FR3059916A1 - PROCESS FOR THE PREPARATION OF ZEOLITHE BETA CRYSTALS IN THE FORM OF A HOLLOW BOX CONTAINING A METAL ON ITS INTERNAL SURFACE AND IN ITS WALLS - Google Patents

Abstract

A process for preparing the material comprising at least one Beta zeolite whose crystals are in the form of hollow nano-boxes, and at least one or more metallic nanoparticles, said nanoparticle (s) metal (s) being located at the same time on the inner surface of said nano boxes and inside the walls of said hollow nano boxes, the process comprising at least the following steps: i) the synthesis of a microporous zinc silicate CIT- 6 of structural type * BEA ii) the introduction of metal nanoparticles in the CIT-6 crystals obtained at the end of step i), iii) the mixing in an aqueous solution of the CIT-6 crystals obtained at the end of step ii) with at least one source of an oxide XO2, at least one source of a oxide Y2O3, and at least one source of at least one alkali metal and / or alkaline earth metal M of valence n, n being an integer greater than or equal to 1 to obtain a gel, the hydrothermal treatment gel obtained at the end of stage iii), at a temperature of between 110 ° C. and 180 ° C., preferably between 130 ° C. and 60 ° C., for a period of between 1 days and 5 days, preferably between 1 and 3 days, to synthesize a beta zeolite whose crystals are in the form of hollow nano boxes, and wherein the one or more metal nanoparticles are located on the inner surface at the same time hollow nano boxes and inside the cavity of said hollow nano boxes.

Description

Technical area

The present invention relates to a new process for the preparation of a material comprising at least one or more metallic nanoparticle (s) and at least one beta zeolite, the crystals of which are in the form of hollow nanoparticles, the or said metallic nanoparticle (s) being located both on the internal surface of said nano boxes and inside the walls of said hollow nano boxes. The material thus obtained can advantageously be used as a metal catalyst and a bifunctional catalyst in all hydrocarbon conversion reactions in the fields of refining and petrochemicals for example and has improved properties, in particular in terms of size selectivity of the reactive thanks to the presence of the beta zeolite.

Prior art

In recent decades, zeolites have been widely used to support and encapsulate metallic nanoparticles (NPs), with advantages over conventional supports such as amorphous oxides. Zeolites consist of TO ₄ tetrahedra (T = Si or Al) linked by their vertices, whose arrangement forms pores and cavities of nanometric size (between 0.4 - 1.2nm), which gives them specific surfaces and very high pore volumes. The zeolitic microcavities (between 0.4 - 1.2 nm) are particularly suitable for confining NPs; they make it possible to limit their size and to prevent the sintering phenomenon, which results in a loss of activity of these catalysts under reaction conditions. The nanoporous zeolite system also imposes size selectivity, by preventing certain molecules from entering the crystal lattice up to the catalytic sites. Consequently, the encapsulation of metallic NPs in the nano cavities of the zeolite can sometimes prevent their deactivation when the reaction is carried out in the presence of large poisons. Finally, the aluminum (Al) content of the zeolites gives them qualities of acid catalyst. Thus, a zeolite in its acid form and containing metallic nanoparticles may have bifunctional catalysis properties: acid catalysis coming from the zeolite and hydro- or dehydrogenating catalysis coming from metallic NPs.

The diffusion of the molecules in the porous network of the zeolite up to the catalytic centers constitutes on the other hand a drawback which affects the speed of reaction and also decreases the volume of zeolite actually used. Indeed, the large molecules will diffuse slowly in the pores of the zeolite and they will be confined essentially to the zeolitic network closer to the external surface, leaving the core of the crystals unoccupied. To optimize molecular transport, it is therefore necessary either to reduce the size of the crystal and thus move towards nano zeolites, or to improve accessibility to catalytic centers. In this context, several classes of zeolites have been developed:

• Nano zeolites, for example, offer a fairly small crystal size (<50 nm), however their synthesis remains a challenge for several zeolitic structures. [Farrusseng, Tuel, New J. Chem., 40 (2016) 3933-3949] • “Hierarchical zeolites” are another class of zeolites in which an additional mesoporous network offers diffusion paths and an additional external surface compared to a classic zeolite.

• Zeolites with unconventional morphologies have also been developed to overcome diffusion problems and limit the size of metallic NPs, such as, for example, nanosheets of ZSM-5, [Kim et al. / J. Catal., 301 (2013) 187-197] and [Verheyen et al. / J. Catal., 300 (2013) 70-80]. In the latter case, the nano sheets have a thickness of between 10 and 100 nm, which optimizes molecular diffusion. The NPs are deposited between the zeolitic nanosheets, which limits their size below a few nm. On the other hand, a large part of the NPs is also deposited on the external surface of the agglomerate of nano sheets.

• “Core-shell” type catalysts in which the metal catalyst (core) is protected by a zeolitic layer (shell). They are generally synthesized from the dissolution of a mesoporous amorphous silica containing NPs and recrystallization of a zeolite on the surface using seeds, mainly zeolites of structure MFI and * BEA, [Pagis et al / Chem. Mater. 28 (2016) 52055223]. On the other hand, this zeolitic layer is generally poly-crystalline, relatively thick (> 200 nm) and often mesoporous, which still leads to problems of diffusion and permeation.

