obtained according to a process as defined in the first object of the invention, in the field of photo-purification of air, purification of confined atmospheres, or photo-reduction of CO ₂ into fuels.

Among the volatile organic compounds degradable by the material of the invention, mention may be made of halogenated hydrocarbons, aromatic hydrocarbons, alkanes, alkenes, alkynes, aldehydes, and ketones, in particular acetone, light alkanes , benzene, toluene, xylene, alcohols, and aldehydes, and preferentially acetone, benzene, xylene, and toluene.

A fifth object of the invention is a process for decontaminating a gaseous medium containing one or more volatile organic compounds, said process being characterized in that the decontamination is carried out by passing said gaseous medium through a material as defined in the second object of the invention or as obtained according to a process as defined in the first object of the invention, under light irradiation.

According to this process, the photocatalysis of the VOCs is carried out instantaneously in the entire volume of the material in accordance with the invention and not only at the surface as in the materials of the prior art. The photocatalytic yields are thus increased. Moreover, the degradation of VOCs is observed instantaneously after UV irradiation, without a latency phase, and is stable over time. The decontamination process allows on the one hand the capture of the impurities contained in the effluents to be treated and on the other hand their degradation.

According to a particular embodiment of the method of the invention, the passage of the gaseous medium is carried out in continuous flow.

The light irradiation can be carried out with at least one irradiation source producing at least one wavelength ranging from approximately 250 to 800 nm.

At the end of the process of the invention, the VOCs are converted at least in part into carbon dioxide (CO ₂ ).

Volatile organic compounds are as defined in the fourth object of the invention.

The process can be implemented in a fixed bed reactor.

Said material is preferably fixed within the reactor, and the gaseous medium containing one or more volatile organic compounds is sent through the photocatalytic material.

During the process, the gaseous medium can be chosen from N ₂ , O ₂ , or air.

The process is generally implemented at a temperature ranging from 20 to 60° C. approximately, and preferably from 25 to 40° C. approximately.

The process is generally implemented at a pressure ranging from 0.05 to 1.5 MPa approximately, and preferably from 0.1 to 0.2 MPa approximately.

The present invention is illustrated by the following exemplary embodiments, to which it is however not limited.

The invention is illustrated by Figures 1 to 5 and the examples which follow.

There represents images of monoliths obtained according to the process of the invention with from left to right: Monolith SiO₂-0.3TiO₂; SiO Monolith₂-0.8TiO₂ ; and SiO Monolith₂-1.2TiO₂(figure a); and scanning electron microscopy (SEM) images showing interconnected macroporosity for the SiO Monolith₂-0.3TiO₂(figures b and c); the SiO Monolith₂-0.8TiO₂(figures d and e); and the SiO Monolith₂-1.2TiO₂(figures f and g).

There represents the differential intrusion (expressed in ml/g) is a function of the distribution of the size of the inter-pore junction windows (expressed in μm) for the monoliths: Monolith SiO ₂ -0.3TiO ₂ ; SiO ₂ -0.8TiO ₂ monolith; and SiO ₂ -1.2TiO ₂ Monolith.

There represents nitrogen (N ₂ ) sorption-desorption isotherms as a function of relative pressure P/P ₀ for the monoliths of the invention: SiO ₂ -0.3TiO ₂ monolith; SiO ₂ -0.8TiO ₂ monolith; and Monolith SiO ₂ -1.2TiO ₂ , and for comparison for a monolith not in accordance with the invention Si(HIPE).

There represents the diagrams obtained by wide-angle X-ray scattering on various materials: nanoparticles of TiO ₂ (Degussa P25) used in the process of the invention heated to 700° C. (figure a); untreated TiO ₂ nanoparticles (Degussa P25) used in the process of the invention (figure b); SiO ₂ -0.3TiO ₂ monolith (figure c); SiO ₂ -0.8TiO ₂ monolith (figure d); and SiO ₂ -1.2TiO ₂ monolith (figure e).

There represents the concentration of acetone (solid circles) and carbon dioxide (open diamonds) as a function of time during the irradiation and dark phases.

