FR2924361A1 - Preparing a powder useful e.g. to catalyze chemical reaction e.g. hydrogenation reactions, comprises preparing a mother liquor, hydrothermal preparation of a soil, and after optional drying step, washing the soil to obtain a powder - Google Patents

Abstract

Process for preparing a powder comprises: (a) preparation of a mother liquor comprising (i) an oxide based constituent or a precursor of the base constituent and an additional constituent (A), or (ii) an oxide based constituent or a precursor of the base constituent incorporating an agent; (b) hydrothermal preparation, from the mother liquor, of a soil of which crude particles have a maximum size less than 100 nm; and (c) after optional drying step, washing the soil to obtain a powder comprising more than 99.3 mass% of the base constituent based on dry material. Process for preparing a powder comprises: (a) preparation of a mother liquor comprising (i) an oxide based constituent or a precursor of the base constituent and an additional constituent (A) of formula (MC), where M is an agent comprising oxoanions, anions of the elements of the column 17 of the periodic table elements and/or hydroxide (OH ->), and C is complement, which is inorganic when the agent is OH ->or (ii) an oxide based constituent or a precursor of the base constituent incorporating the agent; (b) hydrothermal preparation, from the mother liquor, of a soil of which crude particles have a maximum size less than 100 nm; and (c) after optional drying step, washing the soil to obtain a powder comprising more than 99.3 mass% of the base constituent based on dry material. An independent claim is included for the powder.

Description

The invention relates to nanoscale particles and catalytic systems for catalyzing chemical reactions and a process for their synthesis. Catalysis involves many reactions, in various technical fields, in particular environmental applications, petrochemistry, or fine chemistry. It consists in modifying the speed of a chemical reaction by bringing the reactants of this reaction into contact with a catalyst, for example platinum, which does not appear in the reaction balance. Generally the catalyst is previously deposited on a support, for example in the form of a nanometric powder, the particle size of which is less than 100 nm. STATE OF THE ART The article "Surface Studies of supported mode / catalysts" - C.R. Henry - Surface science reports 31 (1998) 231-325 thus describes the use of single crystals as support for a noble metal in catalytic reactions. The article "Effect of the interface structure on the high-temperature morphology of supported metal clusters" H. Graoui, S. Giorgio, CR Henry - Philosophical Maqazine B, 2001, Vol.81, No.11, 1649-1658 also describes the use of micrometric magnesia (MgO) monocrystals of cubic or exponent planes of families {0,0,1} and {1,1,1} as a palladium support. Finally the article "Some recent results on metal / support interaction effects in NM / CeO2 catalysts" - S. Bernai, JJ Calvino, MA Cauqui, JM Gatica, JA Perez Omil, JM Pintado - Catalysis today 50 (1999) 175-206 describes the use in catalysis of single crystals of CeO 2, of cubic structure, as support platinum, palladium or rhodium. In particular, he studies the effect of temperature on the nature of the interactions between the support and the catalyst. On the other hand, the use of substantially spherical zirconia particles as a catalyst support is known. The article describes hydrothermal processes for obtaining nanometric zirconia powders whose particles have anisotropic forms. The outline of these particles is not smooth. No information is provided on the purity of the powders obtained. The article Synthesis of ZrO2 and TiO2 nanocrystalline powders by hydrothermal process - Material Science and Enqineerinq C23 (2003) p1033-1038 also describes, without mentioning catalysis, hydrothermal processes for obtaining titanium dioxide and zirconia powders. . The particles have rod and needle shapes. In addition, the particle sol obtained at the end of the hydrothermal processes is only centrifuged and washed with distilled water in a conventional manner, which makes it impossible to obtain a purity of the powder of less than 0.7%. .

SUMMARY AND OBJECT OF THE INVENTION An object of the invention is to provide a powder or a catalytic system which would make it possible to increase the efficiency of the catalysis reactions and a process making it possible to manufacture this powder or catalytic system industrially. According to the invention, this object is achieved by means of a process for the synthesis of a powder comprising the following successive steps: A) preparation of a mother liquor comprising: i. an oxide base component or a precursor of said base component and an additional component of formula MC, said additional component being composed of: - an agent M selected from the group of oxoanions, anions of the elements of column 17 of the periodic table elements, OH- hydroxide, and mixtures thereof, and - a complement C, this complement being inorganic when the agent is OH- hydroxide; or ii. an oxide base component or a precursor of said base component incorporating an agent M selected from the group of oxoanions, anions of the elements of column 17 of the periodic table of elements, and mixtures thereof; B) preparing, from said mother liquor and hydrothermally, a sol whose raw particles have a maximum size less than 100 nm; C) washing the soil so as to obtain, after a possible drying step, a powder comprising more than 99.3%, preferably more than 99.5%, more preferably more than 99.7%, preferably still more than 99.9%, of said basic constituent, in percentages by weight on the basis of dry matter. Preferably, the 100% complement consists of impurities.

Classically, the purity of the powders of particles with a maximum size of less than 100 nm produced by the hydrothermal route is limited. Indeed, an impurity content of less than 1% is not considered useful. However, the inventors have discovered that the powders consisting of primary particles having a maximum size of less than 100 nm and having an impurity content of less than 0.7% are particularly suitable as catalysts, or as catalysts for a catalyst in a catalytic system. More specifically, the inventors have discovered that the effectiveness of these powders decreases much more slowly over time when the content of impurities is lower, this efficiency being substantially identical immediately after manufacture irrespective of the purity. The powders manufactured according to a process according to the invention can thus have a better maintenance over time of the catalytic performances (reaction rate and / or selectivity) than that of the powders according to the prior art. The invention also relates to a powder manufactured according to a synthesis method according to the invention.

Stage C) preferably comprises at least one washing operation chosen from - filtration, with or without prior resuspension of the soil, - dialysis, with or without prior resuspension of the soil, - purification at using ion exchange resins, with or without re-suspending the soil, - rinsing, - acid-base neutralization, - and combinations of these techniques. If necessary, the washing operations should be repeated as many times as necessary until the desired purity is achieved. Acid-base neutralization is the preferred washing operation. It preferably comprises a suspension in an aqueous solution having a pH less than or greater than the zero charge point of said base component, depending on whether the impurities to be removed are cationic or anionic, and maintaining said suspension at a temperature below its temperature. boiling point at 1 bar for a period longer than 10 minutes, and filtration. The pH is preferably lower than or higher than said zero point of charge of at least 2 pH units, preferably, if possible, still less than or greater than said zero point of charge, of at least 4 pH units. Preferably still, the temperature is maintained less than 30 minutes. The pH can in particular be adjusted by adding hydrochloric acid and / or nitric acid and / or perchloric acid or by addition of ammonia (NH 4 OH). In one embodiment, step C) comprises at least one rinsing comprising a suspension of the product resulting from the preceding step in a solvent at a temperature below its boiling point at 1 bar for a duration greater than 10 minutes, then a filtration. The rinsing is preferably carried out in water having a degree of purity at least equal to that of the osmosis water or in an alcohol of purity greater than 98% by volume. The duration of the rinsing is preferably between 10 and 30 minutes. The implementation of a washing step C) allows the skilled person, possibly multiplying the aforementioned operations, to reach the required level of purity. The method according to the invention may also comprise one or more of the following steps: D) drying; E) calcining said powder so as to remove residual moisture and any organic species present in the particle powder; F) after step C) or step D) or step E), deagglomeration of the powder, in order to emit any agglomerates in the final powder.

A calcination step E) advantageously makes it possible to eliminate the residual moisture and to further improve the purity of the powder by eliminating the organic constituents. Preferably, the powder is calcined for at least 1 hour. More preferably, the calcination temperature is greater than 300 ° C and preferably less than 500 ° C. Advantageously, the proportions of the exposed crystalline planes are preserved. Preferably, in step A), the base component and / or the precursor of this base component comprise more than 99.5%, preferably more than 99.8%, more preferably more than 99.9% by weight. %, of said base component and / or said precursor of this basic component, respectively, in weight percent based on the dry matter. Advantageously, the purification is simplified. The selection of very pure raw materials to bring a basic constituent and / or a precursor of this basic constituent does not, to date, lead, by the hydrothermal route, to a powder with a low level of impurities according to the invention. invention without a washing step. In other words, it is still necessary that the method according to the invention comprises a washing step. In one embodiment, the base component and / or the precursor of the base component added to the mother liquor are chosen so that the basic constituent of the powder has, for more than 95%, preferably more than 95%. %, more preferably for more than 99%, more preferably for substantially 100%, a monoclinic structure, in weight percent based on dry matter. The base component or the precursor of the base component added to the mother liquor may in particular be chosen so that the base component is one of: HfO 2, ZrO 2, ThO 2, Eu 2 O 3, Sm 2 O 3, Fe 3 O 4, Bi 2 O 3, SnO 2, Nb 2 O 5, As203, MoO3, WO3, preferably from: HfO2, ZrO2, MoO3, WO3, more preferably from ZrO2 and / or HfO2, more preferably so that the base component is ZrO2 zirconia.

The pH of the mother liquor may be adjusted to be less than 4 if the basic constituent in question is chosen from HfO 2, ZrO 2, ThO 2, Eu 2 O 3, Sm 2 O 3, Fe 3 O 4, Bi 2 O 3 or less than 2, if the basic constituent under consideration is selected from HfO2, ZrO2, ThO2, EU203, Sm203, Fe3O4, Bi2O3, SnO2, Nb2O5, AS2O3, M003, WO3, OR greater than 10.

The method according to the invention may also comprise one or more of the following optional characteristics: in step A), the mother liquor may comprise at least 0.5% of the basic constituent of monoclinic structure, in particular of zirconia, and or a precursor of said basic constituent, in percentages by weight on the basis of the mother liquor. The precursor of the zirconia is a zirconium salt or a partially hydrolysed zirconium derivative, preferably a zirconium oxyhydroxide. The mother liquor comprises a solvent other than water, for example ethanol. - The content of said other solvent is less than 50% by weight of the liquid phase. In step A), the source of said base component of monoclinic structure and / or of said precursor of said basic component of monoclinic structure (that is to say the raw materials used to introduce said constituent and / or said precursor ) comprises more than 99.5%, preferably more than 99.8%, more preferably more than 99.9%, of said monoclinic structure base component and / or said precursor of said monoclinic structure base component, respectively as a percentage by weight on the basis of the dry matter. Advantageously, the purification is simplified. In step A), at least one soluble agent in an aqueous medium selected from the group of oxoanions, anions of the elements of column 17 of the periodic table of the elements, hydroxide OH- is added to the mother liquor. the pH of the mother liquor is basic, and their mixtures. In particular, the aqueous soluble agent may be selected from the group consisting of sulfate (SO42-), carbonate (CO32-), phosphate (PO43-), fluoride (F), chloride (Cr), perchlorate (CIO4), borate (BO33-), nitrate (NO3), hydroxide (OH-) and mixtures thereof or, if the pH is less than 4 in the group consisting of chloride (Cr), perchlorate (C1O4), nitrate (NO3), and mixtures thereof. - The agent content is greater than 10-4 mol / l, preferably greater than 10-3 mol / l, more preferably greater than 10-2 mol / l. The pH of the mother liquor is adjusted by means of an acid chosen from the group of organic and inorganic acids and their mixtures, or by means of a base chosen from the group of organic bases, inorganic bases and their mixtures. In one embodiment, the mother liquor is composed of water, agent (s), said basic constituent of monoclinic structure and / or said precursor of said basic constituent, other possible solvents, acid (s), ) or possible baseline (s), any surfactant (s) and any deflocculant (s). In step B), the mother liquor is heated in a closed vessel at a temperature above the boiling point at 1 bar, preferably between 100 ° C. and 300 ° C. - In step B), the rate of rise in temperature is less than 250 ° C / h, or even less than 200 ° C / h. In step B), the temperature is preferably maintained at least 1 hour and more preferably less than 200 hours. In step B), the pressure in the closed vessel, or reactor, is greater than or equal to the boiling pressure of the mother liquor. In step D), the drying temperature is less than 500 ° C., preferably less than 200 ° C. - In step D), the drying time is adjusted so that the particle powder has, at the end of this step, a residual moisture of less than 1% by weight. The process comprises an additional step subsequent to step C), in which the particles of the powder are doped by means of a dopant chosen from the group consisting of oxoanions, anions of column 17 and cations of the column. 1, and their mixtures. The dopant may in particular be chosen from silicates, phosphates, sulphates, chlorides, fluorides, sodium and potassium.

In the case where the pH of the mother liquor is less than 4 and where the agent is introduced in the form of an additive of formula MC, said additive is composed of: said agent M, and a complement C selected in the group consisting of all positively charged organic molecules whose organic radical consists of a group of atoms selected from the group consisting of carbon, hydrogen, oxygen, and nitrogen; cations of the elements of columns 1 and / or 2, preferably the cations of the elements of column 1; cations of the following elements: Aluminum (Al), Manganese (Mn), Iron (Fe), Yttrium (Y), Copper (Cu), Silver (Ag), Cerium (Ce); - Ammonium (NH4 +) In the case where the pH of the mother liquor is greater than 10 and where the agent is introduced in the form of an additive, said additive of formula MC is composed of: -dudit agent M and a complement C chosen from the group consisting of all the positively charged organic molecules whose organic radical is constituted by a group of atoms selected from the group consisting of carbon, hydrogen, oxygen, and nitrogen; - cations of the elements of column 1; - Ammonium (NH4 +) The parameters of the synthesis process according to the invention are preferably adjusted so that - the primary particles have maximum sizes greater than 10 nm and / or - the impurities are such that, in percentages of mass of dry matter: SiO 2 <0.1%, preferably SiO 2 <200 ppm, more preferably SiO 2 <100 ppm and / or - Al 2 O 3 <0.1%, preferably Al 2 O 3 <200 ppm, more preferably Al 2 O 3 <100 ppm and / or - MgO <0.1%, preferably MgO <200 ppm, preferably MgO <100 ppm, more preferably MgO <50 ppm and / or - CaO <0.1%, preferably CaO < 200 ppm, more preferably CaO <100 ppm and / or - Na2O <0.1%, preferably Na2O <500 ppm, preferably Na2O <200 ppm, more preferably Na2O <100 ppm and / or - Cl- <0 , 1%, preferably Cl- <500 ppm, preferably Cl- <200 ppm, more preferably Cl- <100 ppm and / or - NO3 <0.1%, preferably NO3 <500 ppm, preferably NO3 <200 ppm, more preferably NO3 <100 ppm and / or - SO42- <0.1%, preferably SO42- <500 ppm, preferably SO42- <200 ppm, more preferably SO42- <100 ppm and / or - Fe2O3 <0.1%, preferably Fe2O3 <200 ppm, preferably Fe2O3 <100 ppm, more preferably Fe2O3 <50 ppm and / or - TiO2 <0.1%, preferably TiO2 <200 ppm, preferably TiO 2 <100 ppm, more preferably TiO 2 <50 ppm. In a first particular embodiment especially for the manufacture of a powder according to the invention comprising zirconia particles having a rod-like shape and exposing a large amount of crystalline planes belonging to the {1,1,1} family , in particular more than 50% or even more than 80% and even more than 85% and / or less than 90% of plans belonging to the family {1,1,1}, in percentages on the basis of the area At total exposure, step A) has the following characteristics: The pH of the mother liquor is less than 4. The mother liquor then comprises a modifying agent selected from the group consisting of oxoanions, anions of the elements of column 17. (Halides) and mixtures thereof, preferably in the group consisting of chloride (Cr), perchlorate (ClO4), nitrate (NO3) and mixtures thereof. In the case where the modifying agent is introduced in the form of an additive of formula MC, this additive is preferably composed of: said modifying agent M, and a complement C chosen from the group consisting of set of positively charged organic molecules whose organic radical consists of a group of atoms selected from the group consisting of carbon, hydrogen, oxygen, and nitrogen; cations of the elements of columns 1 and 2, preferably the cations of the elements of column 1; cations of the following elements: Aluminum (Al), Manganese (Mn), Iron (Fe), Yttrium (Y), Copper (Cu), Silver (Ag), Cerium (Ce); - Ammonium (NH4 +) - More preferably, the concentration of modifying agent in the mother liquor is between 10-4 mol / l and 10 mol / l.