• Based on core-shell type catalysts, zeolitic hollow nano boxes, with encapsulated NP, represent a class of materials recently developed, for the moment essentially for MFI-structured zeolites [Pagis et al / Chem. Mater. 28 (2016) 5205-5223]. Unlike the "heart-shell", the box comes from a zeolitic single crystal. Consequently, walls whose thickness does not exceed a few tens of nanometers thick, are exclusively microporous, which decreases the path of diffusion of the molecules while keeping the selectivity of size. Thus, the box protects the NP encapsulated inside from sintering and poisoning phenomena.

The nano zeolitic hollow boxes with encapsulated NPs, which constitute this last class of materials were developed essentially for the zeolites of MFI structure and have already demonstrated capacities of size selectivity in hydrogenation of substituted aromatics. Thus, a Pt @ silicalite-1 type catalyst has demonstrated remarkable resistance to CO poisoning during the oxidation of propylene [S. Li et al / ACS Catal. 4 (2014) 4299-4303]. Given their potential and the many promising prospects, there is interest in extending this class of materials to other zeolitic structures, in particular structures with larger pores, such as for example the zeolites of structure * BEA.

The beta zeolite is a structure zeolite * BEA, of the “large pore” type which has two different pore systems delimited by 12 tetrahedra with openings of 0.55 x 0.55 and 0.64 x 0.76 nm. This gives it a high pore volume of about 0.23 mL / g. The beta zeolite can be synthesized with very varied Al contents (8 <Si / AI <°°) and can therefore be used in a large number of acid catalysis reactions. [S. Mintova et al. / Microporous and Mesoporous Materials 90 (2006) 237-245]

The synthesis of beta nano-boxes of beta zeolite from a zeolitic single crystal has been described for the first time by Okubo et al. [K. lyoki et al / Chem. Asian J., (2013), 8, 1419-1427] by seeking a method for preparing the beta zeolite without using a structuring agent. The synthesis is carried out in the presence of CIT-6 germs, a microporous zincosilicate with the same structure as the beta zeolite developed by Davis et al [T. Takewaki et al / Topics in Catalysis, 9, (1999), 35-42], CIT-6 is obtained by hydrothermal crystallization of a reaction mixture containing a source of silica, a source of Zn, lithium hydroxide LiOH , and the structuring agent typically used by the beta zeolite, namely tetraethylammonium hydroxide TEAOH.

In the synthesis of nano hollow beta boxes according to Okubo et al, the CIT-6 crystals, preferably untreated and not calcined, are dispersed in a basic solution of soda (NaOH), sodium aluminate (NaAIO ₂ ) and silica (SiO ₂ ) to form a gel. The gel obtained is then subjected to a hydrothermal treatment for a period of between 20 and 76 hours, at a temperature between 140 and 150 ° C. During this hydrothermal treatment, the CIT-6 crystals function like germs, which provide an active surface for the growth of the beta zeolite, once they have the same structure. There are two parallel phenomena: i) the growth of the beta zeolite around the crystals of CIT6 and in the presence of additional sources of aluminum and silicon in the reaction mixture, and ii), the dissolution of CIT-6, releasing silicic species and TEA ⁺ cations. The balance between the dissolution rates of the CIT-6 and the crystallization of the beta zeolite allows the formation of hollow nano-boxes. In the end, the complete dissolution of CIT-6 leads to the formation of hollow nano-boxes of beta zeolite.

Throughout the rest of the text, the term hollow nanoparticles is understood to mean a zeolite crystal comprising a macroporosity in the form of a single internal cavity. The hollow box morphology is demonstrated by scanning electron microscopy (SEM) and also by the presence of hysteresis on the nitrogen adsorption isotherms of the zeolite whose crystals are in the form of nano hollow box , hysteresis which does not exist on the nitrogen adsorption isotherm of the initial CIT-6. This type of isotherm is similar to the type IV isotherm (a) of the IUPAC classification: the presence of hysteresis is due to capillary condensation in pores with a diameter greater than 4 nm [Thommes et al / Pure Appl . Chem. 87 (9-10) (2015), 1051-1069]. In the case of the invention, the porosity corresponds to the interior cavity of the nano boxes.

The dimensions of the nano box are determined from scanning electron microscopy (SEM) images using ImageJ software for example. In particular, the size of the nano boxes is calculated from the average of the sizes determined on a number of measurements that those skilled in the art deem necessary and sufficient and preferably, on a population of at least 100 crystals.

Preferably, the size of the boxes is between 500 and 1200 nm, preferably between 600 and 1000 nm and preferably between 700 and 1000 nm and the thickness of the wall of said nano boxes is between 50 and 300 nm, preferably between 90 and 400 nm and preferably between 90 and 150 nm.

