FR2984304A1 - ALUMINA MATERIAL WITH A MULTICHARGE STRUCTURE COMPRISING ALUMINUM OXIDE BINDER HAVING GOOD MECHANICAL RESISTANCE AND PROCESS FOR PREPARING THE SAME - Google Patents

Abstract

A material, in the form of a millimetric object, consisting of large alumina particles having a median diameter of between 10 and 200 μm, said large particles being spaced, from one interface to another, of less than 10 μm small particles of alumina having a median diameter of between 0.5 and 10 μm, said small particles being distant, from one interface to another, of less than 5 μm and being located in the space between the large ones particles and a binder consisting of aluminum oxide, said binder being located in the space between said small and large particles, said material having a mesoporous volume, measured by mercury porosimetry, between 0.2 and 2 mL / g, a macroporous volume, measured by mercury porosimetry, of between 0.05 and 0.2 mL / g and a BET specific surface area of between 90 and 350 m 2 / g.

Description

TECHNICAL FIELD The present invention relates to the field of alumina-based materials, in the form of a millimetric object, porous and having good mechanical strength. More specifically, it relates to a multiscale structure material resulting from a hierarchical assembly of small and large particles preferentially spherical, micrometric and porous alumina-based and a binder consisting of aluminum oxide as well as its preparation process. PRIOR ART Alumina-based materials consisting of up to a single class of particles bound together by a binder in the form of millimetric and porous objects are widely described in the prior art, such as, for example, in patent US6780817. The particles that constitute these materials are particles of alumina or particles originating from solid precursors of alumina (oxides, oxyhydroxides or aluminum hydroxides), and in this case which do not disperse during the process of setting form, and which will therefore be present within the final material. The binder used is also a solid precursor of alumina but with a better dispersion capacity than those of the particles, during the forming process. The textural properties (BET surface area, mesoporous and macroporous volume, mesopore diameter) of the material are directly related to the textural properties of the particles and the binder used in the preparation of said materials. In addition, the mesoporous and macroporous volumes are influenced by the amount of particles and binder. In fact, a large macroporous volume can be obtained if the binder content is low, and in this case the mechanical properties of the material are strongly degraded. An improvement in the mechanical strength can be sought by increasing the binder content, which implies a decrease in the macroporous volume but also a decrease in the amount of particles within the material. In this case, the mesoporosity of the material may be degraded due to a small amount of porous particles, and therefore the mesoporosity provided to the material and from said particles is less important. The characteristics of the mesopores are attributed to the organization of the alumina crystallites which depends on the preparation process and in particular on the synthesis of the alumina precursor or on the forming of millimetric objects. The improvement of the textural properties of said particles such as the increase of the specific surface area, the mesoporous volume or the size of the mesopores may be sought and obtained by synthesis or post-synthesis treatment. However, these characteristics partially degrade during formatting.

OBJECT AND INTEREST OF THE INVENTION The present invention relates to a material based on alumina, in the form of a millimetric object, with a multiscale structure, porous and having a good mechanical strength which results from a hierarchical assembly of small and large particles micrometric and porous alumina and a binder consisting of aluminum oxide ensuring the cohesion of said material and its preparation process. The interest of the invention relates to the hierarchical organization of small and large micrometric particles porous alumina, which maximizes the content of particles within the material and therefore, to increase the mesoporous volume of said material. The hierarchical organization also minimizes the amount of binder because the space between the large particles is filled by the small particles, and therefore, the binder content necessary to ensure the cohesion of the material is minimized. This binder makes it possible to ensure a minimal macroporous volume but sufficient for accessibility to the porosity of said micrometric particles. On the other hand, the hierarchical organization ensures proper mechanical properties. Moreover, an improvement of these properties can be obtained by the use of a specific binder. In particular, an object of the present invention relates to a material, in the form of a millimeter object, consisting of large particles of alumina having a median diameter of between 10 and 200 μm, said large particles being distant, from an interface to the other, less than 10 μm, small particles of alumina having a median diameter of between 0.5 and 10 μm, said small particles being spaced from one interface to another by less than 5 μm and being located in the space between the large particles, and a binder consisting of aluminum oxide, said binder being located in the space between said small and large particles, said material having a mesoporous volume, measured by mercury porosimetry , between 0.2 and 2 mUg, a macroporous volume, measured by mercury porosimetry, of between 0.05 and 0.2 mUg and a BET specific surface area of between 90 and 350 m 2 / g.