In a second particular embodiment particularly intended for the manufacture of a powder according to the invention comprising particles of zirconia having a rod-like shape and exposing a large quantity of crystalline planes belonging to the {1,1,0} family , in particular more than 35%, or even more than 50% and / or less than 70% of plans belonging to the family {1,1,0}, in percentages based on the total exposed area, step A) has the following characteristics: - The pH of the mother liquor is greater than 10. - The mother liquor comprises a modifying agent chosen from the group consisting of oxoanions except borate (BO33-), carbonate (CO32-), nitrate ( NO 3), perchlorate (CIO 4), and mixtures thereof; preferably selected from the group of sulfate (SO42-), phosphate (PO43-) and mixtures thereof. In the case where the modifying agent is introduced in the form of an additive, this additive, of formula MC, is preferably composed of: said modifying agent M and a complement C chosen from the group consisting of all the positively charged organic molecules whose organic radical consists of a group of atoms selected from the group consisting of carbon, hydrogen, oxygen, and nitrogen; - cations of the elements of column 1; - Ammonium (NH4 +) - The concentration of modifying agent in the mother liquor is greater than 10 -10 mol / l, with a pH between 10 and 14.

In a third particular embodiment particularly intended for the manufacture of a powder according to the invention comprising zirconia particles having a platelet-shaped and exposing a large amount of crystalline planes belonging to the family {1,0,0}, in particular more than 35% or more than 50% and / or less than 60% of plans belonging to the family {1,0,0}, in percentages based on the total exposed area; following characteristics: - The pH of the mother liquor is greater than 10. - The mother liquor comprises a modifying agent selected from the group consisting of oxoanions, anions of the elements of column 17 (halides), hydroxide (OH- ) and mixtures thereof, preferably selected from the group consisting of hydroxide (OH-), carbonate (CO32-), fluoride (F), chloride (Cr), perchlorate (C1O4), borate (BO33) -), nitrate (NO3) and their mixtures. More preferably, the modifying agent is hydroxide (OH-). In the case where the modifying agent is introduced in the form of an additive, this additive, of formula MC, is preferably composed of: said modifying agent M and a complement C chosen from the group consisting of all of the positively charged organic molecules whose organic radical consists of a group of atoms selected from the group consisting of carbon, hydrogen, oxygen, and nitrogen; - cations of the elements of column 1; Preferably, the concentration of modifying agent in the mother liquor is less than 10 mol / l if the modifying agent is chosen from the group of borate (BO33-), carbonate ( CO32-), nitrate (NO3), perchlorate (CI04), hydroxide (OH-), anions of column 17 (halides), and mixtures thereof. More preferably, the modifying agent concentration is less than 10-3 mol / l if the modifying agent is selected from the group of oxoanions except borate (BO33-), carbonate (CO32-), nitrate (NO3 ), perchlorate (C1O4), and mixtures thereof. In general, the invention also relates to a powder synthesized according to a synthetic process according to the invention. The particular embodiments of the process according to the invention which have just been described make it possible to manufacture powders according to the invention, the primary particles of which have the shape of a rod which exhibits, for more than 35%, preferably more than 50%, preferably more than 80%, preferably more than 85% and, preferably, less than 90%, of the {1,1,1} families, the primary particles then being referred to as rods {1,1,1}, and / or - the shape of an exposing wafer, for more than 35%, preferably more than 50%, and preferably less than 60%, of the families belonging to the family { 1.0.0}, the primary particles then being called platelets {1.0.0}, and / or - the shape of an exposing rod, for more than 35%, preferably more than 50%, and , preferably, less than 70%, of the families belonging to the family {1,1,0}, the primary particles then being called rods {1,1,0} As percentages based on the total exposed area. These primary particles advantageously expose a high percentage of so-called rare planes, ie planes other than the most thermodynamically stable plane, and in particular a high percentage of {1,1,0} and {1,1} family planes. 1,0,0}. These crystalline planes are useful for catalyzing or supporting catalysts to expose crystalline planes to which the reaction to be catalyzed is sensitive. Whatever their morphology, the primary particles of the powder according to the invention preferably exhibit for at least 90%, preferably at least 95%, preferably for substantially 100%, family plans {1,1,0} , {1,1,1} and {1,0,0}. The particles of the powder may be primary particles. They may also occur, at least in part, as secondary particles. The powder according to the invention is in particular intended to serve as catalyst or support for a catalyst to form a catalyst system according to the invention. The powder according to the invention can thus itself serve as a catalyst, the particles being uncoated with a catalyst, especially for catalyzing a reaction sensitive to families of {1,1,0}, {1,0,0} and {1,1,0}. The particles can also be used coated with a catalyst, forming a catalytic system, in particular to catalyze a reaction sensitive to families of {1,1,0} and {1,1,1} planes.

The invention also relates to a catalyst system comprising a powder of support particles to which crystallites of a catalyst adhere, said powder being in accordance with the invention. The inventors have in fact discovered that this catalytic system advantageously makes it possible to exhibit a high percentage of so-called rare planes, that is to say of planes other than the most thermodynamically stable plane, and in particular in the case of a catalyst selected from the metals of columns 8, 9 and 10 and of cubic structure, a high percentage of {1,0,0} family planes. Catalyst systems comprising platelet-like primary particles predominantly showing {1,0,0} family planes are particularly advantageous if the catalyst is selected from the metals of columns 8, 9 and 10 and of cubic structure.

The catalytic system according to the invention may have one or more of the following characteristics: Preferably, the primary particles are monocrystalline. The catalyst deposited on the surface of the support may be a metal, preferably chosen from the group of metals of columns 8, 9 and 10 of the periodic table of elements, preferably chosen from the group of metals of columns 8, 9 and 10 of the periodic table of elements and having a cubic structure, and mixtures thereof. Preferably, the metal is platinum. The catalyst may also be an oxide, preferably chosen from lanthanum oxide and / or transition metal oxides, for example V 2 O 5 or Cr 2 O 3. and / or the oxides of the elements of the columns 14 and 15, preferably the oxides of tin (Sn), lead (Pb) and / or bismuth (Bi). The catalyst may also be a carbide, preferably selected from transition metal carbides, such as, for example, molybdenum carbide and / or tungsten carbide.

The catalyst may also be a sulphide, preferably selected from transition metal sulphides, preferably molybdenum sulphides and tungsten sulphides, optionally doped with cobalt or nickel (CoMoS for example). Preferably, the catalyst is a metal selected from the group of metals of columns 8, 9 and 10 of the periodic table and is chosen so that at least 80%, preferably at least 90%, more preferably at least 95% by number, more preferably substantially all of the catalyst crystallites have a truncated cuboid or octahedron shaped raft. With a metal of cubic structure of the columns 8, 9 or 10, the size of the crystallites is preferably between 1.5 nm and 10 nm, preferably between 3 nm and 10 nm. Advantageously, it is thus possible to obtain mainly crystallites of said metal, in particular platinum, which is mainly in one or both of the truncated cubo-octahedral and raft forms. With a cubic metal structure of columns 8, 9 or 10, the truncated cubo-octahedron crystallites are formed on the {1,1,0} and {1,1,1} exposed planes of the support particles, the crystallites in form of raft forming on the exposed planes {1,0,0} of these particles, in particular when said particles have a monoclinic structure.

Preferably, the catalyst is selected so that at least 80%, preferably at least 90%, more preferably at least 95% by number, more preferably substantially all of the catalyst crystallites of the catalyst system have a shape which is not a spherical or spherical shape truncated according to the contact plane with the support. Advantageously, the performance of the catalytic system is improved. The size of the crystallites of the catalyst is preferably between 1.5 nm and 10 nm, preferably between 3 nm and 10 nm. Preferably, the catalyst is less than 10%, preferably less than 7%, more preferably less than 5% of the mass of the catalyst system. A content of 1% of catalyst is generally suitable. Powder containing rods predominantly showing {1,1,0} family planes and platelets with mainly {1,0,0} family planes are particularly advantageous.

The invention therefore also relates to a process using a powder according to the invention for catalyzing a chemical reaction. In particular, when the morphology of the particles of a powder according to the invention is not spherical, for example because the particles are rods or platelets, a powder according to the invention can be advantageously used to catalyze a reaction sensitive to the structure. The invention therefore also relates to a process using a powder according to the invention, the primary particles not being coated with catalyst, for catalyzing a reaction chosen from hydrocarbon conversion reactions including selective oxidation reactions, reactions of hydrogenation, dehydrogenation reactions, hydrogenolysis reactions, isomerization reactions, dehydrocyclization reactions, and reforming reactions, or, after at least partial recovery, of said primary particles with a catalyst, - when the catalyst is selected from the group consisting of the metals of columns 8, 9 and 10 of the periodic table of the elements and exhibiting a cubic structure, and mixtures thereof, and in particular when the basic constituent of the support particles is zirconia, to catalyze a reaction selected from the following group: a. a response where more than 26%, or even more than 35%, or more than 51% of exposed shots belong to the {1,0,0} family and / or more than 35%, or even more than 45%, or more than 66% or more than 70% of the exposed planes belong to the family {1,1,1}, in percentages based on the total exposed surface of the catalyst, are sought and / or, b. a reaction selected from hydrocarbon conversion reactions including selective oxidation reactions, hydrogenation reactions, dehydrogenation reactions, hydrogenolysis reactions, isomerization reactions, dehydrocyclization reactions, and reaction reactions. reforming, c. selective hydrogenation reactions, and particularly selective hydrogenation reactions of molecules containing at least one carbonyl function CO, and reactions of selective hydrogenation of molecules containing at least two double bonds; d. methane formation reactions (methanation reactions); e. Fischer-Tropsch synthesis reactions (including methanol synthesis reactions), ie the formation of oxygenated hydrocarbons from carbon monoxide (CO), hydrogen (H2), and / or organic molecules (biomass for example) ; regardless of the catalyst, provided that the base component of the carrier particles is monoclinic, to catalyze a reaction selected from hydrocarbon conversion reactions including selective oxidation reactions, hydrogenation reactions, dehydrogenation reactions , hydrogenolysis reactions, isomerization reactions, dehydrocyclization reactions, and reforming reactions.

In particular, when the catalyst is a metal of cubic structure of columns 8, 9 and 10, the powders and catalytic systems according to the invention comprise primary particles whose base constituent is selected from the group formed by zirconia (ZrO 2 ), hafnium oxide (HfO2), a mixture of zirconia and hafnium oxide, molybdenum oxide and tungsten oxide are well suited to the catalysis of a plan-sensitive reaction. (s) crystalline (s) exposed families {1,1,1} and / or {1,0,0}.

The catalytic system according to the invention is also particularly well suited to catalyze a reaction taking place at a temperature below 500 ° C. When the reaction takes place at a temperature below 500 ° C, the maximum size of the primary support particles can be advantageously kept below 100 nm, and the size of the catalyst crystallites carried by the support can be kept below 10 nm. In addition, the crystallites can, throughout the reaction, expose other crystalline planes than those of the family of thermodynamically thermodynamically stable crystalline planes. In the particular case where the catalyst is a cubic metal of columns 8, 9 and 10, the family of thermodynamically most stable planes in temperature is the family {1,1,1}.

DEFINITIONS A catalytic system is a set consisting of a support, in the form of powder, and a catalyst fixed on said support.

For the sake of clarity, the term crystallite is used herein to refer to a catalyst crystal that has grown on a support and the term particle to refer to a support powder. Structurally, however, a support primary particle may be a crystal. For convenience, the term crystal is used to denote either a primary carrier particle or a crystallite. Conventionally, when a particle or a crystallite is constituted by a homogeneous single crystal whose lattice planes have a uniform orientation throughout its volume, the particle or crystallite is said to be monocrystalline. A basic constituent of a particle is said to have a monoclinic structure when at least 90% of its volume is of monoclinic structure. A particle is said to have a smooth contour when, in an observation plane making it possible to visualize, at a scale of approximately 10 nm / cm, a top view of the particle, the line of sight being thus substantially according to the length of the particle, the edges defining the contour of the particle do not have or few steps (recesses) and / or corners (asperities). For example, in the case of a rod having a hexagonal cross section, this means that each of the 6 sides of this section is substantially rectilinear. A smooth outline may also be characterized by a proportion of exposed planes belonging to families other than the families {1,1,1}, {1,1,0} and {1,0,0} less than 10% of the total area of the exposed planes. Preferably, according to the invention, this proportion is less than 5%, preferably substantially zero. A powder is a set of particles. These particles may be primary, that is to say, not associated with other particles, or secondary, that is to say constituted by agglomerates or aggregates of primary particles. In an agglomerate, the association of the primary particles results from a low intensity bond, for example by charge effects or by polarity. In an aggregate, the association between the primary particles is stronger than in the case of an association in the form of agglomerates. For example, the primary particles can be chemically bonded to one another. Breakage of agglomerates into smaller agglomerates or primary particles is therefore easier than breakage of aggregates into smaller aggregates or primary particles. In the present description, by support is meant a support in the form of a powder. The basic component of a carrier particle is the major constituent, preferably greater than 90%, preferably more than 99%, by weight of this particle. Preferably, the carrier particles of the invention consist of a base component and impurities.