Brief description of the invention

The present invention relates to a new process for the preparation of a material comprising at least one beta zeolite whose crystals are in the form of hollow nano boxes, and at least one or more metallic nanoparticle (s), the or said metallic nanoparticle (s) being located both on the internal surface of said nanoparticles and inside the walls of said hollow nanoparticles, said preparation process comprises at least the following steps:

i) the synthesis of a microporous zinc-silicate CIT-6 of structural type * BEA ii) the introduction of metallic nanoparticles into the crystals of CIT-6 obtained at the end of step i), iii) the mixture in a aqueous solution of the crystals of CIT-6 obtained in step ii) with at least one source of an oxide XO ₂ , at least one source of an oxide Y2O3, and at least one source of at least one alkali metal and / or alkaline earth M of valence η, n being an integer greater than or equal to 1 to obtain a gel, iv) the hydrothermal treatment of the gel obtained at the end of step iii), at a temperature between 110 ° C and 180 ° C, preferably between 130 ° C and 160 ° C, for a period of between 1 days and 5 days, preferably between 1 and 3 days, to synthesize a beta zeolite whose crystals are in the form of nano hollow boxes, and in which the said metallic nanoparticle (s) are located at the same time on the internal surface of the n ano hollow boxes and inside the cavity of said nano hollow boxes.

An advantage of the present invention is to provide a process which makes it possible, by implementing a sequence of specific steps, to obtain a material comprising at least one beta zeolite whose crystals are in the form of nano hollow boxes, and at least one or more metallic nanoparticle (s) located both on the internal surface of the nano hollow boxes and inside the cavity of said nano hollow boxes.

Detailed description of the invention

According to the invention, a process for the preparation of a material comprising at least one Beta zeolite, the crystals of which are in the form of hollow nano-boxes, and at least one or more metallic nanoparticle (s), is described, said metallic nanoparticle (s) being located both on the internal surface of said nano boxes and inside the walls of said hollow nano boxes, the method comprising at least the following steps:

i) the synthesis of a microporous zinc-silicate CIT-6 of structural type * BEA ii) the introduction of metallic nanoparticles into the crystals of CIT-6 obtained at the end of step i), iii) the mixture in a aqueous solution of the CIT-6 crystals obtained in step ii) with at least one source of an oxide XO ₂ , at least one source of an oxide Y ₂ O ₃ , and at least one source of at least one alkali and / or alkaline earth metal M of valence η, n being an integer greater than or equal to 1 to obtain a gel, iv) the hydrothermal treatment of the gel obtained at the end of step iii), at a temperature comprised between 110 ° C and 180 ° C, preferably between 130 ° C and 160 ° C, for a period of between 1 days and 5 days, preferably between 1 and 3 days, to synthesize a beta zeolite whose crystals are present under the form of hollow nanoparticles, and in which the said metallic nanoparticle (s) are located both on the internal surface of s nano hollow boxes and inside the cavity of said nano hollow boxes.

According to the invention, step i) consists in synthesizing a CIT-6 zincosilicate, precursor of nano boxes of beta zeolite. Preferably, said step i) is implemented by following the protocol described by Davis et al. [Takewaki et al / Topics in Catalysis, 9, (1999), 35-42].

Preferably, step i) of synthesis of CIT-6 comprises a step of forming a gel by mixing in aqueous medium at least one source of XO ₂ , at least one source of zinc, at least one organic nitrogen compound R, R being the tetraethylammonium ion (TEA ⁺ ), and at least one source of at least one alkali and / or alkaline earth metal M of valence η, n being an integer greater than or equal to 1.

Preferably, X is a tetravalent element chosen from the group formed by the following elements: silicon, germanium, titanium, preferably silicon.

Preferably, the source of zinc is chosen from inorganic zinc salts and preferably from zinc nitrates, zinc chlorides, zinc sulfates, zinc hydroxides, zinc alkoxides, zinc acetate, zinc citrate or oxalate and preferably the source of zinc is zinc acetate.

Preferably, the source of at least one alkali and / or alkaline earth metal M is chosen from the oxides of one or more alkali and / or alkaline earth metal (s) chosen from lithium, sodium, potassium, calcium, magnesium oxide and the mixture of at least two of these metals and preferably a lithium oxide.

The mixture has the following molar composition:

XO ₂ / Zn between 10 and 50, preferably between 20 and 35,

H ₂ O / XO ₂ between 1 and 100, preferably between 25 and 40,

R / XO ₂ between 0.2 and 0.9, preferably between 0.4 and 0.7,

M ₂ / _n O / XO ₂ between 0.005 and 0.1, preferably between 0.01 and 0.05, in which X, M and R have the above definition.

Preferably, step i) then comprises a hydrothermal treatment step of the gel thus obtained, preferably carried out in the absence of agitation and at a temperature between 130 ° C and 160 ° C, preferably between 140 ° C and 150 ° C, until the formation of CIT-6 crystals. The time required to obtain crystallization generally varies between 1 and 10 days, preferably between 2 and 7 days. The structure of the CIT-6 obtained is isomorphic to that of the beta zeolite, which constitutes a key criterion for the recrystallization of the latter on the CIT-6, as described in step (iv).

At the end of the hydrothermal treatment, when said CIT-6 is formed, the solid phase is filtered, washed and dried. The drying is generally carried out at a temperature between 20 and 150 ° C, preferably between 60 and 120 ° C, for a period between 5 and 24 hours.

At the end of step i) of the process according to the invention, crystals of microporous zincosilicate CIT-6 of structural type * BEA are obtained.