Another subject of the present invention relates to a process for the preparation of said material which comprises at least the following steps: a) the mixing of 10 to 60% by weight of small alumina particles with a median diameter of between 0.5 and 10 μm and from 40 to 90% by weight of large alumina particles having a median diameter of between 10 and 200 μm, 5 to 50% by weight of a solid alumina precursor, the percentages of small and large alumina particles being expressed with respect to the total amount of micrometric particles, and the percentage of solid precursor of alumina being expressed relative to the total amount of solid, and an acid in solution selected from acetic acid, hydrochloric acid , sulfuric acid, formic acid, citric acid and nitric acid, alone or as a mixture, in proportions such that the ratio of the mass of acid introduced into the solid precursor mass of introduced alumina between 2.5 and 115% by weight; b) shaping the mixture from step a); c) drying the raw material obtained at the end of step b) and; d) calcination of the dried material from step c).

An advantage of the present invention is to provide a material having a maximized proportion of small and large porous particles due to the hierarchical organization of said particles within the material and thus to provide a material having a very wide range of texture, in particular a mesoporous volume can reach the value of 2mUg much greater than the mesoporous volume values generally observed for alumina-based materials consisting of up to a single class of particles collected by a binder of the prior art. Another advantage of the present invention is to provide a material having enhanced strength properties. This is due to the combined effect of the hierarchical organization of the small and large particles of alumina material according to the invention and the cohesion of said material provided by the presence of a specific binder. In particular, the material according to the invention has a maximized lateral crushing strength up to very high values of force, up to 100 N for a material according to the invention of cylindrical shape, with a diameter of 2 mm. and of length equal to 4 mm, which are force values much greater than those observed for alumina materials consisting of a single class of particles collected by a binder of the prior art. Another advantage of the present invention is also to provide a method of preparation for obtaining said material having increased mechanical strength properties by mixing in a step a) micrometric particles of alumina with a solid precursor of alumina and an acidic solution having a very high ratio (introduced acid mass) / (mass of introduced alumina solid precursor), leading to the in situ generation of a specific binder at the end of a calcination step dried and shaped material.

Another advantage of the present invention is also to provide a method of preparation allowing: a good sliding of the micrometric particles of alumina with respect to each other during the mixing step a) which limits on the one hand, the degradation of the particles and thus maintains a high porosity of the material and which also limits the generation of micrometric defects within the mixture and thus improves its mechanical strength; a good flow of the mixture obtained in step a) during step b) which limits the generation of geometric defects and micrometric defects on the surface of the raw material thus improving the appearance and strength of the material as a millimetric object obtained according to the invention; the formation of defects related to the densification of the binder during steps c) and d) is minimized because of its location in the small space between the micrometric particles of alumina which improves the mechanical strength of the material in form millimetric object obtained according to the invention. DESCRIPTION OF THE INVENTION The present invention relates to a material, in the form of a millimeter object, consisting of large particles of alumina having a median diameter of between 10 and 200 μm, said large particles being distant, from an interface to the other, less than 10 μm, small alumina particles having a median diameter of between 0.5 and 10 μm, said small particles being distant, from one interface to another, of less than 5 μm and being localized in the space between the large particles and a binder consisting of aluminum oxide, said binder being located in the space between said small and large particles, said material having a mesoporous volume, measured by mercury porosimetry, included between 0.2 and 2 ml / g, a macroporous volume, measured by mercury porosimetry, of between 0.05 and 0.2 ml / g and a BET specific surface area of between 90 and 350 m 2 / g. Throughout the remainder of the text, by mesoporous volume is meant the volume of pores whose diameter is between 3.6 and 50 nm. According to the invention, the mesoporous volume is measured by mercury porosimetry, the method of which is described below. More particularly, the term mesoporous volume, a pore volume corresponding to the volume of mercury which is entered into the pores with a diameter of between 3.6 and 50 nm. Throughout the remainder of the text, the term macroporous volume, the volume of pores whose diameter is between 50 and 7000 nm. According to the invention, the macroporous volume is measured by mercury porosimetry, the method of which is described below. More particularly, by macroporous volume, a pore volume corresponding to the volume of mercury which is entered into the pores with a diameter of between 50 and 7000 nm. Throughout the rest of the text, the volume measured by mercury porosimetry is the volume measured by the mercury penetration technique in which the Kelvin-Washburn law is applied, which gives the relationship between the pressure and the diameter of the smallest pore in which the mercury penetrates at said pressure, the wetting angle and the surface tension according to the formula: P = (4ycos0) / d, where y is the surface tension, 0 is the wetting angle of the mercury with the material, d represents the diameter of the pore, and P the pressure. Mercury porosimetry is performed according to ASTM D4284-83, using a surface tension of 480 dyne / cm and a contact angle of 140 °. The angle of wetting was taken equal to 140 ° following the recommendations of the book "Techniques of the Engineer, treated analysis and characterization, P 1050-5, written by Jean Charpin and Bernard Rasneur".