An agent which modifies an aqueous-soluble, chemically-interacting constituent of the mother liquor with the oxide of the solid element forming the base component, or the precursor of this oxide present in the mother liquor, is strong enough to its presence or absence in the mother liquor may (especially depending on its quantity and the pH of the mother liquor) influence the morphology of the particles obtained after the hydrothermal treatment. The modifying nature of an agent thus depends on the nature, or even the crystallographic structure, of the oxide of the solid element or of the precursor of this oxide, the concentration of the agent in the mother liquor and the pH of the the latter. For example, the agents selected from the group of oxoanions, anions of the elements of the column 17 of the periodic table of the elements, the hydroxide OH-, and their mixtures, are modifying only if the oxide of the element solid or the precursor of this oxide are of monoclinic structure and in the areas of concentration and pH described below. The maximum size of a primary particle of a powder is defined by its largest dimension measured in an observation plane generally produced by electron microscopy. The maximum size of a set (or powder) of primary particles corresponds to the maximum size of the particle of this set having the largest maximum size. The size of catalyst crystallites deposited on the surface of the support particles is defined by their average surface area. It can be determined from the dispersion value of this catalyst, measured in particular by hydrogen chemisorption, when the catalyst belongs to the group of metals of cubic structure of columns 8, 9 and 10 or by measurement from observations of electron microscopy, especially when the catalyst belongs to the group of carbides, sulphides and oxides identified as catalysts, described in more detail below.

The percentiles or percentiles 50 (D50), and 90 (D90) are the particle sizes of a powder corresponding to percentages by weight, 50% and 90%, respectively, on the cumulative particle size distribution curve of the particles of the powder, the particle sizes being ranked in ascending order. For example, 90% by weight of the particles of the powder are smaller than D90 and 10% of the particles by weight are larger than D90. Percentiles can be determined using a particle size distribution using a sedigraph. D50 corresponds to the median size of a set of particles, that is to say the size dividing the particles of this set into first and second populations equal in mass, these first and second populations comprising only particles having a size upper or lower respectively to the median size. By size of a particle of a powder, the size of particles determined by a sedigraphic analysis carried out to characterize a particle size distribution is conventionally understood. Sedigraphy can for example be performed using a sedigraph Sedigraph 5100 Micromeritics company. A family of crystalline planes or crystalline form, denoted {h, k, l}, is classically called a set of planes presenting a particular atomic arrangement, that is to say a specific arrangement of specific atoms. This definition therefore excludes atomic arrangements extending along a curved surface. The plane of the family {h, k, l}, characterized by a particular orientation defined by its Miller indices (h, k, l), is called a crystalline plane, conventionally designated by its Miller indices (h, k, l). . The family of crystalline planes {h, k, l} thus comprises the set of planes (h, k, l), (h, l, k), (k, h, l), (k, l, h) , (I, h, k), (I, k, h) as well as the combinations obtained by changing the sign of one, two or three Miller indices. For example, the family of crystal planes {1,0,0} is the set of crystal planes (1,0,0), (0,1,0), (0,0,1), (-1, 0.0), (0 -1, 0) and (0.0, -1). An exposed crystal plane of a crystal is a crystalline plane in contact with the outside, unlike a plane attached to a support for example. Only primary particles are considered when reference is made to an exposed crystalline plane of a particle. When reference is made to carrier particles in a catalytic system, the exposed planes are considered to include the at least partially covered planes of catalyst. In other words, to evaluate the percentages of crystalline planes exposed by particles, the presence of the catalyst on the surface of these particles is not taken into consideration. A family of crystalline planes exposed {h, k, l} is the set of exposed crystalline planes of said family. For example, the family of exposed crystalline planes {1,0,0} is the set of exposed crystalline planes, and only these, among the crystalline planes (1,0,0), (0,1,0), (0,0,1), (-1,0,0), (0, -1,0) and (0,0, -1). An exposed crystalline plane is said to be favorable when its contact with reactants promotes a chemical reaction.

A family of planes is said to be favorable when it includes at least one favorable crystalline plane. A family of planes is said to be the most favorable when it has the most favorable crystalline plane.

The growth axis is the main direction of growth of a crystal. The growth axis is not necessarily related to the exposed planes of the crystal. A primary particle has a rod shape when it meets the following three conditions: 1) 1 <L / W 5 10, where L is the length of the primary particle, i.e. its largest dimension, and W = (W1 + W2) / 2, where W1 and W2 are the respective lengths of the smallest axis and the longest axis of the smallest possible ellipse in which a cross-section of the primary particle can be written is ie a section perpendicular to the direction of the length of the primary particle, and 2) W2 <1.5W1, and 3) the cross section is substantially constant over the entire length of the rod, substantially polygonal and has at least 4 sides. The cross section of a rod primary particle according to the invention preferably has less than 8 sides. More preferably, the primary rod-shaped particles are such that 1.5 5 L / W 5 5, preferably 25 L / W 4. FIG. 1 is a diagram of a primary particle in the form of of a stick. A primary particle has a platelet form when it meets the following three conditions: 1) L / W1 5 10 and L / W2 5 5, where L is the length of the primary particle, i.e. to say its largest dimension, and W1 and W2 are the respective lengths of the smallest axis and the largest axis of the smallest possible ellipse in which may be inscribed a cross-section of the primary particle, that is, ie, a section perpendicular to the direction of the length of the primary particle, and 2) W2> 1.5 W1, preferably W2> 2 W1, and 3) the cross section is substantially constant over the entire length of the plate, is substantially polygonal and has at least 4 sides.20 The cross section of a wafer particle according to the invention preferably has less than 8 sides. More preferably, the wafer-shaped primary particles are such that LWL 581 and LW254.

Figure 2 shows the schematic of a primary particle in the form of a wafer. Particles in the form of platelets or rods are therefore distinguished in particular spherical shapes, fibrous, son, filaments, needles or cubes.

A raft corresponds to a flat cross-sectional shape of substantially constant general shape over its entire thickness (for example always octahedral whatever the cross sectional plane considered). More specifically, a crystallite has a raft shape when it meets the following four conditions: 1) W / E 5 10 where E denotes the crystallite thickness, i.e., its smallest dimension, and W = (W1 + W2) / 2 where W1 and W2 are the respective lengths of the smallest axis and the largest axis of the smallest possible ellipse in which a transverse section of the crystallite may be inscribed, that is, -describe a section perpendicular to the direction of the crystallite thickness, and 2) W2 <1.5 W1, preferably W2 <1.2 W1, and 3) the thickness is substantially constant, and 4) the cross section raft-shaped crystallite is substantially polygonal and has at least 4 sides. Preferably, it comprises less than 8 sides.

More preferably, the raft-shaped crystallites are such that 45W / E56. Figure 7 shows a schematic of a crystallite in the form of a raft.

A cubo-octahedron is an isotropic form, that is to say having no preferred growth axis, and facet, the number of faces being greater than 14 and preferably less than 26. A truncated cubo-octahedron is a cubo-octahedron that is not whole, but cut along a contact plane with a support. Figures 6a and 6b show photographic snapshots and diagrams of a crystallite in the form of a truncated cubo-octahedron, in top and side views, respectively. Oxoanion is an oxide-containing anion, of the form QOX n-Q being a metal (eg silicon) or a non-metal (eg carbon, phosphorus, sulfur), where n is a higher integer or equal to 1 and x being equal to (n + w) / 2, with w the valence of the metal or non-metal considered. By impurities is meant the inevitable constituents introduced involuntarily and necessarily with the raw materials or resulting from reactions with these constituents. Impurities are not necessary constituents, but only tolerated. For example, the compounds forming part of the group of oxides, nitrides, oxynitrides, carbides, oxycarbides, carbonitrides and metallic species of sodium and other alkalis, iron, vanadium and chromium are impurities if their presence is not desired. Hafnium oxide is not considered an impurity when the desired product is zirconia or a product based on zirconia and hafnium oxide. Similarly, MoO2 and MoO3 molybdenum oxides are not considered impurities if MoO3 is the basic constituent of the particles. Finally, in the same way, the tungsten oxides WO2 and WO3 are not considered as impurities if WO3 is the basic constituent of the particles. In a powder according to the invention, the impurities may be incorporated into the primary particles of the powder according to the invention or may constitute independent particles. In the present description, these two types of impurities are not distinguished. By definition, a hydrothermal process is a well-known method of heating an aqueous solution, in a closed vessel, to a temperature above the boiling point at atmospheric pressure and at a pressure equal to or greater than the minimum required pressure. to avoid boiling the aqueous solution.

The term "sol" conventionally refers to a dispersion or suspension of colloidal particles in a liquid phase (for example in an aqueous medium). Particles in a soil are usually little or not agglomerated. The zero load point, PCN, is defined as follows. The surface charge of an oxide in suspension, 6, results from acid-base equilibria. It therefore depends on the pH and the ionic strength of the solution. 6 can be positive, negative or zero depending on the conditions of the environment. The zero point of charge defines the pH of the medium for which the charge 6 vanishes. At pH <PCN, the charge 6 is positive. At pH> PCN, 6 is negative. The value of the NCP is directly related to the nature of the oxide.

The hourly volume velocity of a gas, denoted VVH, is defined as the volume of gas passing per unit volume of the reactor and per unit of time. It is expressed in h-1. A reaction is said to be sensitive to the structure when it is modified not only according to the nature of the catalyst and the surface area of the catalyst accessible to the reagents, but also depending on the morphology and the exposed crystalline planes of the catalyst. Unless otherwise mentioned, the percentages used to characterize the proportion of exposed plans always refer to percentages related to the total area of the exposed planes expressed in m2.

Unless otherwise stated, the percentages used to characterize a composition always refer to mass percentages on the basis of dry matter. When a column is referenced to identify elements, it is implicitly referred to as a column of the periodic table of elements. BRIEF DESCRIPTION OF THE FIGURES Other features and advantages of the invention will become apparent upon reading the following detailed description and examining the accompanying drawing in which Figure 1 shows a schematic diagram of a primary particle. in the form of a stick, FIG. 2 is a diagram of a primary particle in the form of a wafer, FIGS. 3a and 3b are photographs of a wand in plan view and in view of side, respectively, Figure 3a being particularly representative of a rod having a smooth contour; Figures 4a and 4b show photographic snapshots of a set of platelets and a wafer, respectively, predominantly exposing the family {1,0,0}; FIGS. 5a and 5b show photographic snapshots of sticks predominantly showing the family {1,1,1}. Figs. 6a and 6b show photographic snapshots and diagrams of a crystallite in the form of a truncated cubo-octahedron, in top and side views, respectively; Figure 7 shows a schematic of a crystallite in the form of a raft.

DETAILED DESCRIPTION The above-mentioned steps A) to F) are now described in detail. Step A) In step A), a stock solution or an aqueous mother suspension, hereinafter called the mother liquor, containing the oxide of the solid element to be manufactured or a precursor of this oxide is prepared. This solution is determined according to the parameters of step B) in order to obtain the desired primary particles. Conventionally, the mother liquor typically contains a solid content (oxide of the element or precursor of this oxide) of between 0.5 and 40% by weight.

The synthesis method may in particular be used to manufacture a monoclinic oxide, in particular chosen from: HfO 2, ZrO 2, ThO 2, Eu 2 O 3, Sm 2 O 3, Fe 3 O 4, Bi 2 O 3, SnO 2, Nb 2 O 5, As 2 O 3, MoO 3, WO 3. Preferably the monoclinic oxide is selected from HfO2, ZrO2, MoO3, WO3, preferably from ZrO2 and HfO2 and mixtures thereof. Preferably, the monoclinic oxide is ZrO 2 zirconia.

The mother liquor may contain the monoclinic oxide (s) to be manufactured. It may also contain a precursor of these oxides. The liquid phase of the mother liquor may be water. However, other water-miscible solvents, for example ethanol, may be included in the liquid phase. Preferably, the content of other solvents is less than 50% by weight of the liquid phase. The aqueous mother liquor also comprises at least one additional component of formula MC, said additional constituent being composed of: an agent M chosen from the group of oxoanions, anions of the elements of column 17 of the periodic table of elements, OH-hydroxide, and mixtures thereof, and a complement C, this complement being inorganic when the agent is OH- hydroxide.

The pH of the mother liquor can be adjusted by adding organic and inorganic acids and / or bases. When the agent is a modifying agent, the pH of the mother liquor, the nature and the amount of the modifying agent influence the morphology as well as the nature and the proportion of the exposed planes of the finally obtained elementary particles. Step B) In step B), the mother liquor is heated in a closed vessel at a temperature above the boiling point to 1 bar, preferably between 100 ° C and 300 ° C, and at a temperature above pressure equal to or greater than the boiling pressure of the mother liquor. Advantageously, this method makes it possible to obtain directly the primary particles of the oxide of the element under consideration. The hydrothermal treatment can be carried out in a batch reactor, called batch batch, or in a continuous reactor. Residence times are generally shorter and temperatures are generally higher in a continuous reactor than in a batch reactor. The reactor is heated to the desired temperature, then the temperature is maintained at least 1 hour and preferably less than 200 hours. The holding time of the temperature may vary depending on the reactor temperature and the concentrations in the mother liquor. It can be determined by routine tests. The choice of the temperature / hold time pair also influences the size of the primary particles. This pair is therefore chosen so as to obtain a maximum primary particle size of less than 100 nm.

The pressure in the reactor is greater than or equal to the boiling pressure of the mother liquor. It can be autogenous, that is to say, correspond to the vapor pressure of the water at the reactor temperature, hydraulic, or can result from the addition of an inert gas such as nitrogen. The pressure is chosen so that the final product obtained is in the desired crystalline phase. The reactor is then cooled to a temperature below the boiling point at 1 bar. The soil obtained is isolated. It contains a liquid fraction and crude particles which have a high content of impurities. The inventors have discovered that these impurities adversely affect the efficiency of the particles whether they are used as a catalyst support or as a catalyst. Step C) Steps A) and B) are known for hydrothermally producing raw particles.

According to the invention, the solid phase of the sol consisting of raw particles is purified, during step C), so that its content of impurities is less than 0.7%, preferably less than 0.5%, of preferably less than 0.3%, more preferably less than 0.1%, in percentages by mass of dry matter. The purification of the raw particles from the soil preferably comprises the following steps C1 to C3:

Step C1, optional, consists in decreasing the amount of the liquid phase and / or purifying said liquid phase of the soil. By reducing the amount of the liquid phase of the soil is meant any operation that makes it possible to eliminate part or all of the liquid phase of said soil. These methods are well known to those skilled in the art and may include - filtration, with or without prior resuspension of the soil, - dialysis, with or without prior resuspension of the soil, - purification with the aid of ion exchange resins, with or without prior resuspension of the soil, - rinsing, - and combinations of these techniques. The term "purification of the liquid phase of the soil" is understood to mean any operation making it possible to reduce the concentration of ionic species in the liquid solution of the soil. These methods are well known to those skilled in the art and may include dialysis, purification methods using ion exchange resins or dilution rinsing. Step C1 does not achieve with the usual procedures the purity levels desired in the final powder. It is therefore preferable to apply a C2 step and / or an additional purification step C3.

Step C2 consists of the removal of cationic impurities. Cationic impurity, a molecule containing elements other than carbon (C), hydrogen (H), oxygen (O) and nitrogen (N), positively charged and contained in the liquid phase of the soil and / or fixed on the raw particles of the ground. The liquid phase of the soil and the raw particles of the soil may not contain cationic impurities, for example in the case of the use of hydrochloric acid (HCI). Step C2 is then useless.