Preferably, the CIT-6 crystals obtained in step i) undergo no ion exchange step and no calcination step subsequent to the hydrothermal treatment step of step i) before being sent to said step ii) of the method according to the invention.

Preferably, the CIT-6 crystals obtained in step i) optionally dried, can advantageously undergo a partial desorption step, before step ii) of introduction of metallic nanoparticles, in order to remove all the water included in its porosity. Desorption makes it possible to facilitate step ii) of introduction of the metallic nanoparticles. The desorption of the CIT-6 crystals obtained at the end of step i) of the process according to the invention is preferably carried out in air or vacuum, at a temperature between 80 and 700 ° C, preferably between 80 and 200 ° C, preferably between 90 and 110 ° C, for a period of between a few hours and several days, preferably between 1 and 24 hours, very preferably between 2 and 5 hours.

According to the invention, the method according to the invention comprises a step ii) of introducing metallic nanoparticles into the CIT-6 crystals, optionally dried and desorbed, obtained at the end of step i).

The introduction or deposition of metallic nanoparticles can be done by different methods, varying the percentage of metal, the nature of the precursors, and other factors.

Step ii) can advantageously be carried out according to all the techniques known to those skilled in the art: impregnation, ion exchange, competitive ion exchange with a preference for impregnation. These techniques are preferably carried out at a temperature between 10 ° C and 90 ° C for a period between 8 and 48 hours. In the case of ion exchange and competitive tonic exchange methods, the exchange can advantageously be carried out by suspending said CIT-6 in one or more times with the ion exchange solution. Preferably, the sample should be washed with distilled water after the exchange.

Preferably, the metallic nanoparticles are chosen from the noble metals of groups 8 to 11 of the periodic table, preferably chosen from Au, Ag, Rh, Pd, Ru, Ir and Pt, the transition metals from groups 3 to 12 , preferably chosen from Co and Ni or metallic alloys, that is to say, a mixture of these elements. For each metal, several metal precursors can be used.

Intermediate treatments such as for example calcination and / or reduction can advantageously be applied between the successive deposits of the different metals.

A metal reduction step can advantageously be carried out by chemical reduction with NaBH _4i LiBH ₄ or KBH ₄ , preferably at a temperature between 10 ° C and 60 ° C. The reduction can advantageously be carried out also by: hydrogen, hydrogen saturated with water vapor, formic acid, ethylene glycol or under UV radiation, or even during a previous calcination step in air.

According to the invention, the method according to the invention comprises a step iii) of mixing in an aqueous solution the crystals of CIT-6 obtained in step ii) with at least one source of an oxide XO ₂ , at least a source of an oxide Y ₂ O ₃ , and at least one source of at least one alkali and / or alkaline-earth metal M of valence η, n being an integer greater than or equal to 1 to obtain a gel.

Preferably, X is a tetravalent element chosen from the group formed by the following elements: silicon, germanium, titanium, preferably silicon, Y is one or more trivalent element (s) chosen from the group formed by the following elements: aluminum, iron, boron, indium and gallium, preferably aluminum, the source of at least one alkali and / or alkaline-earth metal M is chosen from the oxides of one or more metals (aux ) alkaline (s) and / or alkaline earth M selected from lithium, sodium, potassium, calcium, magnesium oxide and the mixture of at least two of these metals, preferably sodium.

Preferably, said step ii) is carried out according to the method described in the literature by Okubo et al. [K. lyoki et al / Chem. Asian J. 8 (2013) 1419-1427],

Preferably, the reaction mixture has the following molar composition:

XO ₂ / Y ₂ O ₃ between 10 and 300, preferably between 30 and 250,

H ₂ O / XO ₂ between 1 and 100, preferably between 15 and 25,

CIT-6 (g) / XO ₂ (g) x100% between 5% and 50%, preferably between 10% and 25%,

M _{2 / n} O / XO ₂ between 0.02 and 0.5, preferably between 0.225 and 0.350, in which X, Y, and M have the above definition.

According to the invention, the method comprises a step iv) which consists in the hydrothermal treatment of the gel obtained in step iii) at a temperature between 110 ° C and 180 ° C, preferably between 130 ° C and 160 ° C, for a period of between 1 days and 5 days, preferably between 1 and 3 days, until the hollow nano boxes of beta zeolite are formed.

During this hydrothermal treatment, the CIT-6 crystals function like germs, which provide an active surface for the growth of the beta zeolite, once they have the same structure. There are two parallel phenomena: i) the growth of the beta zeolite around the crystals of CIT-6 and in the presence of additional sources of aluminum and silicon in the reaction mixture, and ii), the dissolution of CIT-6 , releasing silicic species and TEA ⁺ cations. The balance between the dissolution rates of the CIT-6 and the crystallization of the beta zeolite allows the formation of hollow nano-boxes. In the end, the dissolution of CIT-6 leads to the formation of the cavity inside the nano boxes of beta zeolite.

At the end of step iv), the beta zeolite obtained is advantageously filtered, washed and then dried. Preferably, the drying is carried out at a temperature between 20 and 150 ° C, preferably between 60 and 120 ° C, for a period between 5 and 20 hours.

The dried solid phase thus obtained at the end of step iv) is analyzed by X-ray diffraction so as to verify the presence of the beta zeolite. This technique also makes it possible to determine the purity of said zeolite obtained by the absence of reflections characteristic of other structures.