Throughout the rest of the text, BET specific surface area is understood to mean a specific surface area determined by nitrogen adsorption according to ASTM D 3663-78 established from the BET method (Brunauer-Emmett-Teller) described in the periodical. "The Journal of the American Society", 60, 309 (1938).

The large particles and the small particles of alumina which constitute the material according to the invention are, by their size, micrometric particles. The material according to the present invention is advantageously constituted by the hierarchical organization of small and large micrometric porous particles of alumina.

The term "micrometric particles" refers throughout the text to all large and small particles. The total amount of micrometric particles is therefore the sum of large and small particles of alumina. Said micrometric particles which constitute the material according to the invention are transition aluminas. In particular, the micrometric alumina particles are transition aluminas chosen from chi, kappa, rho, eta, theta, gamma and delta alumina, alone or as a mixture, and preferably the transition alumina is alumina. gamma. The small and large particles or micrometric particles that constitute the material according to the present invention are advantageously porous particles. Preferably, said particles are of rounded shape and even more preferably of spherical shape.

Said small and large particles are advantageously agglomerates of nanometric crystallites of alumina. These crystallites are in the form of platelets, fibers or "blocks" whose dimensions are advantageously between 2 and 150 nm, preferably between 4 and 150 nm and preferably between 4 and 100 nm.

The crystallites of alumina are the elementary entities which constitute the micrometric particles. The inter-crystallite space is responsible for the presence of the mesoporosity within said particles. The size of the crystallite agglomerates constituting the micrometric particles is analyzed by scanning electron microscopy (SEM). In particular, the average size of the crystallite agglomerates that make up the micrometric particles is determined from scanning electron microscopy (SEM) images of the material according to the invention using SMile View software. SEM images must be made on the material in fracture mode and with a secondary electron detector.

According to the invention, the large particles of alumina constituting the material according to the invention have a median diameter of between 10 and 200 μm, preferably between 10 and 100 μm, and preferably between 10 and 80 μm. The term "median diameter" refers to the D50, i.e., the diameter of the equivalent disc such that 50% by number of said coarse particles has a size less than said diameter. The distribution of the particle size of said large particles of alumina within the material according to the invention is represented by the dimension DX, defined as the diameter of the equivalent disc such that X% in number of said large particles has a size less than said diameter. More precisely, the distribution of the particle size of said large particles is advantageously represented by the three dimensions D10, D50 and D90. Said material according to the invention advantageously has a distribution of the particle size of said large particles of alumina within said material such that the diameter D10 is at most 3 times lower than the median diameter D50 and preferably at most 2 times smaller than the median diameter D50 and such that the diameter D90 is at most 3 times greater than the median diameter D50 and preferably at most 2 times greater than the median diameter D50. The diameter of the large particles is determined, in the material according to the invention, from scanning electron microscopy (SEM) images of the material according to the invention, on a polished section, using the SMile View software. The median diameter is calculated from the average of the diameters determined on a number of measurements that the person skilled in the art deems necessary and sufficient and preferably on at least 200 measurements.

According to the invention, said large particles are spaced, from one interface to another, less than 10 μm, preferably less than 8 μm, and preferably less than 6 μm. The distance between the interface of a large particle and the interface of the closest large particle is determined from scanning electron microscopy (SEM) images of the material according to the invention, on a polished section, at using the SMile View software. The measurements are made on a number of measurements that the person skilled in the art deems necessary and sufficient and preferably on 50 images in order to determine an average distance from one interface to the other. Said large particles advantageously have a median diameter less than 1/10 times the mean diameter of the smallest dimension of the material according to the invention, and preferably less than 1/50 times the average diameter of the smallest dimension of said material, so to ensure complete filling of the space of said material. The large particles constituting the material according to the invention can advantageously be composed of two populations of particles having distinct median diameters. Said large particles may optionally comprise a proportion of particles, called "lower fat" particles, having a median diameter of between 10 and 60 μm and preferably between 15 and 30 μm and a proportion of particles called "larger" particles having a median diameter between 60 and 200 pm and preferably between 100 and 180 pm.