The product resulting from the preceding process step (step B) or step C1)) is suspended in an aqueous solution having a pH below the zero point of charge (PCN) of the oxide of the final element under consideration. Preferably, this pH is less than 2 pH units to said PCN, more preferably less than 4 pH units to said PCN.

The PCN values of the principal oxides, as well as a method of determination, can be consulted for example in the pages 255 to 308 of the book De la Solution à l'Oxyde, J.-P. Jolivet, CNRS Editions, Paris (1994). The pH of the suspension is regulated by addition of organic and / or inorganic acid. Of the acids, nitric acid and / or perchloric acid are preferred.

This suspension is maintained, preferably with stirring, at a temperature below its boiling point at 1 bar, preferably at room temperature, for a period preferably of greater than 10 minutes and preferably less than 30 minutes. This suspension is then filtered by any means known to those skilled in the art. It is possible to repeat step C2 several times if the desired cationic species purity requires it.

Step C3 consists of the removal of anionic impurities. An anionic impurity is a molecule containing elements other than carbon (C), hydrogen (H), oxygen (O) and nitrogen (N), negatively charged and contained in the liquid phase of the soil and / or bound to the crude particles of the soil. ground.

The liquid phase of the soil and the raw particles of the soil may not contain anionic impurities, for example in the case of the use of ammonia (NH4OH). Step C3 is then useless. The product resulting from the preceding process step (B) or C1 or C2) is suspended in an aqueous solution having a pH greater than the zero point of charge of the oxide of the final element under consideration. Preferably, this pH is greater than 2 pH units to said PCN, more preferably 4 pH units higher than said PCN, if the PCN is less than 10. The pH of the suspension is regulated by adding an organic base and / or or inorganic. Among these bases, it is preferred to use ammonia NH 4 OH.

This suspension is maintained, preferably with stirring, at a temperature below its boiling point at 1 bar, preferably at room temperature, for a period preferably of greater than 10 minutes and preferably less than 30 minutes. This suspension is then filtered by any means known to those skilled in the art. It is possible to repeat step C3 several times if the purity required anionic species requires. It is possible to alternate or reverse steps C2 and C3. Since the crude particles always have at least cationic or anionic impurities, at least one of C2 and C3 is carried out to reduce the impurity content to less than 0.7%. Between each step C1, C2 and / or C3, an optional C4 rinse in a solvent, preferably in water having a degree of purity at least equal to that of the osmosis water or in an alcohol of purity greater than 98 % by volume, preferably greater than 99% by volume, can be achieved. This operation consists of suspending the product from the preceding step in this solvent, preferably with stirring, at a temperature below its boiling point at 1 bar, preferably at room temperature, for a period of preferably greater than 10 minutes and preferably less than 30 minutes.

This suspension is then filtered by any means known to those skilled in the art.

At the end of step C), the product obtained consists of moist purified particles. Step D) In step D), after purification, the liquid fraction is removed by drying. A purified particle powder is thus obtained according to the invention. Any method well known to those skilled in the art can be used. The drying temperature is generally less than 500 ° C, preferably less than 200 ° C. The drying time is adjusted so that the powder of purified particles has, at the end of this step, a residual moisture of less than 1% by weight. Step E) During an optional step E), the powder is calcined, preferably for at least one hour. The calcination temperature is preferably greater than 300 ° C. and preferably less than 500 ° C. in order to avoid the transformation of the exposed planes towards the thermodynamically most stable plane. This step E) makes it possible to eliminate the residual moisture and any organic species present in the powder of purified particles obtained at the end of step D).

A final deagglomeration step may also be added after step E) or step D), in order to emit any agglomerates in the final powder.

Particular Process for the Manufacture of Zirconia Particles The synthesis process according to the invention described above (stages A) to E)) is advantageously used to manufacture a powder of primary particles according to the invention. In the case of zirconia, the process then has the following particulars: Step A) The hydrothermal preparation of a sol of particles smaller than 100 nm (steps A) and B)) is a known technique of skilled in the art described for example in the book De la Solution with Oxide, J.-P. Jolivet, CNRS Editions, Paris (1994).

In step A), a stock solution or an aqueous mother slurry, hereinafter called mother liquor, containing zirconia or a precursor of zirconia is prepared. This solution is determined according to the parameters of step B) in order to obtain the desired primary particles. The mother liquor contains at least 0.5% zirconia or an equivalent amount of precursor, in percentages by weight based on the mother liquor. The precursor of the zirconia may be a zirconium salt or a partially hydrolysed zirconium derivative. Preferably, this precursor is a zirconium oxyhydroxide, also called hydrated zirconia or zirconium hydroxide, without being limited thereto. This compound can be obtained by precipitation of zirconium salts in basic or neutral medium. Suitable zirconium oxyhydroxide powders are commercially available, for example, from the company SEPR Saint Gobain ZirPro (France) under the trade designation ZHO. These powders have a median diameter of 20 microns.

The liquid phase of the mother liquor may be water. However, other water-miscible solvents, for example ethanol, may be included in the liquid phase. Preferably, the solvent content is less than 50% by weight of the liquid phase. The aqueous mother liquor may comprise a modifying agent.

The agent may be added as an additive or provided by the precursor of the zirconia. The aqueous soluble agent may be selected from the group of oxoanions, anions of the elements of column 17 (halides) and hydroxide (OH-) and mixtures thereof, preferably from the group consisting of sulfate ( SO42-), carbonate (CO32-), phosphate (PO43-), fluoride (F), chloride (Cr), perchlorate (CI04), nitrate (NO3), borate (BO33-), hydroxide (OH-) and their mixtures. The agent can be corrosive and therefore requires suitable equipment.

The pH of the mother liquor, the nature and amount of the modifying agent influence the morphology as well as the nature and the proportion of the exposed planes of the finally obtained elementary particles. Surprisingly, the inventors have discovered that mother liquor pHs of between 4 and 10 do not make it possible to obtain primary particles in platelet or rod form according to the invention, whatever the preparation conditions used. The pH of the mother liquor can be adjusted by adding organic and inorganic acids and / or bases.

To manufacture zirconia particles of the invention having a rod shape and exhibiting a large amount of crystalline planes belonging to the family {1,1,1}, especially more than 50% or more than 80% and even more than 85%, and less than 90%, of {1,1,1} planes, the pH of the mother liquor is made acidic and less than 4.

The modifying agents used are then preferably chosen from the group consisting of oxoanions, anions of the elements of column 17 (halides) and their mixtures, preferably from the group consisting of chloride (Cr), perchlorate (ClO 4) and nitrate (NO3) and their mixtures In the case where the modifying agent is introduced in the form of an additive of formula MC, this additive is preferably composed of: - said modifying agent M and - a complement C selected in the group consisting of all positively charged organic molecules whose organic radical is constituted by a group of atoms selected from the group consisting of carbon, hydrogen, oxygen, and nitrogen; cations of the elements of columns 1 and 2, preferably the cations of the elements of column 1; cations of the following elements: Aluminum (Al), Manganese (Mn), Iron (Fe), Yttrium (Y), Copper (Cu), Silver (Ag), Cerium (Ce); - Ammonium (NH4 +) The proportion of exposed crystal planes belonging to the {1,1,1} family is regulated by the pH and by the amount of the modifying agent. For maximum sizes greater than 10 nm, the proportions of the exposed crystalline planes do not advantageously depend on the maximum size of the primary particles. At a fixed modifying agent content, the amount of exposed crystal planes of the {1,1,1} family decreases as the pH increases.

The concentration of modifying agent in the mother liquor is between 10-4 mol / l and 10 mol / l. This concentration modifies the proportions of the different families of exposed crystalline planes. Routine tests allow the concentration to be adjusted according to the desired proportions.

To manufacture zirconia particles of the invention having a rod shape and exhibiting a large amount of crystalline planes belonging to the family {1,1,0}, especially more than 35% or more than 50% and / or less 70% of plans belonging to the family {1,1,0}, the pH of the mother liquor is made basic and greater than 10.

The modifying agents used are then selected from the group of oxoanions except borate (BO33-), carbonate (CO32-), nitrate (NO3), perchlorate (CI04), and mixtures thereof; preferably selected from the group of sulfate (SO42-), phosphate (PO43-) and mixtures thereof. In the case where the modifying agent is introduced in the form of an additive of formula MC, this additive is composed of: said modifying agent M - a complement C chosen from the group consisting of all the organic molecules positively charged, the organic radical consisting of a group of atoms selected from the group consisting of carbon, hydrogen, oxygen, and nitrogen; - cations of the elements of column 1; - Ammonium (NH4 +) The proportion of exposed crystal planes of the {1,1,0} family is regulated by the pH and by the amount of modifying agent. For maximum sizes greater than 10 nm, the relative proportions of the different crystalline planes exposed do not depend on the size of the primary particles. At a fixed pH, the proportion of exposed crystal planes belonging to the {1,1,0} family increases if the concentration of modifying agent increases.

In order to obtain a particle powder which is substantially exclusively in the form of said rods, the concentration of modifying agent is greater than 10-1 mol / l, preferably with a pH of between 10 and 14. The concentration of modifying agent can be however, lower if the pH increases: A concentration of 0.01 mol / l is thus suitable for a pH of 13. If the amount of modifying agent is not sufficient, platelets according to the invention will be obtained. The domains of rods displaying {1,1,0} family planes and platelets with {1,0,0} family planes are indeed continuous. In other words, for a given pH, there is a limit value below which platelets will be obtained and above which rods will be obtained.

For producing zirconia particles of the invention having a platelet-like shape and exhibiting a large amount of crystalline planes belonging to the family {1,0,0}, especially more than 35% or more than 50% and / or less of 60% of {1,0,0} family plans, the pH of the mother liquor is made basic and greater than 10. The modifying agents used are chosen from the group constituted by the oxoanions, the anions of the elements of the column 17 (halides), hydroxide (OH-) and mixtures thereof, preferably selected from the group consisting of hydroxide (OH-), carbonate (CO32-), fluoride (F), chloride (Cr ), perchlorate (CI04), borate (BO33-), nitrate (NO3) and mixtures thereof. More preferably, the modifying agent is hydroxide (OH-). In the case where the modifying agent is introduced in the form of an additive of formula MC, this additive is composed of: said modifying agent M and a complement C chosen from the group consisting of all the organic molecules positively charged, the organic radical consisting of a group of atoms selected from the group consisting of carbon, hydrogen, oxygen, and nitrogen; - cations of the elements of column 1; - Ammonium (NH4 +) The proportion of exposed crystalline planes belonging to the family {1,0,0} is regulated by the pH and by the amount of modifying agent. For sizes greater than 10 nm, the relative proportions of the different crystalline planes exposed do not depend on the size of the primary particles.

With a fixed modifying agent content, the proportion of exposed crystalline planes belonging to the {1,0,0} family increases with increasing pH. At fixed pH, and if the modifier is selected from oxoanions except borate (BO33-), carbonate (CO32-), nitrate (NO3), perchlorate (ClO4), and mixtures thereof, such as 5042 - and PO43-, the proportion of exposed crystalline planes belonging to the family {1,0,0}, or even the morphology of the particles, varies with the amount of modifying agent. In particular, this proportion decreases if the concentration of modifying agent increases. In order to obtain a particle powder which is substantially exclusively in the form of said platelets, the concentration of the abovementioned modifying agents must always be less than 10 -3 mol / l. Furthermore, in general, in the pH range between 10 and 14, for modifying agent concentrations of between 10-3 and 10-1 mol / l, routine tests make it possible to adjust the concentration of modifying agent. to adjust the proportion of exposed crystalline planes and morphology (platelets or rods). At a fixed pH, and if the modifying agent is selected from the group of borate (8033-) carbonate (CO32-), nitrate (NO3), perchlorate (C1O4), anions of the elements of column 17 of the table Periodic element, and their mixtures, the concentration of modifying agent does not change the proportion of exposed crystalline planes belonging to the family {1,0,0}. However, preferably, the concentration of modifying agent is less than 10 mol / l.

In step B), the mother liquor is heated in a closed vessel at a temperature above the boiling point of 1 bar, preferably between 100 ° C. and 300 ° C. The morphology, nature and proportions of exposed crystalline planes of the primary particles do not depend on the temperature of the hydrothermal treatment, but these temperature values allow a short treatment time (to obtain the desired size and morphology), allowing an exploitation on an industrial scale of the invention.

The reactor is heated to the desired temperature, then the temperature is maintained at least 1 hour and preferably less than 200 hours. The choice of the temperature / hold time pair also influences the size of the primary particles. This pair is therefore chosen so as to obtain a maximum primary particle size of less than 100 nm. The pressure in the reactor is equal to or greater than the boiling pressure of the mother liquor. This pressure may be autogenous, that is to say, correspond to the steam pressure of the water at the reactor temperature, hydraulic, or may result from the addition of an inert gas such as nitrogen.

The pressure is determined so that the final product obtained is in the desired crystalline phase. The final product is then in monoclinic form. The hydrothermal treatment can be carried out in a so-called batch reactor or in a continuous reactor. The residence times are generally shorter and the temperatures are generally higher in a continuous reactor than in a batch reactor. At constant pressure and temperature, an increase in the maintenance time leads to an increase in the size of the primary particles. At a constant holding time, an increase in temperature leads to an increase in the size of the primary particles.

The reactor is then cooled to a temperature below the boiling point at 1 bar.

The resulting soil is then isolated. It contains a liquid fraction and crude particles of monoclinic zirconia which have a high content of impurities. The inventors have discovered that these impurities adversely affect the efficiency of the particles whether they are used as a catalyst support or as a catalyst.

In microscopic observation, the primary particles have a smooth outline, even when their size is greater than 50 nm. Advantageously, the selectivity of the catalysis is improved.

The inventors explain this advantageous characteristic in that the rate of temperature rise of the reactor used for the hydrothermal treatment is less than 250 ° C., preferably less than 200 ° C. per hour. According to the invention, the solid phase of the sol consisting of crude particles of monoclinic zirconia is purified, during step C), so that its content of impurities is less than 0.7%, preferably less than 0.5%, preferably less than 0.3%, more preferably less than 0.1%, in percentages by weight of the dry matter. The known purification methods can be used in step C). The purification of the raw particles of the soil preferably comprises one or more of the following steps C1 to C3: The optional step C1 consists of reducing the quantity of the liquid phase and / or purifying said liquid phase of the soil. By reducing the amount of the liquid phase of the soil is meant any operation that makes it possible to eliminate part or all of the liquid phase of said soil. These methods are well known to those skilled in the art and may include - filtration, with or without prior resuspension of the soil, - dialysis, with or without prior resuspension of the soil, - purification with the aid of ion exchange resins, with or without prior resuspension of the soil, - rinsing, - and combinations of these techniques.