The hollow box morphology is demonstrated by scanning electron microscopy (SEM) and in transmission and also by the presence of hysteresis on the nitrogen adsorption isotherms of the zeolite, hysteresis which is not present on the initial CIT-6 nitrogen adsorption isotherm. This type of isotherm is similar to a type IV isotherm (a) of the IUPAC classification: the presence of hysteresis is due to capillary condensation in pores with a diameter greater than 4 nm [M. Thommes et al / Pure Appl. Chem. 87 (9-10) (2015) 1051-1069]. In the case of the invention, the porosity corresponds to the interior cavity of the nano-boxes.

The dimensions of the nano box are determined from scanning electron microscopy (SEM) images using Image J software. In particular, the size of the nano boxes is calculated from the average of the sizes determined on a number of measurements that a person skilled in the art considers necessary and sufficient and preferably, on a population of, at least, 100 crystals.

Preferably, the size of the boxes is between 500 and 1200 nm, preferably between 600 and 1000 nm and preferably between 700 and 1000 nm and the thickness of the wall of said nano boxes is between 50 and 300 nm, preferably between 90 and 400 nm and preferably between 90 and 150 nm.

Step iv) and more specifically the specific sequence of steps i), ii), iii) and iv) allows a beta zeolite to be obtained, the crystals of which are in the form of hollow nano-boxes, in which the metallic nanoparticle (s) are deposited both on the internal surface of said nano boxes and inside the walls of said hollow nano boxes of beta zeolite. The presence of nanoparticles on the internal surface and inside the walls of nano hollow boxes of beta zeolite is confirmed by photos of Transmission Electron Microscopy (TEM) coupled with a Tomography analysis followed by a 3D reconstruction. Transmission electron microscopy also makes it possible to verify that the size distribution of the metallic nanoparticles and the size of said metallic nanoparticles are a function of many factors such as the nature of the precursor, the deposition method, the reduction method, etc.

Preferably, the beta zeolite obtained by step iv) optionally undergoes a calcination step. Preferably, the calcination is preferably carried out in air at a temperature between 200 and 700 ° C, preferably between 500 and 700 ° C, preferably between 450 and 650 ° C, very preferably between 500 and 600 ° C for a period of between a few hours and several days, preferably between 5 and 24 hours, preferably between 5 and 15 hours and very preferably between 7 and 13 hours.

It is also advantageous to obtain the proton form of the beta zeolite obtained according to the method of the invention. Said proton form can be obtained by carrying out an ion exchange with compounds such as chloride, sulphate or ammonium nitrate. The ion exchange can advantageously be carried out by suspending said beta zeolite in one or more times with the ion exchange solution, (operating conditions: concentration, volume of solution, T, etc.). zeolite can be calcined before or after ion exchange, or between two stages of ion exchange. The zeolite is preferably calcined before the ion exchange, in order to remove any organic substance included in its porosity, insofar as the ion exchange is thereby facilitated.

The beta zeolite obtained according to the method of the invention and the crystals of which are in the form of nano hollow boxes, in which one or more metallic nanoparticles are deposited on the internal surface at the same time and inside the walls can be used after ion exchange as a metallic catalyst and as a bifunctional catalyst, in the fields of refining and petrochemicals for example.

For example, when used as a catalyst, the material obtained according to the process of the invention can advantageously be associated with an inorganic matrix, which can be inert or catalytically active. The inorganic matrix can be present simply as a binder to hold together the small particles of the zeolite in the various known forms of the catalysts (extrudates, pellets, balls, powders), or else can be added as diluent to impose the degree of conversion process which would otherwise progress at an excessively rapid rate, leading to fouling of the catalyst as a result of significant formation of coke. Typical inorganic matrices are in particular support materials for catalysts such as silica, the different forms of alumina, magnesia, zirconia, oxides of titanium, boron, titanium, zirconium, aluminum phosphates, kaolinic clays, bentonites, montmorillonites, sepiolite, attapulgite, fuller's earth, synthetic porous materials such as SiO2-AI ₂ O ₃ , SiO ₂ -ZrO ₂ , SiO ₂ -ThO ₂ , SiO ₂ -BeO , SiO ₂ -TiO ₂ or any combination of these compounds. The inorganic matrix can be a mixture of different compounds, in particular an inert phase and an active phase.

The material obtained according to the preparation process of the invention can advantageously be shaped according to all the methods known to those skilled in the art. We can use, for example, pelletizing or extrusion or the form of beads. The material prepared according to the process of the invention and which is at least partly in acid form is generally put into the form of extrudates or beads for use.

The invention is illustrated by the following examples which are in no way limiting in nature.

Example 1: synthesis of CIT-6

CIT-6 crystals are synthesized according to a protocol from the literature [T. Takewaki et al / Topics in Catalysis, 9, (1999), 35-42], Pour into a polypropylene beaker 0.239 g of LiOH (98%, Aldrich) and 54 g of distilled water. Then added 54.6 g of a TEAOH solution at 35% by weight in water and after stirring for 15 minutes, 1.328 g of zinc acetate Zn (CH ₃ COO) 2-2H ₂ O is added. After 15 minutes with stirring, 30 g of silica (Ludox HS-40) are added dropwise while raising the temperature to 40 ° C and keeping the mixture under vigorous stirring for 1 hour. The gel is, whose molar composition is 1 SiO ₂ : 0.03 Zn (CH ₃ COO) ₂ : 0.05 LiOH: 0.65 TEAOH: 30 H ₂ O, is then divided into four identical fractions, each of they being transferred into a Teflon jacket and then the shirt placed in a 48 milliliter autoclave.