In the case where said large particles are composed of "coarse" particles and "larger" particles, said material according to the invention advantageously has a distribution of the particle size of said "coarse" particles of alumina within said material such as that the diameter D10 is at most 3 times smaller than the median diameter D50 and preferably at most 2 times less than the median diameter D50 and such that the diameter D90 is at most 3 times greater than the median diameter D50 and preferably at most 2 times greater at the median diameter D50 and a distribution of the particle size of said "larger" particles of alumina in said material such that the diameter D10 is at most 3 times smaller than the median diameter D50 and preferably at most 2 times less than the median diameter D50 and such that the diameter D90 is at most 3 times greater than the median diameter D50 and preferably at most 2 times greater than the median diameter D50. According to the invention, the small particles which constitute the material according to the invention have a median diameter of between 0.5 and 10 μm, preferably between 0.5 and 6 μm and, preferably, between 0.5 and 3 μm. pm.

The term "median diameter" refers to the D50, i.e., the diameter of the equivalent disc such that 50% by number of said small particles has a size smaller than said diameter. The distribution of the particle size of said small particles of alumina within the material according to the invention is represented by the dimension DX, defined as the diameter of the equivalent disk such that X% by number of said small particles has a size smaller than said diameter. More precisely, the distribution of the particle size of said small particles is advantageously represented by the three dimensions D10, D50 and D90. Said material according to the invention advantageously has a distribution of the particle size of said small particles of alumina within said material such that the diameter D10 is at most 3 times smaller than the median diameter D50 and preferably at most 2 times smaller than the median diameter D50 and such that the diameter D90 is at most 3 times greater than the median diameter D50 and preferably at most 2 times greater than the median diameter D50. The diameter of said small particles is determined, in the material according to the invention, from scanning electron microscopy (SEM) images of the material according to the invention on a polished section, using SMile View software. The median diameter is calculated from the average of the diameters determined on a number of measurements that the person skilled in the art deems necessary and sufficient and preferably on at least 200 measurements. According to the invention, said small particles are spaced, from one interface to another, less than 5 μm, preferably less than 3 μm, and preferably less than 2 μm. The distance between the interface of a small particle and the interface of the smallest particle closest, is determined by scanning electron microscopy (SEM) images of the material according to the invention, on a polished section, using SMile View software. The measurements are made on a number of measurements that the person skilled in the art deems necessary and sufficient and preferably on 50 images in order to determine an average distance from one interface to the other. Said small particles advantageously have a median diameter less than 1/5 times the median diameter of the large particles, and preferably a median diameter less than 1/10 times the median diameter of the large particles. This makes it possible to fill the space between said large particles, said small particles being located in the space between said large particles. In addition, the proportion of said small particles is determined so as to fill the space between said large particles. The proportions of the small and large particles of alumina constituting the material according to the invention are expressed as surface percentages with respect to the total surface area of the SEM image of the material according to the invention, so as to indicate the area that the small and large particles occupy within the material, as well as the proportion of coarse fat particles having a median diameter of between 10 and 60 μm. The surface percentages of the micrometric particles are determined from the SEM image analysis of the material according to the invention, on a polished section. The resolution of the scanning electron microscope (SEM) is advantageously adapted to the median diameter of said micrometric particles so as to have an image representative of all the material according to the invention. The area occupied by the small particles is determined as follows: the diameter of all the small particles is measured using SMile View software, in order to calculate the area occupied by each of said small particles. The sum of each of the areas occupied by each of said small particles corresponds to the total area occupied by all of said small particles in the SEM image. The area occupied by the large particles is determined from the GIMP and Analysis software as follows: the edges of each of the large particles are delimited using the GIMP software. The area occupied by all of said large particles is then expressed as a percentage from the Analysis software. The proportion of small and large particles occupying the surface of the SEM image of the material according to the invention is expressed as a percentage by surface area relative to the total surface area of the SEM image of the material according to the invention. It is calculated according to the formula: (area occupied by small or large particles / total area of the SEM image) x100. The surface percentages are determined on a number of images that those skilled in the art deem necessary and sufficient and preferably on 10 images.