The term "purification of the liquid phase of the soil" is understood to mean any operation making it possible to reduce the concentration of ionic species in the liquid solution of the soil. These methods are well known to those skilled in the art and may include dialysis, purification methods using ion exchange resins or dilution rinsing.

Step C2 consists of the removal of cationic impurities. Cationic impurity, a molecule containing elements other than carbon (C), hydrogen (H), oxygen (O) and nitrogen (N), positively charged and contained in the liquid phase of the soil and / or fixed on the raw particles of the ground. The product resulting from the preceding process step (B) or Cl) is suspended in an aqueous solution having a pH of less than 7, more preferably less than 5. The pH of the suspension is adjusted by adding organic and / or inorganic acid. Among the acids, nitric acid, perchloric acid and / or hydrochloric acid (HCl) are preferred. This suspension is maintained, preferably with stirring, at a temperature below its boiling point at 1 bar, preferably at room temperature, for a period greater than 10 minutes, and preferably less than 30 minutes. This suspension is then filtered by any means known to those skilled in the art.

It is possible to repeat step C2 several times if the desired cationic species purity requires it. The number of steps C2 can be determined by routine tests. Preferably, step C2 is not repeated, a single step C2 being sufficient for zirconia.

Step C3 consists of the removal of anionic impurities. An anionic impurity is a molecule containing elements other than carbon (C), hydrogen (H), oxygen (O) and nitrogen (N), negatively charged and contained in the liquid phase of the soil and / or bound to the crude particles of the soil. ground. The product resulting from the preceding process step (B) or Cl or C2) is suspended in an aqueous solution having a pH greater than 11. The pH of the suspension is adjusted by adding an organic base and / or or inorganic. Among these bases, it is preferred to use ammonia NH 4 OH.

This suspension is maintained, preferably with stirring, at a temperature below its boiling point at 1 bar, preferably at room temperature, for a period of preferably greater than 10 minutes and preferably less than 30 minutes.

This suspension is then filtered by any means known to those skilled in the art. It is possible to repeat step C3 several times, if the purity required anionic species requires it. The number of C3 steps can be determined by routine tests. Preferably, step C3 is not repeated, a single step C3 being sufficient for zirconia. It is possible to alternate or reverse steps C2 and C3. These steps C2 and C3 are conventionally referred to as acid-base neutralization steps. Between steps C1, C2 and / or C3, an optional C4 rinse in a solvent, preferably a water having a degree of purity at least equal to that of osmosis water or an alcohol of purity greater than 98% by volume, preferably greater than 99% by volume, can be achieved. This operation consists in suspending the product resulting from the preceding step in this solvent, preferably with stirring, at a temperature below its boiling point at 1 bar, preferably at room temperature, for a duration greater than 10 minutes, preferably between 10 and 30 minutes. This suspension is then filtered by any means known to those skilled in the art. At the end of step C), the purified product obtained is substantially composed of monoclinic zirconia particles. Preferably, the purification is sufficient so that the powder obtained after step D) is constituted for more than 99.3%, preferably for more than 99.5%, preferably for more than 99.7%. %, more preferably for more than 99.9% of zirconia, the 100% complement consisting of impurities, in percentages by mass on the basis of the dry matter. In step D), after purification, the liquid fraction is removed by drying.

A purified particle powder is thus obtained according to the invention. Any method well known to those skilled in the art can be used. The drying temperature is generally less than 500 ° C, preferably less than 200 ° C. The drying time is adjusted so that the powder of purified particles has at the end of this step a residual moisture of less than 1% by weight.

During this optional step, the zirconia powder is preferably calcined for at least one hour. The calcining temperature is preferably greater than 300 ° C and preferably less than 500 ° C. This step makes it possible to eliminate the residual moisture present in the powder obtained at the end of step D), as well as any organic species remaining in said powder, without modifying the morphology and the nature of the particles, or the proportions of exposed crystalline planes.

The pore volume of the agglomerates of the primary particles of the zirconia powder obtained is generally between 0.1 and 0.2 cm 3 per gram of said powder. A disagglomeration step F) can be carried out in order to emulsify the agglomerates of this powder.

The primary particles have a maximum size of less than 100 nm, preferably less than 90 nm, preferably less than 80 nm and greater than 10 nm, preferably greater than 20 nm. Advantageously, in the case where the powder is used as a catalyst support, this characteristic allows a regular arrangement of the catalyst crystallites and a uniform growth of the exposed planes. The impurity content of the powder according to the invention is preferably less than 0.7%, preferably less than 0.5%, preferably less than 0.3%, more preferably less than 0.1%, percentages by mass of dry matter.

The impurities are preferably such that, in percentages by mass of dry matter: SiO 2 <0.1%, preferably SiO 2 <200 ppm, more preferably SiO 2 <100 ppm and / or - Al 2 O 3 <0.1%, preferably Al2O3 <200 ppm, more preferably Al2O3 <100 ppm and / or - MgO <0.1%, preferably MgO <200 ppm, preferably MgO <100 ppm, more preferably MgO <50 ppm and / or - CaO <0.1%, preferably CaO <200 ppm, more preferably CaO <100 ppm and / or - Na2O <0.1%, preferably Na2O <500 ppm, preferably Na2O <200 ppm, more preferably Na2O < 100 ppm and / or - Cl- <0.1%, preferably Cl- <500 ppm, preferably Cl- <200 ppm, more preferably Cl- <100 ppm and / or - NO3 <0.1%, preferably NO3 <500 ppm, preferably NO3 <200 ppm, more preferably NO3 <100 ppm and / or - SO42- <0.1%, preferably SO42- <500 ppm, preferably SO42- <200 ppm, more preferably SO42- <100 ppm and / or - Fe2O3 <0.1%, from reference Fe2O3 <200 ppm, preferably Fe2O3 <100 ppm, more preferably Fe2O3 <50 ppm and / or - TiO2 <0.1%, preferably TiO2 <200 ppm, preferably TiO2 <100 ppm, more preferably TiO2 < 50 ppm.

Each primary particle is constituted, excluding impurities, of a crystal having, for more than 95%, preferably for more than 97%, more preferably for more than 99% of its mass, more preferably for substantially 100%, a monoclinic structure. The complement of zirconia in monoclinic form is stabilized zirconia in quadratic or cubic form. Few or no single crystalline defects are observed for primary particles smaller than 80 nm. The powder has the following characteristics: Specific area of the particles, calculated by the BET method, greater than 20 m 2 / g, preferably greater than 50 m 2 / g, preferably greater than 70 m 2 / g and / or less than 100 m 2 / g g; and / or pore volume of the agglomerates of the primary particles of the powder of between 0.1 and 0.2 cm 3 per gram of said powder; and / or Percentile 50, D50, corresponding to the median size, of approximately 1 μm and 90%, D90 <10 μm, the size measurements being carried out by sedigraphy and the percentages being mass percentages; and / or - Loss on ignition after calcination at 1100 ° C of the powder: <3% - Unpacked density of about 1 g / cm3.

Moreover, more than 95% by number of the primary particles of the powder have a shape different from a spherical or spherical truncated shape. More specifically, at least 90%, preferably at least 95%, more preferably at least 99% by number of dry matter, more preferably substantially all the particles of the powder have a wafer and / or rod form. In general, on a powder directly obtained at the end of step D), these percentages relate to only one of these two forms. A mixture of different powders is however possible to change the respective percentages of platelets and rods. Surprisingly, the inventors have discovered that such platelet and / or rod zirconia particles exhibit families of rare crystal planes {1,1,0} and {1,0,0}. In addition, they have also found that such particles allow the development of cubic structure metal crystallites of columns 8, 9 and 10, and in particular platinum, which expose the families of {1,1,1} and {1 , 0.0}. Other methods are also possible for making nanoscale, monoclinic, monocrystalline powders, and exposing families of crystalline planes selected from {1,0,0}, {1,1,0} and {1,1,1 }. In particular, processes by the sol gel method, by combustion of a metal salt or oxidation of the metal, or by calcination at high temperature of a precursor of the product in question are possible. The parameters of these methods are then adapted according to the exposed crystal planes or morphologies of the desired primary particles. Similarly, the invention extends in particular to primary particles of hafnium oxide (HfO2). In general, the invention extends to primary particles consisting, excluding impurities, of zirconia and hafnium oxide, the proportion of hafnium oxide may be between 0 and 100%. Furthermore, the invention extends to the use of other powders than a single crystal powder of zirconia (ZrO 2), in particular to a molybdenum oxide powder (MoO 3) and / or a powder of tungsten oxide (WO3) which can also be obtained hydrothermally. These powders are monoclinic, nanometric, and monocrystalline. The morphologies and crystal planes of the primary particles of molybdenum oxide and tungsten oxide are identical to those of the zirconia according to the invention.

The primary particles of hafnium oxide, hafnium oxide and zirconia, molybdenum oxide, tungsten oxide, can be prepared in a manner similar to the synthesis method described above for the particles of zirconia. The inventors have also observed that certain dopants also make it possible to improve the catalytic performances in reactions that are sensitive to the structure carrying out the conversion of hydrocarbons using acidic or basic catalysts, such as, for example, dehydration reactions, elimination reactions, amination reactions, hydrogenation reactions, alkylation reactions. Preferably, the dopant is selected from the group consisting of oxoanions, column 17 anions (halides), column 1 (alkaline) cations, and mixtures thereof. More preferably, the oxoanions are chosen from silicates, phosphates and sulphates, the halides are selected from chlorides and fluorides, and the alkalis are selected from sodium and potassium.

The dopant can be combined with the powder according to the invention by any method known to those skilled in the art, for example by an impregnation process.

The particles according to the invention may advantageously be used in the context of a catalytic system.

Methods for growing catalyst crystallites are known. Preferably, the support is impregnated with a solution, aqueous or not, containing a precursor of the catalyst. The impregnated support then undergoes a maturation step allowing the impregnation solution to penetrate by capillarity into the pores of the support. The duration of this step is generally greater than 5 hours. The impregnated support is then dried by any drying means known to those skilled in the art, such as for example by steaming, under vacuum or not. The drying temperature is generally less than 500 ° C, the drying time being adjusted so that the impregnated support has, at the end of this step, a residual moisture of less than 1% by weight.

The impregnated and dried support then undergoes an optional calcination step, generally at a temperature above 300 ° C and below 500 ° C, and generally for a dwell time greater than 1 hour. The calcination step makes it possible to eliminate any binders contained in the support and originating from the impregnating solution.

Preferably, the drying step and the calcination step are carried out in a single operation. The impregnated support dried and optionally calcined then undergoes an optional activation operation, known to those skilled in the art. This step is carried out under a controlled atmosphere, adjusted to the selected catalyst (for example under a sulfurizing mixture for a sulfide-based catalyst), generally at a temperature of between 100 ° C. and 500 ° C., for a duration of plateau greater than 1 hour. After activation, the support can be qualified as a catalytic system. Optionally, this activation step can be carried out directly in the catalytic reactor. An additional optional step of passivation, known to those skilled in the art, can be performed after activation, in particular to facilitate the transport of the catalytic system. Generally, this step is carried out under gas sweep in an oxidizing medium, at temperatures below 100 ° C. A reactivation step must then be carried out in the reactor. The amount of catalyst deposited on the support particles is preferably limited so as to avoid excessive crystallite growth. An excess of catalyst can indeed lead to a change in the nature and quantity of the planes exposed by the crystallites.

Preferably, the catalyst represents less than 10%, preferably less than 7%, preferably still less than 5% of the mass of the catalyst system. A content of 1% of catalyst is generally suitable. Preferably, the process is determined so that the size of the catalyst crystallites is greater than 1.5 nm. The inventors have in fact discovered that a size greater than 1.5 nm makes it possible to prevent the crystallites from taking a spherical or spherical shape truncated by a plane of contact with the support particle, in particular when the catalyst is a metal of cubic structure of columns 8, 9 and 10 of the periodic table of elements, and in particular platinum.

Particular Process for the Production of a Catalytic System Preferably, in order to manufacture a catalytic system according to the invention, a manufacturing process comprising the following steps is used: I) Preparation of a solution based on catalyst (s) or catalyst precursor (s); II) impregnating a support powder according to the invention by means of the solution prepared in step I); III) Maturation to allow the impregnating solution to penetrate by capillarity into the pores of the support; IV) drying the impregnated support; V) calcination of the dried support; VI) Activation of the catalyst; VII) Optionally, passivation of the catalyst; VIII) Optionally, before use in the catalysis reaction, activation in the reactor preferably under the same conditions as those of step VI). In step I), in the case of platinum as a catalyst, a solution of platinum precursor, such as platinum tetramine nitrate Pt (NH 3) 4 (NO 3) 2 (for example manufactured by Sigma Aldrich) can be prepared. . In step II), the impregnation can be carried out according to a conventional procedure without excess solution. In step III), the maturation preferably lasts at least 5 hours and more preferably lasts about 8 hours. It is carried out at a temperature below 100 ° C, more preferably at room temperature. In step IV), the drying is preferably carried out in an oven at 110 ° C for at least 12 hours, so that the residual moisture of the support is less than 1% by weight.

In step V), the calcination is preferably carried out in air and, more preferably, for at least one hour, preferably for about 2 hours, and preferably at a temperature of between 300 ° C and 500 ° C. The rise ramp is preferably less than 10 ° C / min, more preferably about 2 ° C / min. Preferably, the calcination is carried out under gas flushing.

In step VI), the catalyst is preferably activated by reduction under hydrogen (H 2), more preferably for at least 1 hour, preferably for about 6 hours, and more preferably at a temperature between 100 ° C and 500 ° C.

The rise ramp is preferably less than 10 ° C / min, more preferably about 2 ° C / min. Preferably, the activation is carried out under hydrogen scavenging. The activation temperature is preferably between 300 and 500 ° C.

In step VII), the catalyst is passivated by flushing with inert gases containing a few ppm of oxygen, preferably under nitrogen containing a few ppm of O 2, preferably at a temperature below 100 ° C, more preferably at a temperature of ambient and, more preferably, for about 1 hour. At the end of step VI) and / or VIII), a catalytic system according to the invention is obtained. This method of preparation is not the only method that can be implemented to prepare a catalyst system according to the invention. In particular, a catalyst deposition by Chemical Vapor Deposition (CVD) may very well be suitable.