The autoclaves are heated for 4 days at 145 ° C in a static oven. Then, the products are washed with distilled water by centrifugation, dried overnight at 80 ° C and mixed to form only one batch.

The dried solid product is analyzed by X-ray powder diffraction: the diffractogram obtained is characteristic of a CIT-6 with a structure * BEA without any other parasitic phase. The difractogram represented in Figure 1 is obtained in XRD on a Bruker-Siemens D5005 device using the Ken and Ka _{2 line} of copper (λ = 1.54184A).

From the position of the diffraction peaks represented by the angle 20, we calculate, by the Bragg relation, the reticular equidistances d _hk i characteristic of the sample. The estimation of the measurement error A (d _h w) on d _h ki is calculated by the Bragg relation as a function of the absolute error Δ (2θ) assigned to the measurement of 20. An absolute error Δ (2Θ ) equal to ± 0.2 ° is commonly accepted. The relative intensity l _re i assigned to each value of d _hk | is measured according to the height of the corresponding diffraction peak. The X-ray diffraction diagram of the crystallized solid CIT-6 of Example 1 comprises at least the lines with the values d _hk i given in Table 1. In the column of d _hki , the mean values of distance between Reticular in Angstroms (Â). Each of these values must be assigned the measurement error A (d _hk i) between ± 0.2Â and ± 0.008Â.

2theta [degrees] d _hk , [A] l / lo 7.16 12.34 f 7.64 11.56 m 11.60 7.62 ff 13.31 6.65 ff 14.63 6.05 ff 16.51 5.37 ff 17.59 5.04 ff 17.66 5.02 ff 20.24 4.38 ff 21.42 4.15 f 22.45 3.96 F 22.79 3.90 ff 24.52 3.63 ff 24.59 3.62 ff 25.30 3.52 ff 26.02 3.42 ff 26.80 3.32 f 27.99 3.19 ff 28.82 3.10 ff 29.48 3.03 f 30.47 2.93 ff 30.90 2.89 ff 32.85 2.72 ff 33.38 2.68 ff 34.56 2.59 ff 36.21 2.48 ff 37.46 2.40 ff 41.11 2.19 ff 42.46 2.13 ff 43.64 2.07 ff 44.41 2.04 ff 49.62 1.84 ff 51.80 1.76 ff

Table 1 in which F signifies Strong, m signifies medium, f signifies weak and ff signifies very weak, the relative intensity l / lo being given in relation to a relative intensity scale where a value of 100 is assigned to the line the most intense of the X-ray diffraction diagram: ff <10; 10 <f <20; 20 <m <60; F> 60

In addition, chemical analysis shows the compositions of Si / Li = 26 and Si / Zn = 17.

N ₂ adsorption measurements were carried out in the usual manner on a “Belsorp Mini” type material. The results are shown in Figure 3.

Example 2: introduction of the Pt nanoparticles on the CIT-6 crystals obtained in Example 1 by impregnation of a solution of (NH ₃ ) ₄ Pt (NO ₃ ) _? as a precursor to Pt

The crystals of CIT-6 obtained in Example 1 are heated to 100 ° C. in static air for 4 h, in order to remove all the adsorbed water. After cooling to 50 ° C, the CIT-6 crystals are immediately weighed and placed in a glass bottle. Then, pour an aqueous solution of (NH ₃ ) ₄ Pt (NO ₃ ) ₂ , C = 0.25M (Pt% w% = 2%) with an adjustable pipette. Mix manually with a spatula for 15 minutes, until all the powder is wet. The closed bottle is left to mature at room temperature for 12 hours. Then, the glass vial containing the CIT-6 crystals is transferred to a drying oven at 80 ° C for 11 h.

Then, a step of chemical reduction of Pt is carried out with a solution of NaBH ₄ , under the conditions: 0.05 gCIT-6 / ml and [NaBH ₄ ] = 5 mmol. Water is poured into the glass flask containing the CIT-6 crystals with stirring at 500 rpm, and added immediately after NaBH ₄ . After one minute with stirring at 500 rpm, the mixture is left for 1 hour at room temperature with stirring at 200 rpm. Then, the product is washed with distilled water and dried overnight at 80 ° C.

The solid product was analyzed by powder X-ray diffraction: the diffractogram obtained is characteristic of a CIT-6 structure * BEA without any other parasitic phase (DRX not shown).

In addition, chemical analysis proves the presence of Platinum.

Transmission electron microscopy images were carried out on the CIT-6 crystals resulting from Example 2, presented in FIG. 4. The dimensions of the platinum nanoparticles are determined from transmission electron microscopy images. using ImageJ software. In particular, the size of the platinum nanoparticles is calculated from the average of the sizes determined on a number of measurements that those skilled in the art deem necessary and sufficient and preferably, on a population of at least 100 nanoparticles of platinum.