The surface percentage of the small particles relative to the total surface of the SEM image of the material according to the invention is advantageously between 5 and 55%, preferably between 10 and 45% and preferably between 10 and 30%. The surface percentage of the large particles is advantageously between 35 and 85%, preferably between 45 and 80% and preferably between 60 and 80% relative to the total surface of the SEM image of the material according to the invention. The complement of the surface fraction, that is to say, the sum of the surface percentages of small and large particles subtracted from 100%, corresponds to the surface percentage occupied by the binder consisting of aluminum oxide and the residual vacuum. . The presence of residual vacuum in the SEM image, that is to say unfilled space, is related to the existence of a macroporous volume within the material. According to the invention, said small particles are located in the space between the large particles. This assembly of small and large particles is at the origin of the hierarchical organization of said micrometric particles within the material. Said material according to the invention advantageously has a hierarchical organization. The hierarchical organization of said micrometric particles within said material leads to a very wide range of texture, in particular of mesoporous volume measured by mercury porosimetry on said material of between 0.2 and 2 ml / g.

According to the invention, a binder consisting of aluminum oxide is located in the space between small and large particles. Preferably, said small particles fill the gap between the coarse particles and said binder fills the residual space between the small and coarse particles and the residual space between the small particles. Said binder fills the space between said micrometric particles so as to ensure the cohesion of the material according to the invention, and consequently, so as to minimize the residual vacuum, that is to say the unfilled space between said particles micrometric. The existence of residual vacuum is responsible for the presence of a macroporous volume within the material. The binder present in the space between the micrometric particles makes it possible to minimize the macroporous volume within the material according to the invention so that said macroporous volume of said material is between 0.05 and 0.2 m 2 / g, of preferably between 0.07 and 0.17 ml / g, and preferably between 0.1 and 0.15 ml / g. Moreover, the binder must not interfere with the hierarchical organization of small and large particles, that is to say, that the binder must not move the micrometric particles apart from each other. Thus, said binder makes it possible to maintain a distance from one interface to another between the small particles of less than 5 μm, preferably less than 3 μm, and preferably less than 2 μm, and a distance from an interface to the other between large particles less than 10 μm, preferably less than 8 μm and preferably less than 6 μm. Thus, the cohesion of the micrometric particles is ensured by the presence of said binder consisting of aluminum oxide.

Said binder is advantageously present in the material according to the invention in the form of particles having a mean size of less than 1 μm. The size of the particles constituting the binder can advantageously be measured, in the material according to the invention, by SEM image analysis of the material according to the invention, with the aid of SMile View software. The SEM images must be made on the material according to the invention in fracture mode and with a secondary electron detector. According to the invention, said material has a mesoporous volume measured by mercury porosimetry of between 0.2 and 2 mUg, preferably between 0.25 and 1.5 mUg and preferably between 0.25 and 1, 0 mUg. According to the invention, said material has a macroporous volume measured by mercury porosimetry of between 0.05 and 0.2 mUg, preferably between 0.07 and 0.17 mUg and preferably between 0.1 and 0.15 mUg. Preferably, said material has a median mesopore diameter measured by mercury porosimetry between 8 and 25 nm, preferably between 10 and 25 nm and preferably between 10 and 23 nm. According to the invention, said material has a BET specific surface area of between 90 and 350 m 2 / g, preferably between 100 and 300 m 2 / g and preferably between 130 and 280 m 2 / g. According to the invention, said material is in the form of a millimeter object. Preferably, the average diameter of the smallest dimension of the material according to the invention is advantageously between 0.5 and 10 mm, preferably between 0.7 and 5 mm, and preferably between 1.0 and 4.0. mm. The dimensions of the millimetric object can be obtained with any measuring tool adapted to the dimensions of said material and in particular with a vernier caliper. Said material is advantageously in the form of a granule or an extrusion and preferably in the form of an extrudate.

The material according to the invention thus has maximized proportions of micrometric particles porous because of this hierarchical organization, resulting in a porous material and having good mechanical strength. The material according to the present invention has excellent strength properties.