It is also possible to deposit the catalyst by selective impregnation, so that the catalyst is deposited only on one or more particular exposed crystalline planes or on exposed planes of particular families of the support particles. All the known methods of selective impregnation are conceivable, the parameters of a selective impregnation being a function of the nature of the support. Generally, selective impregnation consists in masking certain planes of the support particles so that they are no longer accessible to the catalyst during the deposition of the latter. It is thus possible to keep exposed only the planes of support particles allowing the growth of crystallites exposing favorable planes or families of planes favorable to the reaction to be catalyzed. Advantageously, the effectiveness of the catalyst is thus optimized. At the same amounts of catalyst, the performance of the catalyst system is improved, with a larger percentage of the catalyst surface exposing useful planes. Conversely, with identical catalytic performances, the quantity of catalyst consumed is reduced. The catalyst deposited on the surface of the support may be a metal, preferably chosen from the group of metals of columns 8, 9 and 10 of the periodic table of elements, preferably chosen from the group of metals of columns 8, 9 and 10 of the periodic table of elements and having a cubic structure, and mixtures thereof. Preferably, the metal is platinum. The catalyst may also be an oxide, preferably chosen from lanthanum oxide and / or transition metal oxides, for example V 2 O 5 or Cr 2 O 3 and / or the oxides of the elements of columns 14 and 15, preferably oxides. tin (Sn), lead (Pb) and / or bismuth (Bi). The catalyst may also be a carbide, preferably selected from transition metal carbides, such as, for example, molybdenum carbide and / or tungsten carbide. The catalyst may also be a sulphide, preferably selected from transition metal sulphides, preferably molybdenum sulphides and tungsten sulphides, optionally doped with cobalt or nickel (CoMoS for example).

Preferably, the catalyst is selected so that at least 80%, preferably at least 90%, more preferably at least 95% by number, more preferably substantially all of the catalyst crystallites of the catalyst system have a shape which is not a spherical or spherical shape truncated according to the contact plane with the support. Advantageously, the performance of the catalytic system is improved. The size of the crystallites of the catalyst is preferably between 1.5 nm and 10 nm, preferably between 3 nm and 10 nm. Preferably, the catalyst is less than 10%, preferably less than 7%, more preferably less than 5% of the mass of the catalyst system. A content of 1% of catalyst is generally suitable.

Preferably, the catalyst is a metal selected from the group of metals of columns 8, 9 and 10 of the periodic table and is chosen so that at least 80%, preferably at least 90%, more preferably at least 95% by number, more preferably substantially all of the catalyst crystallites have a truncated cuboid or octahedron shaped raft. With a metal of cubic structure of the columns 8, 9 or 10, the size of the crystallites is preferably between 1.5 nm and 10 nm, preferably between 3 nm and 10 nm. Advantageously, it is thus possible to obtain mainly crystallites of said metal, in particular platinum, which is mainly in one or both of the truncated cubo-octahedral and raft forms. With a cubic metal structure of columns 8, 9 or 10, the truncated cubo-octahedron crystallites are formed on the {1,1,0} and {1,1,1} exposed planes of the support particles, the crystallites in form of raft forming on the exposed planes {1,0,0} of these particles, in particular when said particles have a monoclinic structure.

CHARACTERIZATION METHODS The following methods can be used to characterize the invention and the tests below are provided for illustrative and not limiting. Morphology and plans The morphology of the crystals can be determined from observations made in High Resolution Transmission Electron Microscopy (METHR) after ultrasonic dispersion of the samples. The determination of the morphology of the primary particles and crystallites, the determination of the exposed crystalline planes and the quantification of the proportion of these planes are carried out as follows: After ultrasonic dispersion of a sample to be characterized, high resolution images of Primary particles or crystallites are made using a transmission electron microscope. A large number of clichés is necessary, generally more than 50. From these clichés, the crystalline planes of each primary particle or crystallite observed are determined in the plane of observation. The orientation of each primary particle or each crystallite being random, it is then possible, assuming that all the primary particles or crystallites observed have the same morphology and the same natures and proportions of crystalline planes exposed, to determine the orientation of each primary particle or crystallite with respect to the plane of observation. With the assumptions made, clichés can be assimilated to different views in different directions of the same primary particle or crystallite. It is then possible to determine the morphology of the primary particles or crystallites observed.

Each image being considered as a view in a precise direction, it is then possible, after determining the morphology of a crystallite or a particle, to select the images showing at least one face exposed perpendicularly to the observation plane. This exposed face is in the form of an edge in said cliche. The corresponding exposed crystalline plane can then be determined. This procedure is used for all exposed faces of the crystallite or particle to determine all of its exposed crystalline planes. The proportion of the exposed crystalline planes of the family {h, k, l} is equal to the sum of all the exposed surfaces of the crystalline planes of the family {h, k, l} divided by the total sum of the exposed surfaces. Finally, after having determined these characteristics, we confirm the hypothesis that the set of primary particles or crystallites observed have the same morphology by verifying by measurements on the two-dimensional projection that the different images can correspond to different views of the same elementary particle or crystallite made according to determined orientations. The same procedure is used to confirm the hypothesis taken about the nature and proportion of exposed crystalline planes by verifying that the nature and proportion of the exposed crystalline planes of the primary particle or crystallite having the determined morphology observed in different directions are identical to those revealed by the different shots taken in the same directions. Chemical analyzes The metal contents are determined by ICP (Induction Coupled Plasma). The determination of Cl- chloride ions is carried out after pyrohydrolysis by ion chromatography. The carbon and sulfur contents (converted to SO42- sulfate content) are measured on a Sulfur Carbon model C, S-MAT 5500 analyzer (sold by Stréhleim Inst.). Measurements of specific surface area and porous volume The textural properties are determined by physical adsorption / desorption of N2 at -196 ° C. The samples are desorbed beforehand under vacuum at 400 ° C. for 2 hours for the calcined supports and at 100 ° C. for 10 hours for the non-calcined supports. The specific area is calculated by the BET method (Brunauer Emmet Teller) as described in Journal of American Chemical Society 60 (1938) pages 309-316.

The pore volume Vp is determined with the BJH method [described by E. P. Barrett, L. G. Joyner, P. H. Halenda, J. Am. Chem. Soc., 73 (1951) 373] applied to the desorption branch of the isotherm. Measurements of the unpacked density The unpacked density is determined by the following method: in a graduated cylinder of 250 cm 3 having a mass Mo (in g), a volume of powder is poured, by means of a funnel, between 240 and 250 cm3. The volume Vo (in cm.sup.3) of powder poured is then read on the test piece and the test piece thus filled is weighed to measure the mass M1 (in g).

The unpacked density, in g / cm3, is determined according to the following relationship: Unpacked Density = (M1-Mo) No. Determination of Crystalline Structure (XRD Analysis) X-ray powder diffraction patterns are obtained on a BRUKER D5005 diffractometer, using copper Ka radiation (1.54060 A). The intensity data are recorded over a range of 3-80 ° with a pitch of 0.02 ° and a counting time of 1s per step. The crystalline phases are identified by comparison with the standard JCPDS files. The crystal structure can be confirmed by other well known methods such as Raman spectroscopy or, locally at the level of a primary particle, by transmission electron microscopy. Measurement of the dispersion of a catalyst belonging to the group of metals of cubic structure of columns 8, 9 and 10 The dispersion of this type of catalyst is determined by hydrogen chemisorption. The chemisorption measurements were performed on an ASAP 2010 device (marketed by Micromeritics). The amount of chemisorbed hydrogen (HCirr) is determined from the extrapolation at zero pressure of the linear branch of the total and reversible adsorption isotherms as described in [G. Bergeret, P. Gallezot, Handbook of Heterogeneous Catalysis, G. Ertl, H. Knezinger, J. Weitkamp Eds., Wiley-VCH, Weinheim, Vol. 2 (1997)].

Assuming a dissociative adsorption of hydrogen on the catalyst belonging to the group of metals of cubic structure of columns 8, 9 and 10, the dispersion is given by the relation: D = 100x2x HCirr ntot HCirr being the number of moles of H2 chemisorbed and the total number of catalyst atoms belonging to the group of metals of cubic structure of columns 8, 9 and 10. Determination of the size of a catalyst crystallite

The size of a catalyst crystallite is defined as the average surface area of the catalyst crystallite. It can be measured from

10 observations made by electron microscopy, especially for catalysts belonging to the group of carbides, sulfides or oxides, and / or by measuring the dispersion of said catalyst at the surface of the support (method used for example for the catalysts belonging to the group cubic structure metals of columns 8, 9 and 10).

The determination of the average surface area from observations by electron microscopy is as follows: The sample to be observed is dispersed under ultrasound in ethanol. The suspension is then deposited on a carbon-coated copper grid. A large number of high resolution snapshots of

20 crystallites are made, usually more than 50. From these clichés, if n; is the number of crystallites with a maximum dimension in the observation plane d, the average surface dimension ds is defined by the following formula: s = En ~ d.3 ds

25

The determination of the average surface area from the measurement of the catalyst dispersion at the surface of the support is given by the following formula, assuming that the crystallites have a cubic form with

30 5 accessible faces: ds = 5000Mca

p it acaNaD5 D being the catalyst dispersion Ca on the surface of the support (expressed in%), pca the density of the catalyst (in g / m3), aca the area occupied by a catalyst atom Ca (in m2), MCa la molar mass of catalyst Ca (in g / mol) and Na the number of Avogadro.

The aca values can be found, for example, in [G. Bergeret, P. Gallezot, Handbook of Heterogeneous Catalysis, G. Ertl, H. Knezinger, J. Weitkamp Eds., Wiley-VCH, Weinheim, Vol. 2 (1997)]. The inventors have verified that the hypothesis of the cubic form is realistic within the framework of the morphologies described in the present description, namely the raft and truncated cubo-octahedron shapes, by verifying that the average surface dimension of a sample of Metallic crystallites of cubic structure of columns 8, 9 or 10 is identical for the two methods described above.

Measurements of the catalytic properties In the case of catalytic systems, for a given catalysis reaction using a given reagent r, the intrinsic velocity per atom of surface catalyst (turnover rate or turnover rate) in mol) ~ reagent • s 1 • gSurface catalyst 1 denoted Vi, is defined by the relation: 100V Catalyst Vi =

D VC catalyst (moleactant • s 1 • gCatalyst 1) designating the specific rate: 100V VC catalyst = ° / ° catalyst where -% Catalyst refers to the mass percentage of catalyst on the basis of the catalytic system, and - V denotes the reaction rate, in 1 g-1, calculated by the following formula: ## EQU1 ## with F ° the molar flow rate of reagent at the inlet of the reactor (moleactant • s 1); the degree of conversion, less than 20% in the context of the catalytic reactions used for the examples and the mass of the catalytic system (g); and D denotes the dispersion of the catalyst determined by hydrogen chemisorption, expressed in%. The molar flow rate of the reagent at the reactor inlet, F0, is calculated according to the following equation: ## EQU1 ## for total reagent flow under normal conditions of temperature and pressure in cm 3. s-,; Pr the reagent partial pressure in Pa and Ptotai the total pressure in Pa. As part of the evaluation of the catalytic properties of the various powders carried out, as described below, on a reaction of selective hydrogenation of crotonaldehyde (2- butenal) Crotyllic alcohol, CrOH, is defined, at a given conversion level, the selectivity towards crotyllic alcohol, CrOH, by the following expression: t SrOOH [cro] 0 - [cro] t with [cro] o, the concentration of crotonaldehyde at the inlet of the reactor, [believes, the concentration of crotonaldehyde at the outlet of the reactor and [CrOH] t, the concentration of the crotyllic alcohol at the instant t at the outlet of the reactor. EXAMPLES Example 1 In a 100 ml PTFE Teflon beaker, 7.4 g of zirconium oxyhydroxide at 50% by weight of zirconia was suspended and stirred in 70 ml [CrOH] of deionized water. This mother liquor is then introduced into a 100 ml PTFE Teflon bomb which is then sealed in a stainless steel autoclave and then placed in a controlled oven at 200 ° C. One hour is necessary to reach the set temperature of 200 ° C in the bomb. The mother liquor is maintained at 200 ° C. for 100 hours and then cooled to room temperature. At this moment, the bomb is extracted from the stainless steel autoclave. This procedure generates a sol consisting of a solid phase and a liquid supernatant. Part of the soil liquid is siphoned off. The rest of the soil is filtered and then washed with 200 ml of deionized water on a Buchner type filter. The cake obtained is then dried in an oven for at least 12 hours at 110 ° C, and then stirred with agate mortar. The solid raw particles are calcined in air for 2 hours (temperature ramp of 2 ° C./min, air flow rate of 100 ml / min, ie a WH speed of 300 h -1) at 550 ° C. The main physico-chemical properties of the powder thus obtained are given in Table 1. This powder has a specific surface area after calcination at 550 ° C. of 80 m 2 / g and presents more than 99% of monoclinic phase, as shown by its diffractogram. X-rays. The primary particles are in a quasi-spherical form. Powder exposes 30% family plans {1,1,1}, 10% family plans {1,1,0}, 10% family plans {1,0,0} and 50% other family plans with Miller indices greater than 1.

Example 2 (Platelets {1,0,0}) In a 100 ml PTFE Teflon beaker, 4 g of NaOH sodium hydroxide was dissolved in 70 ml of deionized water, and 7.4 g of zirconium oxyhydroxide at 50% by weight of zirconia. The concentration of OH-modifying agent is 0.5 mol / l. This mother liquor is then introduced into a 100 ml PTFE Teflon bomb which is then sealed in a stainless steel autoclave and then placed in a controlled oven at 200 ° C. One hour is necessary to reach the set temperature of 200 ° C in the bomb. The mother liquor is maintained at 200 ° C. for 100 hours and then cooled to room temperature. At this moment, the bomb is removed from the stainless steel autoclave. This procedure generates a sol consisting of a solid phase and a liquid supernatant. Some of the liquid is siphoned off.

The rest of the sol is then resuspended and the pH is adjusted to 8 by addition of 0.1 N hydrochloric acid. The suspension is then filtered and then washed with 200 ml of deionized water on a Buchner type filter. The cake thus obtained is then resuspended in 500 ml of deionized water and the pH is adjusted to 5 by addition of 0.1 N hydrochloric acid. The suspension is then filtered and then washed with 200 ml of deionized water. on a Buchner type filter. The cake obtained is resuspended in 500 ml of deionized water and the pH is adjusted to 11 by adding ammonia (NH 4 OH) to 1 N. The suspension is then filtered and then washed twice with 200 ml of deionized water. on a Buchner type filter.