Examples

The raw materials used in the examples are listed below:
- Dodecane, purity ≥ 99%, Sigma-Aldrich,
- Hydrochloric acid (HCl), 37% by mass, Sigma-Aldrich,
- Tetraethylorthosilicate (TEOS), purity ≥ 99%, Sigma-Aldrich,
- Nanoparticles of TiO ₂ , Degussa P25, Sigma-Aldrich (85% by mass of TiO ₂ in anatase form and 15% by mass of TiO ₂ in rutile form, nanoparticle size 20 nm on average),
- Tetradecyltrimethyl ammonium bromide (TTAB), Alfa Aesar,
- Dichloromethane, ACS-Reagent RPE, Carlo Erba,
- Deionized water, obtained using a Milli-Q type water purification system.

Unless otherwise stated, all materials were used as received from manufacturers.

Claims (15)

Method for preparing a material in the form of a porous monolith comprising a matrix of silicon oxide, and nanoparticles of TiO ₂ , said method comprising:
i) a step for preparing an acidic aqueous phase comprising at least one cationic surfactant and TiO ₂ nanoparticles,
ii) a step of adding a silica precursor to the acidic aqueous phase of step i),
iii) a step of adding an oily phase to the acidic aqueous phase of step ii) to form an emulsion,
iv) a polycondensation step,
v) a step for removing the oily phase, and
vi) a calcining step. Process according to Claim 1, characterized in that the TiO ₂ nanoparticles have a size ranging from 5 to 40 nm. Process according to Claim 1 or 2, characterized in that the titanium dioxide (TiO ₂ ) of the nanoparticles of step i) comprises at least 60% by mass of titanium dioxide in anatase form, relative to the total mass of titanium dioxide. Process according to any one of the preceding claims, characterized in that the TiO ₂ nanoparticles represent at least 0.5% by mass, relative to the total mass of acidic aqueous phase of step i). Method according to any one of the preceding claims, characterized in that step i) is carried out according to the following sub-steps:
i-1) a sub-step of preparing an aqueous phase comprising water and the cationic surfactant,
i-2) a sub-step of adding TiO ₂ nanoparticles to the aqueous phase, and
i-3) a sub-step of acidification of the aqueous phase. Process according to any one of the preceding claims, characterized in that the oily phase comprises one or more compounds chosen from linear and branched alkanes having at least 9 carbon atoms and in that the oily phase represents from 50 to 80% by volume, relative to the total volume of the emulsion. Method according to any one of the preceding claims, characterized in that step vi) comprises:
- a first calcination sub-step vi-1) comprising heating from an initial temperature T _i to a temperature T _{c 1} ranging from 160 to 200°C, with a heating rate ranging from 0.5 to 3°C /min, then maintaining the temperature T _{c 1} for 3 to 10 hours, and
- a second calcination sub-step vi-2) comprising heating from temperature T _c1 to temperature T _c2 ranging from 600 to 800°C, with a heating rate ranging from 0.5 to 2°C/min, then maintaining the temperature T _c2 for 3 to 10 hours. Material in the form of a self-supporting porous monolith, characterized in that it is obtained according to a process as defined in any one of the preceding claims, in that it comprises a matrix of silicon oxide, and TiO ₂ nanoparticles, and in that:
- said material comprises macropores, mesopores, micropores, and walls forming an interconnected porous network, and
- the TiO ₂ nanoparticles are distributed in a statistical manner on the surface of the macropores, and on the surface and in the volume of said walls. Material according to Claim 8, characterized in that the said material comprises from 5% to 30% by mass of TiO ₂ , relative to the total mass of material. Material according to Claim 8 or 9, characterized in that it has a specific surface ranging from 500 to 1200 m ² /g. Material according to any one of Claims 8 to 10, characterized in that it has an apparent density of at least 0.06 g/cm ³ . Material according to any one of Claims 8 to 11, characterized in that it has a porosity of at least 80%. Use of a material as defined in any one of Claims 8 to 12, as a photocatalyst. Use of a material as defined in any one of claims 8 to 12, in the field of photo-purification of air, purification of confined atmospheres, or photo-reduction of CO ₂ in fuels. Process for decontaminating a gaseous medium containing one or more volatile organic compounds, said process being characterized in that the decontamination is carried out by passing said gaseous medium through a material as defined in any one of Claims 8 to 12, under light irradiation.