This is due to the combined effect of the hierarchical organization of the small and large particles of alumina material according to the invention and the cohesion of said material provided by the presence of a specific binder. In particular, the material according to the invention has a mechanical resistance to lateral crushing determined by the grain-to-grain (EGG) crushing test advantageously between 50 and 100 N, preferably between 60 and 100 N and so preferred between 80 and 100 N, expressed in force in the case of the materials according to the invention in the form of granules; and a mechanical resistance to lateral crushing advantageously between 0.8 and 2.5 daN / mm, preferably between 0.85 and 2 daN / mm and preferably between 0.85 and 1.8 daN / mm, expressed as force per unit length in the case of the materials according to the invention in the form of extrudates. By mechanical resistance to lateral crushing, the mechanical strength of the material according to the invention determined by the grain-to-grain (GGE) crushing test. This is a standardized test (ASTM D4179-01) that involves subjecting a material in the form of a millimeter object, such as a granule or extrusion, to a compressive force that causes the rupture. This test is therefore a measure of the tensile strength of the material. The analysis is repeated on a number of solids taken individually and typically on a number of solids between 10 and 200. The average of the lateral forces of rupture measured is the average EGG which is expressed in the case of the granules in unit of force (N), and in the case of extrusions in unit of force per unit length (daN / mm). Another object of the present invention relates to a process for preparing said material comprising at least the following steps: a) mixing 10 to 60% by weight of small alumina particles with a median diameter of between 0.5 and 10 μm and 40 to 90% by weight of large particles of alumina with a median diameter of between 10 and 200 μm, of 5 to 50% by weight of a solid alumina precursor, the percentages of small and large particles of alumina being expressed relative to the total amount of micrometric particles, and the percentage of solid precursor of alumina being expressed relative to the total amount of solid, and an acid in solution selected from acetic acid, the hydrochloric acid, sulfuric acid, formic acid, citric acid and nitric acid, alone or as a mixture, in proportions such as the ratio of the mass of acid introduced into the solid precursor mass of alumina introduced it is between 2.5 and 115% by weight, b) the shaping of the mixture resulting from step a), c) the drying of the green material obtained at the end of step b) and, d) calcination of the dried material from step c). The large particles and the small particles of alumina, referred to as the "micrometric particles" used in the mixing step a) of the preparation process according to the invention are advantageously transition aluminas chosen from chi, kappa alumina. , rho, eta, theta, gamma and delta, alone or in a mixture, and preferably gamma-alumina, or are advantageously solid precursors of alumina chosen from hydrargillite, gibbsite, nordstrandite, bayerite, boehmite or pseudo-boehmite. In the case where the micrometric particles are solid precursors of alumina, said precursors must be little dispersible in the acid solution introduced in the mixing step a) of the preparation process, that is to say in such a way that that the median diameter of said micrometric particles is little reduced during step a) and that the hierarchical organization of small and large particles can be observed within the material at the end of step d) of preparation according to the characterization techniques described in detail above. In this case, at the end of step d) of calcination, a phase transformation takes place so as to obtain the transition aluminas within said material. Throughout the rest of the description of the preparation process, the micrometric alumina particles used in the mixing step a) of the preparation process are transition aluminas or solid precursors of alumina which are poorly dispersible in the acid solution used. in said step a) of the preparation process, and which will be converted to transition alumina at the end of step d) of calcination.

In step a) of mixing the preparation process according to the invention, small particles of alumina with a median diameter of between 0.5 and 10 μm are mixed with large particles of alumina with a median diameter of between 10 and 200 pm. Preferably, said large particles of alumina have a median diameter of between 10 and 100 μm, and preferably between 10 and 80 μm.

Said large particles mixed in step a) of the preparation process according to the invention may advantageously be composed of two populations of particles having distinct median diameters. Said large particles may optionally comprise a proportion of particles, called "lower fat" particles, having a median diameter of between 10 and 60 μm and preferably between 15 and 30 μm and a proportion of particles called "larger" particles having a median diameter between 60 and 200 pm and preferably between 100 and 180 pm. Preferably, said small alumina particles have a diameter of between 0.5 and 6 μm and preferably between 0.5 and 3 μm.

Said micrometric alumina particles mixed in step a) of the preparation process are advantageously used in powder form. The size distribution of said micrometric alumina particles in the powders used in said mixing step a) is measured by laser diffraction granulometry, based on the Mie diffraction theory (GBJ de Boer, C. de Weerd, D Thoenes, HWJ Goossens, Charact 4 (1987) 14-19). The distribution of the particle size of the micrometric alumina particles in the powders is represented by the dimension DvX, defined as the diameter of the equivalent sphere such that X% by volume of said particles has a size less than said diameter.