The cake obtained is then dried in an oven for at least 12 hours at 110 ° C, and then stirred with agate mortar. The powder obtained is calcined under air for 2 hours (temperature ramp of 2 ° C./min, air flow rate of 100 ml / min, ie a hourly volume velocity VVH of 300 h -1) at 400 ° C. The main physico-chemical properties of the powder thus obtained are given in Table 1. The powder obtained has more than 99% of monoclinic phase as shown by its X-ray diffractogram. The morphology of the powder is given in Table 2. The primary particles are in the form of platelets exposing up to 50% of the {1,0,0} family planes. The other plans exposed by these particles are family plans {1,1,0} at 35%, family plans {1,1,1} at 10% and family plans with Miller indices greater than 1 at 5%. EXAMPLE 3 (Sticks {1,1,0}) In a 100 ml PTFE Teflon beaker, 14.2 g of Na 2 SO 4 sodium sulphate and then 4 g of NaOH soda are dissolved in 70 ml of deionized water and then suspends and stirred 7.4 g of zirconium oxyhydroxide at 50% by weight of zirconia. The concentration of SO42- modifying agent is 0.5 mol / l and the concentration of hydroxide (OH-) is 0.5 mol / l. This mother liquor is then introduced into a 100 ml PTFE Teflon bomb which is then sealed in a stainless steel autoclave and then placed in a controlled oven at 200 ° C. One hour is necessary to reach the set temperature of 200 ° C in the bomb. The mother liquor is kept at 200 ° C. for 100 h and then cooled to room temperature. At this moment, the bomb is removed from the stainless steel autoclave. This procedure generates a sol consisting of a solid phase and a liquid supernatant. Some of the liquid is siphoned off. The rest of the soil is filtered and then washed twice with 200 ml of deionized water on a Buchner type filter. The cake thus obtained is then resuspended in 500 ml of deionized water and the pH is adjusted to 5 by addition of 0.1 N hydrochloric acid. The suspension is then filtered and then washed with 200 ml of deionized water. on a Buchner type filter. The cake obtained is resuspended in 500 ml of deionized water and the pH is adjusted to 11 by adding ammonia to 1 N. The suspension is then filtered and then washed twice with 200 ml of deionized water on a filter. Buchner type. The cake obtained is then dried in an oven for at least 12 h at 110 ° C, and then stirred with agate mortar. The powder obtained is calcined in air for 2 hours (temperature ramp of 2 ° C./min, air flow rate of 100 ml / min, ie a WH hourly speed of 300 h -1) at 400 ° C.

The main physico-chemical properties of the powder thus obtained are given in Table 1. The powder obtained has more than 99% of monoclinic phase, as shown by its X-ray diffractogram. The morphology of the powder is given in Table 2 The primary particles are in the form of rods exposing up to 75% of the {1,1,0} families. Other plans exposed by these particles are family plans {1,0,0} at 5%, family plans {1,1,1} at 15%, and family plans with Miller indices greater than 1 at 5%. Example 4 (Sticks {1,1,0}). In a 100 ml PTFE Teflon beaker, 5 g of Na 2 SO 4 sodium sulphate and then 4 g of NaOH soda are dissolved in 70 ml of deionized water, and 7.4 g of oxyhydroxide are suspended and stirred. zirconium at 50% by weight of zirconia. The concentration of SO42- modifying agent is 0.25 mol / l and the concentration of hydroxide (OH-) is 0.5 mol / l. This mother liquor is then introduced into a 100 ml PTFE Teflon bomb which is then sealed in a stainless steel autoclave and then placed in a controlled oven at 200 ° C. One hour is necessary to reach the set temperature of 200 ° C in the bomb. The mother liquor is maintained at 200 ° C. for 100 hours and then cooled to room temperature. At this moment, the bomb is removed from the stainless steel autoclave. This procedure generates a sol consisting of a solid phase and a liquid supernatant. Some of the liquid is siphoned off. The rest of the soil is filtered and then washed twice with 200 ml of deionized water on a Buchner type filter. The cake thus obtained is then resuspended in 500 ml of deionized water and the pH is adjusted to 5 by addition of 0.1 N hydrochloric acid. The suspension is then filtered and then washed with 200 ml of deionized water. on a Buchner type filter. The cake obtained is resuspended in 500 ml of deionized water and the pH is adjusted to 11 by adding ammonia to 1 N. The suspension is then filtered and then washed twice with 200 ml of deionized water on a filter. Buchner type. The cake obtained is then dried in an oven for at least 12 hours at 110 ° C, and then stirred with agate mortar. The powder obtained is calcined under air for 2 hours (temperature ramp of 2 ° C./min, air flow rate of 100 ml / min, ie a hourly volume velocity VVH of 300 h -1) at 400 ° C. The main physico-chemical properties of the powder thus obtained are given in Table 1. The powder obtained is present more than 99% of monoclinic phase as shown by its X-ray diffractogram. The morphology of the powder is given in Table 2 The primary particles are in the form of rods exposing up to 60% of the {1,1,0} family planes. The other plans exposed by these particles are family plans {1,0,0} at 25%, family plans {1,1,1} at 10%, and family plans with Miller indices greater than 1 at 5%. EXAMPLE 5 (Sticks {1,1,1}) 7.4 g of zirconium oxyhydroxide containing 50% by weight of zirconia in 70 ml of hexagonal zirconium hydroxide were suspended and stirred in a 100 ml PTFE Teflon beaker. a solution of 5N hydrochloric acid. The concentration of Cl- modifying agent is 1 mol / l. This mother liquor is then introduced into a 100 ml PTFE Teflon bomb which is then sealed in a stainless steel autoclave and then placed in a controlled oven at 200 ° C. One hour is needed to reach into the bomb at set temperature. The mother liquor is maintained at 200 ° C. for 100 hours and then cooled to room temperature. At this moment, the bomb is removed from the stainless steel autoclave. This procedure generates a sol consisting of a solid phase and a liquid supernatant. Some of the liquid is siphoned off. The pH of the sol is then adjusted to 8 by addition of ammonia to 1 N. The suspension is then filtered and then washed twice with 200 ml of deionized water on a Buchner type filter. The cake obtained is resuspended in 500 ml of deionized water and the pH is adjusted to 11 by adding ammonia to 1 N. The suspension is then filtered and then washed twice with 200 ml of deionized water on a filter. Buchner type. The cake obtained is then dried in an oven for at least 12 hours at 110 ° C, and then stirred with agate mortar. The powder obtained is calcined under air for 2 hours (temperature ramp of 2 ° C./min, air flow rate of 100 ml / min, ie a hourly volume velocity VVH of 300 h -1) at 400 ° C. The main physico-chemical properties of the powder thus obtained are given in Table 1. The powder obtained has more than 99% of monoclinic phase as shown by its X-ray diffractogram. The morphology of the powder is given in Table 2. The primary particles are in the form of rods exposing up to 65% of the {1,1,1} families. The other plans exposed by these particles are family plans {1,1,0} at 10%, family plans {1,0,0} at 20%, and family plans with Miller indices greater than 1 at 5%. Example 6 (Sticks {1,1,0} having a level of impurities higher than that of Example 3) In a 100 ml PTFE Teflon beaker, 14.2 g of sodium sulphate Na.sub.2 SO.sub.4 are dissolved, then 4 g of sodium hydroxide NaOH in 70 ml of deionized water, and 7.4 g of zirconium oxyhydroxide containing 50% by weight of zirconia are then suspended and stirred. The concentration of SO42- modifying agent is 0.5 mol / l and the concentration of hydroxide (OH-) is 0.5 mol / l. This mother liquor is then introduced into a 100 ml PTFE Teflon bomb which is then sealed in a stainless steel autoclave and then placed in a controlled oven at 200 ° C. One hour is necessary to reach the set temperature of 200 ° C in the bomb. The mother liquor is kept at 200 ° C. for 100 h and then cooled to room temperature. At this moment, the bomb is removed from the stainless steel autoclave. This procedure generates a sol consisting of a solid phase and a liquid supernatant. Some of the liquid is siphoned off. The rest of the soil is filtered on a Buchner type filter. The cake thus obtained is then resuspended in 500 ml of deionized water and the pH is adjusted to 5 by addition of 0.1 N hydrochloric acid. The suspension is then filtered on a Buchner type filter and then washed with 200 ml of permutated water on a Buchner type filter. The suspension is then filtered and then washed twice with 200 ml of deionized water on a Buchner type filter. These operations make it possible to lower the impurity content to less than 0.7%, which makes Example 6 in accordance with the invention. The impurity content, however, was maintained at a level higher than that of Examples 1 to 5 in order to evaluate the effect of the impurities, as described below. The cake obtained is then dried in an oven for at least 12 h at 110 ° C, and then stirred with agate mortar. The powder obtained is calcined under air for 2 hours (temperature rise ramp of 2 ° C./min, air flow rate of 100 ml / min, ie a 300 hr hourly volume velocity VVH) at 400 ° C. The main physico-chemical properties of the powder thus obtained are given in Table 1. The level of anionic impurities such as Cl- and SO42- is greater than the level of these same impurities in Example 3 having undergone a washing step. complete. The powder obtained has more than 99% monoclinic phase. The morphology of the powder is given in Table 2. The primary particles are in the form of rods exposing up to 75% the {1,1,0} families. The other planes exposed by these particles are family plans {1,0,0} at 5%, family plans {1,1,1} at 15% and plan families with Miller indices greater than 1 at 5%. Example 7 (Sticks {1,1,0} not undergoing any extensive purification step) In a 100 ml PTFE Teflon beaker, 14.2 g of Na 2 SO 4 sodium sulphate are dissolved, followed by 4 g of NaOH soda. in 70 ml of deionized water, and 7.4 g of zirconium oxyhydroxide containing 50% by weight of zirconia are then suspended and stirred. The concentration of SO42- modifying agent is 0.5 mol / l and the concentration of hydroxide (OH-) is 0.5 mol / l. This mother liquor is then introduced into a 100 ml PTFE Teflon bomb which is then sealed in a stainless steel autoclave and then placed in a controlled oven at 200 ° C. One hour is necessary to reach the set temperature of 200 ° C in the bomb. The mother liquor is kept at 200 ° C. for 100 h and then cooled to room temperature. At this moment, the bomb is removed from the stainless steel autoclave. This procedure generates a sol consisting of a solid phase and a liquid supernatant. Some of the liquid is siphoned off.

The rest of the soil is filtered on a Buchner type filter. The cake thus obtained is then resuspended in 500 ml of deionized water. The suspension is then filtered on a Buchner type filter and then washed with 200 ml of deionized water on a Buchner type filter. The suspension is then filtered and then washed twice with 200 ml of deionized water on a Buchner type filter.

This filtration-washing operation does not make it possible to reach high levels of purity (<0.7%), as in the powders of the invention. The cake obtained is then dried in an oven for at least 12 h at 110 ° C, and then stirred with agate mortar. The powder obtained is calcined under air for 2 hours (temperature rise ramp of 2 ° C./min, air flow rate of 100 ml / min, ie a 300 hr hourly volume velocity VVH) at 400 ° C. The main physicochemical properties of the powder thus obtained are given in Table 1. The level of ionic impurities such as Na + (expressed as Na2O) and SO42- is greater than the level of these same impurities in Example 3 The powder obtained has more than 99% monoclinic phase. The morphology of the powder is given in Table 2. The primary particles are in the form of rods exposing up to 75% the {1,1,0} families. The other planes exposed by these particles are family plans {1,0,0} at 5%, family plans {1,1,1} at 15% and plan families with Miller indices greater than 1 at 5%.

Table 1 Ex Na2O SiO2 Fe2O3 CaO Al2O3 TiO2 MgO NO3 CI-SO42 Area Content (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) sp m2 phase (m 'g) monoclinic (% by weight) 1 <100 <100 <50 <50 <50 <50 <50 <100 <100 <100 80> 99 2 <100 <100 <50 <50 <50 <50 <50 <100 <100 <100 50> 99 3 <100 <100 <50 <50 <50 <50 <50 <100 <100 <100 65> 99 4 <100 <100 <50 <50 <50 <50 <50 <100 <100 <100 60> 99 <100 <100 <50 <50 <50 <50 <50 <100 <100 <100 100> 99 6 <100 <100 <50 <50 <50 <50 <50 <100 500 2000 65 > 99 7 5000 <100 <50 <50 <50 <50 <50 <100 <100 2500 65> 99 Example 1 shows that impurities of less than 0.7% are possible, without thorough washing, by hydrothermal, when the mother liquor does not contain an agent selected from the group of oxoanions, anions of column 17, hydroxide OH- and mixtures thereof. The invention does not however relate to the products obtained from such mother liquors.

Table 2 Ex Morphology Size Maximum Nature and Proportion (%) of families of exposed planes of primary particles (nm) {1,1,1} {1,1,0} {1,0,0} Other 1 Quasi-15 30 10 10 50 Spherical 2 Pads 80 10 35 50 5 3 Sticks 50 15 75 5 5 4 Sticks 60 10 60 25 5 5 Sticks 15 65 10 20 5 6 Sticks 50 15 75 5 5 7 Sticks 50 15 75 5 5 Preparation of systems Catalysts: The powders of Examples 1, 2, 3, 5 and 6 were used in the manufacture of catalytic systems using platinum as a catalyst and having a platinum mass content of about 1%.

Their preparation was carried out by impregnation, without excess solution, of the powder of each example with a solution of platinum tetramine nitrate Pt (NH 3) 4 (NO 3) 2 (from Sigma Aldrich). This method, easy to implement, is well known to those skilled in the art. After impregnation of the powder, it is left to mature for 8 hours at room temperature, in order to allow the solution to penetrate by capillarity into the pores. The different impregnated powders are dried in an oven at 110 ° C. for a dwell time of 12 hours. The various dried impregnated powders are calcined in air for 2 hours (temperature ramp of 2 ° C./min, air flow rate of 100 ml / min, ie a hourly volume velocity VVH of 300 h -1) at a temperature of 400.degree. ° C. The catalyst is then activated by reduction under H 2 for 6 h (temperature ramp of 2 ° C./min, flow rate of H 2 of 100 ml / min in the reactor used, ie a hourly volume velocity VVH of 300 h -1) at a temperature of activation temperature of 300 ° C. The catalyst thus activated is then passivated under N 2 (containing a few ppm of O 2) at ambient temperature for 1 h.

The crystallite size was greater than 1.5 nm and less than 10 nm. Catalytic tests were carried out on a hydrogenation reaction of ortho-xylene (1,2-dimethylbenzene) in a fixed-bed open-pyrex reactor operating at low conversion and at atmospheric pressure. the following procedure: 20 mg of the catalyst system (in this case the zirconia powder of Examples 1, 2, 3, 5, 6, coated with platinum) and 50 mg of ground quartz (80-125 μm) are placed in the reactor and activated at 300 ° C (temperature ramp 4 ° C / min) under H2 (flow rate of 27 cm3 / min, an hourly volume velocity VVH of 80 h -1). The temperature is then set at 200 ° C. and the hydrogen-orthoxylene reaction mixture is introduced into the reactor. The ortho-xylene partial pressure is 4 kPa (condenser temperature 50 ° C). The activity of the catalytic system stabilizes after 2 hours. The reaction temperature is then set at 100 ° C.