The platinum nanoparticles are clearly visible in the hollow nano-boxes of the beta zeolite. Their position on the internal walls of said hollow nano boxes of beta zeolite and in the walls of said hollow nano boxes is confirmed by Tomographic analysis, followed by 3D reconstruction. We can estimate that there is a population of nanoparticles between 10-20nm, which are above all clearly on the surface.

Example 3: Synthesis of the hollow beta nanoparticles with platinum nanoparticles on the internal surface and inside the walls of the hollow nanoparticles from the crystals of CIT-6 of Example 2.

Nano boxes of beta zeolite in its sodium form (Na-beta) are synthesized according to the protocol described by Okubo et al. [K. lyoki et al / Chem. Asian J., 8, (2013), 1419-1427). 0.764 g of NaOH (98%, Aldrich) and 12 g of distilled water are poured into a polypropylene beaker. The mixture is left under magnetic stirring until complete dissolution of NaOH. 0.064 g of NaAIO ₂ is then added and the mixture is stirred for 10 minutes. 0.2 g of crystals of CIT-6 are added with metallic nanoparticles obtained according to Example 2, and the mixture is left stirring for 5 min. Finally, 1.8 g of Aerosil 200 silica are gradually added, and the mixture is stirred manually with a spatula until a dense gel is formed. The gel is then transferred to a Teflon jacket and the shirt is placed in a 100 milliliter autoclave. The molar composition of the gel is 1 SiO ₂ : 0.01 AI ₂ O ₃ : 0.3 Na ₂ O: 20 H ₂ O, where SiO ₂ is composed of 10% by weight of CIT-6 and 90% by weight of Aerosii 200.

The autoclave is heated for 22 hours at 150 ° C in an oven. Then, the product is washed with distilled water by centrifugation and then dried overnight at 80 ° C.

The dried solid product was analyzed by X-ray powder diffraction: the diffractogram is characteristic of a pure beta zeolite. The diffractogram of example 3, which is represented in Figure 2, was obtained in XRD on a Bruker-Siemens D5005 device using the line Κα _r and Ka ₂ of copper (λ = 1.54184Â).

From the position of the diffraction peaks represented by the angle 20, we calculate, by the Bragg relation, the reticular equidistances dhkl characteristic of the sample. The estimation of the measurement error A (dhkl) over dhkl is calculated by the Bragg relation as a function of the absolute error Δ (2θ) assigned to the measurement 20. An absolute error Δ (2Θ) equal to ± 0 , 2 ° is commonly accepted. The relative intensity Irel assigned to each value of dhkl is measured according to the height of the corresponding diffraction peak. The X-ray diffraction diagram of the crystallized solid of Example 3 comprises at least the lines with the values dhki given in Table 2. In the column of d _hk i, the mean values of inter-reticular distances in Angstoms have been indicated ( AT). Each of these values must be assigned the measurement error A (d _h ki) between ± 0.2Â and ± 0.008Â.

2theta [degrees] d _h ki [Â] l / lo 7.09 12.46 f 7.66 11.54 m 11.66 7.58 ff 13.31 6.65 ff 14.51 6.10 ff 14.54 6.09 ff 16.41 5.40 ff 18.02 4.92 ff 18.46 4.80 ff 21.27 4.17 f 22.32 3.98 F 25.17 3.53 ff 25.80 3.45 ff 26.81 3.32 f 28.64 3.11 ff 29.38 3.04 f 30.26 2.95 ff 33.20 2.70 ff 34.48 2.60 ff 35.95 2.50 ff 37.19 2.42 ff 43.33 2.09 ff 48.14 1.89 ff 49.28 1.85 ff 52.95 1.73 ff 55.27 1.66 ff 55.95 1.64 ff 61.63 1.50 ff 67.22 1.39 ff 72.69 1.30 ff

Table 2 in which F means Strong, m means medium, f means weak and ff means very weak, the relative intensity l / lo being given in relation to a relative intensity scale where a value of 100 is assigned to the line the most intense of the X-ray diffraction diagram: ff <10; 10 <f <20; 20 <m <60; F> 60

In addition, chemical analysis confirms the presence of Pt.

N ₂ adsorption measurements were carried out in the usual manner on “Belsorp Mini” equipment. The results are given in Figure 3. The morphology in a hollow box is directly demonstrated by the presence of hysteresis in the nitrogen adsorption / desorption isotherm. This type of isotherm belongs to type IV (a) of the lUPAC classification: the presence of hysteresis is due to capillary condensation in pores with a diameter greater than 4 nm [M. Thommes et al / Pure Appl. Chem. 87 (9-10) (2015) 1051-1069], In the case of the invention, the porosity corresponds to the interior cavity of the nanobins.

Transmission electron microscopy images were carried out on the crystals resulting from Example 3, presented in Figure 5. The morphology of the hollow box is highlighted: the size of the boxes is between 500 and 1200 nm and preferably between 700 and 1000 nm, and the thickness of the wall of said nano boxes is between 50-300 nm and preferably between 90 and 150nm. The dimensions of the platinum nanoparticles are determined from transmission electron microscopy images using the ImageJ software. In particular, the size of the platinum nanoparticles is calculated from the average of the sizes determined on a number of measurements that those skilled in the art deem necessary and sufficient and preferably, on a population of at least 100 nanoparticles of platinum.