More precisely, the distribution of the particle size of said particles is represented by the three dimensions Dv10, Dv50 and Dv90. The term "median diameter" used in the description of the preparation process according to the invention denotes the Dv50, that is to say the diameter of the equivalent sphere such that 50% by volume of said particles has a size less than said diameter.

The distribution of the particle size of the small particles is such that the diameter Dv10 is at most 3 times smaller than the median diameter Dv50 and preferably at most 2 times less than the median diameter Dv50 and such that the diameter Dv90 is at most 3 times greater than the diameter. median Dv50 and preferably at most 2 times greater than the median diameter Dv50.

The distribution of the particle size of the larger particles and the particle size distribution of the "coarse" particles is advantageously identical to the particle size distribution of the small particles. Thus, the particle size distribution of the small particles is preferably monomodal, and the particle size distribution of the coarse particles may be monomodal or bimodal, in the case where said coarse particles are composed of said larger and lower coarse particles. The textural properties of said micrometric alumina particles, such as the BET surface area, the mesoporous volume and the mesopore median diameter, are respectively determined by nitrogen adsorption and mercury porosimetry. The BET surface area, the mesoporous volume and the pore size distribution of the micrometric particles constituting the material according to the invention can be very variable depending on the process for preparing the alumina precursor used to prepare the micrometric particles and the operating conditions. .

Said micrometric particles advantageously have a BET specific surface area of between 100 and 480 m 2 / g, preferably between 120 and 300 m 2 / g, and preferably between 120 and 250 m 2 / g. Said micrometric particles advantageously have a mesoporous volume of between 0.2 and 2.5 micrograms, preferably of between 0.3 and 2.0 ml / g, and preferably of between 0.3 and 1.5 ml / g. . Said micrometric particles advantageously have a median mesopore diameter of between 7 and 25 nm, preferably between 7 and 24 nm, and preferably between 7.5 and 23 nm. The proportions of small and large particles introduced into the mixture of step a) is an essential criterion of the preparation process according to the present invention because said proportions define the compactness of the mixture of small and large particles. Said proportions lead to the hierarchical organization of small and large particles within the material obtained at the end of said preparation process. More specifically, the proportions of small and large particles are chosen so as to have a mixture of small and large particles which leads to a satisfactory compactness. Said compactness of the mixture of small and large particles is determined by a conventional packed fill density (TDD) measurement. At the end of this measurement, it is possible to calculate the compactness (C) of the mixture of powders from the formula: C DRT / Dg, where DRT is the packed filling density of the mixture of powders and Dg is the density grain of said mixture of powders. From the compactness (C), it is possible to determine the porosity (E) of the mixture of powders where E = 1 - C. The porosity gives information on the volume of voids between the small and the large particles, and consequently, allows to define the proportion of binder necessary to fill the space between said small and large particles. This also makes it possible to define the proportion of solid precursor of alumina necessary to obtain said proportion by binding.

According to step a) of mixing the preparation process according to the invention, 10 to 60% and preferably 15 to 35% by weight of small alumina particles with a median diameter of between 0.5 and 10 μm and 40 to 90% and preferably 65 to 85% by weight of large alumina particles with a median diameter of between 10 and 200 μm, the percentages being expressed relative to the total amount of micrometric particles. If the proportion of coarse particles is less than 40% by weight, the hierarchical organization of micrometric particles of alumina will not be optimal given the large amount of small particles and the maximization of the proportion of micrometric particles of alumina. can not be reached because of a less compact organization. On the other hand, if the proportion of large particles is too high and in particular greater than 90% by weight, the quantity of small particles will not be sufficient to fill the voids between the large particles, and the hierarchical organization of micrometric particles of Alumina will not be optimal because of a less compact organization.