The performance of the catalytic system tested is measured 200 minutes after the introduction of the reagents. The following Table 3 gives the results obtained: Table 3 Reaction insensitive to the structure: Hydrogenation of ortho-xylene Example Powder used Size of the crystallites Vi (molortno-xylenes 1 • gPlatinum (system as platinum support (nm) -1 catalytic ) of catalyst surface 8 Example 1 1.7 2.0x10-7 9 Example 2 3.8 1.8x10-7 Example 3 1.5 2.1x10-7 11 Example 5 2.1 1.7x10-7 These results show that the catalytic performances are not significantly improved by the morphology of the support particles The catalytic properties were then studied on a reaction of selective hydrogenation of crotonaldehyde (2-butenal) crotyllic alcohol, CrOH. The hydrogenation of crotonaldehyde is studied in a vapor phase at atmospheric pressure at 100 ° C. in a fixed-bed open microreactor.The hydrogen is bubbled into the liquid crotonaldehyde contained in a saturator at room temperature. The reaction then passes through a condenser maintained at 0 ° C. in a thermostatic bath, thereby fixing the crotonaldehyde partial pressure at 1.1 × 10 3 Pa. Typically, 50 mg of the test catalytic system manufactured as described above are placed in a reactor and diluted in 50 mg of ground quartz (80-125 μm). The catalyst is then activated at 300 ° C. under H 2 for 6 hours (heating ramp of 2 ° C./min, flow rate of H 2 of 100 ml / min, ie a hourly volume velocity V VH of 900 h -1). The temperature is then set at 100 ° C. and the reaction mixture is introduced into the reactor. The temperature regulation is carried out thanks to a thermocouple located in a thermowell at the level of the catalytic bed. The composition of the mixture leaving the reactor is analyzed every 25 minutes by gas chromatography (HP 4890). The chromatograph is equipped with a flame ionization detector (FID) on a CP-SIL 5CB column (length 25 m, inner diameter 0.32 mm, and film thickness 0.5 lm). The analysis conditions are as follows: column temperature: 40 ° C., carrier gas: nitrogen, and overpressure at the top of the column: 3 × 10 4 Pa.

The intrinsic velocity per Vicro surface catalyst atom and the selectivity for crotyl alcohol SCrOH are measured. The following Table 4 gives the results obtained: TABLE 4 Structural Sensitivity Reaction: Hydrogenation of Crotonaldehyde Example Powder used as Vi SCrOH (%) (catalytic catalyst support system 1) (mOlcrotonalhyde • S • surface gPlatin) 8 Example 1 4.5 × 10 -4 18 9 Example 2 9.6 × 10 -4 Example 3 3.9 × 10 -4 14 12 Example 6 2.5 × 10 -4 8 10 Example 9 thus makes it possible to considerably improve the catalytic properties with intrinsic speed. Vi double and selectivity, SCrOH 55% greater than those of the reference powder. However, as described above, the support powder of Example 2 differs from that of Example 1 by the morphology of the primary particles. A comparison of these examples (catalytic systems of Examples 8 and 9) shows that the anisotropic morphology gives better results. A comparison of the catalytic systems 9, 10 and 12, however, shows that not all the anisotropic morphologies are suitable for a reaction sensitive to the determined structure. The catalyst system 10 obtained from the rod-shaped carrier powder of Example 3 thus exhibits few planes to which the reaction would be sensitive, the catalytic performance being similar to that of Example 8. A comparison of Examples 9 and 5 10, shows that among the anisotropic morphologies tested, the platelet morphology is the only one that is effective.

The inventors have also found that this difference in efficiency results from the crystalline planes exposed by the primary particles of the support. The inventors have thus discovered that it is advantageous to choose particles exhibiting particular crystalline planes. In the present case, in the context of a hydrogenation reaction of crotonaldehyde, it is advantageous to choose primary support particles exhibiting a large area of planes belonging to the {1,0,0} family, these particles making it possible to formation of raft crystallites themselves exposing a large area of planes belonging to the {1,0,0} family. A comparison of the catalytic performances of the catalytic system of Example 10, with the support of the powder of Example 3 and of the catalytic system of Example 12 supported by the less pure powder of Example 6, shows that the purity of the carrier powder is high, the better the catalytic performance. Example 10 demonstrates the benefit of the advanced wash purification step. The intrinsic speed is in fact improved by 56% and the selectivity with respect to the crotyllic alcohol is improved by 75%. As it is now clear, the invention provides particles which, directly or in catalytic systems, make it possible, by virtue of thorough purification, to improve the efficiency of the catalysis of structurally sensitive reactions.

Of course, the invention is not limited to the embodiments described and shown. In particular, a powder according to the invention could be used, for example, as a mineral filler, as an abrasive or in the field of filtration.

Claims (42)

1. A method for synthesizing a powder comprising the following successive steps: A) preparing a mother liquor comprising: i. an oxide base component or a precursor of said base component and an additional component of formula MC, said additional component being composed of: - an agent M selected from the group of oxoanions, anions of the elements of column 17 of the periodic table elements, OH- hydroxide, and mixtures thereof, and a complement C, this complement being inorganic when the agent is OH- hydroxide; or ii. an oxide base component or a precursor of said base component incorporating an agent M selected from the group of oxoanions, anions of the elements of column 17 of the periodic table of elements, and mixtures thereof; B) preparing, from said mother liquor and hydrothermally, a sol whose raw particles have a maximum size less than 100 nm; C) washing the soil so as to obtain, after a possible drying step, a powder comprising more than 99.3% of said base component, in weight percentage on the basis of the dry matter. 2. Method according to the preceding claim, wherein said powder comprises more than 99.5% of said base component, in weight percent based on the dry matter. 3. Method according to the preceding claim, wherein said powder comprises more than 99.7% of said basic constituent, in weight percent based on the dry matter. 4. Method according to the preceding claim, wherein said powder comprises more than 99.9% of said base component, in weight percent based on the dry matter. The process according to any one of the preceding claims, wherein in step A) the amount of agent is greater than 104 mol / l. 6. Method according to the preceding claim, wherein in step A), the amount of agent is greater than 10-3 mol / l. 7. Method according to the preceding claim, wherein in step A), the amount of agent is greater than 10 -2 mol / l. 8. A method according to any one of the preceding claims, wherein step C) comprises at least one operation selected from - filtration, with or without resuspension of the soil, - dialysis, with or without resuspension soil pretreatment, - purification with ion exchange resins, with or without prior resuspension of the soil, - rinsing, - acid-base neutralization, - combinations of these techniques. 9. Method according to the preceding claim, wherein step C) comprises at least one acid-base neutralization. A process according to any one of the two immediately preceding claims, wherein the acid-base neutralization comprises suspending in an aqueous solution having a pH lower or higher than the zero charge point of said base component depending on whether the impurities at eliminate cationic or anionic, maintaining said suspension at a temperature below its boiling point at 1 bar for a period greater than 10 minutes, and filtration. 11. Method according to the preceding claim, wherein the pH is below or above said zero point of charge of at least 2 pH units. 12. Method according to the preceding claim, wherein the pH is less than or greater than said zero point of charge if possible of at least 4 pH units. 13. A process according to any one of the three immediately preceding claims, wherein the temperature is maintained less than 30 minutes. 14. A process according to any one of the four immediately preceding claims, wherein the pH is adjusted by adding hydrochloric acid and / or nitric acid and / or perchloric acid or adding ammonia. The process according to any one of claims 1 to 14, wherein step C) comprises at least one rinse comprising suspension in a solvent at a temperature below its boiling point at 1 bar for a period of time. more than 10 minutes, followed by filtration. 16. Method according to the preceding claim, wherein the rinsing is carried out in water having a degree of purity at least equal to that of the osmosis water or in an alcohol of purity greater than 98% by volume. 17. A method according to any one of the two immediately preceding claims, wherein the duration of the rinsing is between 10 and 30 minutes. A method according to any one of the preceding claims, wherein in step A) the base component and / or precursor of said base component comprises more than 99.5% of said base component and / or said precursor of said basic component, respectively, as a percentage by weight on the basis of the dry matter. 19. The method according to the preceding claim, wherein, in step A), the base component and / or the precursor of said base component comprise more than 99.8% of said base component and / or said precursor of said constituent of basis, respectively, as a percentage by mass on the basis of dry matter. The method according to the preceding claim, wherein, in step A), the base component and / or the precursor of said base component comprises more than 99.9% of said base component and / or said precursor of said component of said base component. basis, respectively, as a percentage by mass on the basis of dry matter. 21. A process according to any one of the preceding claims, wherein the base component and / or the precursor of the base component added to the mother liquor are chosen so that the basic constituent of the powder has, for more than 95 %, a monoclinic structure, in percentage by weight on the basis of dry matter. 22. Method according to the preceding claim, the basic constituent of the powder having, for more than 97%, a monoclinic structure. 23. Method according to the preceding claim, the basic constituent of the powder having, for more than 99%, a monoclinic structure. 24. Method according to the preceding claim, the basic constituent of the powder having, for 100%, a monoclinic structure. 25. A process according to any one of the preceding claims, wherein the base component or precursor of the base component added to the mother liquor is selected so that the base component is one of: HfO2, ZrO2, ThO2, Eu203, Sm203, Fe3O4, Bi2O3, SnO2, Nb2O5, As2O3, M003, WO3. 26. The method according to the preceding claim, wherein the base component or the precursor of the base component added in the mother liquor are chosen so that the base component is an oxide among: HfO2, ZrO2, MoO3, WO3. 27. The method according to the preceding claim, wherein the base component or the precursor of the base component added in the mother liquor are chosen so that the base component is an oxide among ZrO2 and / or HfO2. 28. The method according to the preceding claim, wherein the base component or the precursor of the base component added in the mother liquor are chosen so that the base constituent is zirconia ZrO2. 29. A process according to any one of the preceding claims wherein the agent is selected from the group consisting of SO42- sulfate, CO32- carbonate, PO43- phosphate, F- fluoride, Cl- chloride, perchlorate. C104, boron B033-, nitrate NO3, hydroxide OH- and mixtures thereof or, if the pH is less than 4 in the group consisting of Cl- chloride, perchlorate C104, nitrate NO3, and their mixtures. 30. The method according to the preceding claim, wherein the pH of the mother liquor is adjusted to be less than 4 if the basic constituent in question is selected from HfO 2, ZrO 2, ThO 2, Eu 2 O 3, Sm 2 O 3, Fe 3 O 4, Bi 2 O 3 or less than 2, if the basic constituent under consideration is selected from HfO 2, ZrO 2, ThO 2, Eu 2 O 3, Sm 2 O 3, Fe 3 O 4, Bi 2 O 3, SnO 2, Nb 2 O 5, As 2 O 3, MoO 3, WO 3, or greater than 10. 31. Process according to any one of the two immediately preceding claims, in which, in the case where the pH of the mother liquor is less than 4 and the agent is introduced in the form of an additive of formula MC, said additive is composed of: said agent M, and a complement C chosen from the group consisting of all the positively charged organic molecules, the organic radical of which consists of a group of atoms selected from the group consisting of carbon, hydrogen, oxygen, and nitrogen; cations of the elements of columns 1 and 2, cations of the following elements: Aluminum Al, Manganese Mn, Fe Iron, Yttrium Y, Copper Cu, Ag Silver, Cerium Ce; - ammonium NH4 + 32. Process according to any one of claims 29 and 30, wherein, in the case where the pH of the mother liquor is greater than 10 and the agent is introduced in the form of an additive, said additive of formula MC is composed of: said agent M and a complement C selected from the group consisting of all of the positively charged organic molecules, the organic radical of which consists of a group of atoms selected from the group consisting of carbon, hydrogen, oxygen, and nitrogen; - cations of the elements of column 1; - ammonium NH4 + 33. Process according to any one of the preceding claims, in which, in step B), the mother liquor is heated in a closed vessel at a temperature above the boiling point at 1 bar and between 100 ° C and 300 ° C, the temperature being maintained at least one hour and the rate of temperature rise being less than 250 ° C / h. 34. The method according to any of the preceding claims, wherein the parameters are adjusted so that the primary particles of said powder have maximum sizes greater than 10 nm. 25 5 10 15 20 30 35 35. Process according to any one of the preceding claims, adapted so that the manufactured powder has an impurity content such that, in percentages by mass of dry matter: SiO 2 <0.1%, and / or Al 2 O 3 <0.1 %, and / or MgO <0.1%, and / or CaO <0.1%, and / or Na2O <0.1%, and / or Cl- <0.1%, and / or NO3 <0, 1%, and / or SO42- <0.1%, and / or TiO2 <0.1%. 36. Process according to the preceding claim, in which the impurities are such that, in percentages by mass SiO 2 <200 ppm, and / or - Al 2 O 3 <200 ppm, and / or - MgO <200 ppm, and / or - CaO <200. ppm, and / or -Na2O <500 ppm, and / or - Cl- <500 ppm, and / or - NO3 <500 ppm, and / or - SO42- <500 ppm and / or - TiO2 <200 ppm. 37. Process according to the preceding claim, in which the impurities are such that, in percentages by mass of dry matter: SiO 2 <100 ppm, and / or Al 2 O 3 <100 ppm, and / or MgO <50 ppm, and / or CaO < 100 ppm, and / or Na2O <100 ppm, and / or Cl- <100 ppm, and / or NO3 <100 ppm, and / or SO42- <100 ppm and / or Fe2O3 <50 ppm and / or TiO2 <50 ppm . dry matter: 38. A method according to any one of the preceding claims, comprising an additional step subsequent to step C), wherein the particles of said powder are doped with a dopant selected from the group consisting of oxoanions, column 17, cations of column 1, and mixtures thereof. 39. Process according to the preceding claim, in which the dopant is chosen from silicates, phosphates, sulphates, chlorides, fluorides, sodium and potassium. 40. Powder manufactured according to a process according to any one of the preceding claims. 41. Process using a powder according to the preceding claim, for catalyzing a chemical reaction. 42. Process according to the preceding claim, the primary particles of said powder not being covered with catalyst, for catalyzing a reaction chosen from hydrocarbon conversion reactions including selective oxidation reactions, hydrogenation reactions, reactions. dehydrogenation reactions, hydrogenolysis reactions, isomerization reactions, dehydrocyclization reactions, and reforming reactions, or, after at least partial recovery, of said primary particles with a catalyst, - when the catalyst is selected in the group formed by the metals of columns 8, 9 and 10 of the periodic table of the elements and having a cubic structure, and the mixtures thereof, and especially when the basic constituent of the support particles is zirconia, to catalyze a selected reaction in the following group: a. a response where more than 26%, or even more than 35%, or more than 51% of exposed shots belong to the {1,0,0} family and / or more than 35%, or even more than 45%, or more than 66% or more than 70% of the exposed planes belong to the family {1,1,1}, in percentages based on the total exposed surface of the catalyst, are sought and / or, b. a reaction selected from hydrocarbon conversion reactions including selective oxidation reactions, hydrogenation reactions, dehydrogenation reactions, hydrogenolysis reactions, isomerization reactions, dehydrocyclization reactions, and reforming reactions , c. selective hydrogenation reactions, and particularly selective hydrogenation reactions of molecules containing at least one carbonyl function CO, and reactions of selective hydrogenation of molecules containing at least two double bonds; d. methane formation reactions; e. Fischer-Tropsch synthesis reactions; regardless of the catalyst, provided that the base component of the carrier particles is monoclinic, to catalyze a reaction selected from hydrocarbon conversion reactions including selective oxidation reactions, hydrogenation reactions, reaction reactions, dehydrogenation, hydrogenolysis reactions, isomerization reactions, dehydrocyclization reactions, and reforming reactions.