The platinum nanoparticles are clearly visible in the hollow nano-boxes of the beta zeolite. Their position on the internal walls of said hollow nano boxes of beta zeolite and in the walls of said hollow nano boxes is confirmed by Tomographic analysis, followed by 3D reconstruction. We can estimate that there is a population of nanoparticles between 10-30nm.

Claims (2)

Process for the preparation of a material comprising at least one beta zeolite, the crystals of which are in the form of hollow nanoparticles, and at least one or more metallic nanoparticle (s), the metallic nanoparticle (s) ( s) being located both on the internal surface of said nano boxes and inside the walls of said hollow nano boxes, the method comprising at least the following steps: i) the synthesis of a microporous zinc-silicate CIT-6 of structural type * BEA, ii) the introduction of metallic nanoparticles into the crystals of CIT-6 obtained at the end of step i), iii) the mixture in an aqueous solution of the CIT-6 crystals obtained in step ii) with at least one source of an oxide XO ₂ , at least one source of an oxide Y ₂ O ₃ , and at least one source of at least an alkali and / or alkaline earth metal M of valence η, n being an integer greater than or equal to 1 to obtain a gel, iv) the hydrothermal treatment of the gel obtained at the end of step iii), at a temperature between 110 ° C and 180 ° C, preferably between 130 ° C and 160 ° C, for a period of between 1 days and 5 days, preferably between 1 and 3 days, to synthesize a beta zeolite whose crystals are present in the form of hollow nanoparticles, and in which the said metallic nanoparticle (s) are located both on the internal surface nano hollow boxes and inside the cavity of said nano hollow boxes. Process according to Claim 1, in which step i) of synthesis of CIT-6 comprises a step of forming a gel by mixing in aqueous medium at least one source of XO ₂ , X being a tetravalent element chosen from the group formed by the following elements: silicon, germanium, titanium of at least one source of zinc chosen from zinc salts, of at least one nitrogenous organic compound R, R being the tetraethylammonium ion (TEA ⁺ ), and at least one source of at least one alkali and / or alkaline earth metal M of valence η, n being an integer greater than or equal to 1, the source of at least one alkali and / or alkaline earth metal M is chosen among the oxides of one or more alkali and / or alkaline-earth metal (s) M chosen from lithium, sodium, potassium, calcium, magnesium oxide and the mixture of minus two of these oxides. 3. The method of claim 2 wherein the source of zinc is zinc acetate. 4. Method according to one of claims 1 to 3 in which the mixture of step i) has the following molar composition: XO ₂ / Zn between 10 and 50, H ₂ O / XO ₂ between 1 and 100, R / XO ₂ between 0.2 and 0.9, M _{2 / n} O / XO ₂ between 0.005 and 0.1 in which X, M and R have the above definition. 5. Method according to claim 2 to 4 wherein step i) then comprises a 10 hydrothermal treatment step of the gel thus obtained, carried out at a temperature between 130 ° C and 160 ° C, for a period between 1 and 10 days. 6. Method according to one of claims 1 to 5 wherein the CIT-6 crystals obtained in step i) undergo a partial desorption step, before step ii) of introduction of metallic nanoparticles said desorption step being carried out in air or vacuum, at 15 a temperature between 80 and 700 ° C, for a period between 1 and 24 hours. 7. Method according to one of claims 1 to 6 wherein step ii of introducing metallic nanoparticles into the crystals of CIT-6 is carried out by impregnation, ion exchange or competitive ion exchange. 8. The method according to claim 7, in which the metallic nanoparticles are chosen from the noble metals of groups 8 to 11 of the periodic table, chosen from Au, Ag, Rh, Pd, Ru, Ir and Pt, the transition metals. groups 3 to 12 chosen from Co and Ni or a mixture of these elements. 9. Method according to one of claims 1 to 8 wherein in step iii), X is a 25 tetravalent element chosen from the group formed by the following elements: silicon, germanium, titanium, Y is one or more trivalent element (s) chosen from the group formed by the following elements: aluminum, iron, boron , indium and gallium, and the source of at least one alkali and / or alkaline earth metal M is chosen from the oxides of one or more alkali and / or alkaline earth metal (s) chosen among the lithium oxides, 30 of sodium, potassium, calcium, magnesium and the mixture of at least two of these metals. 10. The method of claim 9 wherein X is silicon, Y is aluminum and M is sodium. 11. Method according to one of claims 1 to 10 in which the reaction mixture of step iii) has the following molar composition: 5 XO2 / Y2O3 between 10 and 300, H ₂ O / XO ₂ between 1 and 100, CIT-6 (g) / XO ₂ (g) x100% between 5% and 50%, M _{2 /} nO / XO ₂ between 0.02 and 0.5, in which X, Y, and M have the above definition. 12. Method according to one of claims 1 to 11, in which the beta zeolite obtained at the end of step iv) is filtered, washed and then dried at a temperature between 20 and 150 ° C, for a period of time included. between 5 and 20 hours. 1/3 DRX 100 ⁵⁰ 4 14 24 34 44 54 64 2theta