Another essential criterion of the preparation process according to the invention is the introduction into step a) of a mixture of a solid precursor of alumina and of an acid in solution allowing the in situ generation of a binder consisting of of aluminum oxide, after step d) of calcination of the preparation process according to the invention. Moreover, the solid precursor of alumina and the acid in solution must be introduced in said step a) in the proportions as claimed. The generation of said binder consisting of aluminum oxide requires a solid precursor of alumina which in the presence of the acidic solution is more reactive than the micrometric alumina particles also introduced in step a) of mixing the preparation process according to the invention. Indeed, the solid precursor of alumina must be capable of dispersing predominantly or dissolving predominantly in the acid solution used and forming particles of average size less than 1 .mu.m between the micrometric particles. In particular, the micrometric alumina particles must be slightly degraded in the presence of said acid solution, that is to say that the median diameter of said alumina particles must be maintained in the presence of the acid in solution introduced in the presence of during step a) of the preparation process according to the invention. The solid precursor of alumina is advantageously chosen from aluminum oxides, aluminum hydroxides and aluminum oxyhydroxides which are soluble or dispersible in the acid solution employed, preferably from aluminum hydroxides and aluminum oxyhydroxides. . Very preferably, said solid alumina precursor is an aluminum oxyhydroxide and more preferably said solid alumina precursor is boehmite or pseudo-boehmite. Said alumina precursor is advantageously in the form of a powder consisting of solid particles having a median diameter, determined by laser diffraction granulometry, between 9 and 80, preferably between 10 and 60 mm and preferably between 15 and 45. m. The particles of the solid precursor of alumina are advantageously constituted by agglomerates of elementary units, called crystallites, whose dimensions are advantageously between 2 and 150 nm, preferably between 4 and 150 nm and preferably between 4 and 100 nm. determined by transmission electron microscopy (TEM). The morphology of the crystallites, the size and the manner in which the crystallites are organized depend mainly on the synthesis route of the alumina precursor used to prepare said micrometric particles. According to step a) of the preparation process according to the invention, the proportion of the solid precursor of alumina is between 5 and 50% by weight relative to the total amount of solid particles, preferably between 10 and 30% by weight, and preferably between 15 and 25% by weight. The total amount of solid particles is the sum of the micrometric particles (small and large alumina particles) and particles of the solid alumina precursor. If the amount of solid precursor of alumina is less than 5% by weight relative to the total amount of solid particles introduced, the amount of aluminum oxide binder which will be formed in situ after step d) of calcination, will not be sufficient to ensure the cohesion of the hierarchical organization of small and large micrometric particles of alumina. Moreover, if the amount of solid precursor of alumina is greater than 50% by weight relative to the total amount of solid particles, the amount of binder consisting of aluminum oxide which will be formed in situ after step d ) of calcination will be greater than the amount necessary to fill the residual voids between the micrometric particles, and therefore, the micrometric particles will be dispersed unorganized within said binder and the proportion of micrometric particles present in the material obtained in the resulting from the preparation process according to the final invention will not be maximum.

According to step a) of the preparation process according to the invention, an acid in solution is chosen from acetic acid, hydrochloric acid, sulfuric acid, formic acid, citric acid and nitric acid, alone or as a mixture, is introduced, provided that after step d) of the preparation process according to the invention, said acid is decomposed and lead to a binder consisting of aluminum oxide . Preferably, said acid is nitric acid. The role of the acid solution is to promote the dispersion of the particles which constitute the solid precursor of alumina. In this way, the particles of the solid precursor of alumina which initially have a median diameter of about ten micron become submicron colloidal particles, thanks to the action of the acid solution associated with the mechanical energy provided during step a) of mixing the process for preparing the material according to the invention. These colloidal particles re-aggregate to form particles having an average size of less than 1 μm. According to step a) of the preparation process according to the invention, the solid precursor of alumina and the acid in solution chosen from acetic acid, hydrochloric acid, sulfuric acid and formic acid. the citric acid and the nitric acid, alone or as a mixture, are introduced in proportions such that the ratio of the mass of acid introduced into the solid precursor mass of introduced alumina is between 2.5 and 115; % by weight, preferably between 7.4 and 74% by weight and preferably between 30 and 62% by weight. The use of a high acid ratio (mass of introduced acid) / (mass of introduced alumina solid precursor) in step a) of the preparation process according to the invention makes it possible at the same time to disperse well the particles having a median diameter of about ten micron which constitute the solid precursor of alumina in submicron colloidal particles, but also to promote the partial dissolution of said solid precursor of alumina and in a minority, micrometric particles alumina. This high acid ratio dissolution favors the formation of alumina species which modify the rheology of the mixture resulting from stage a) of the preparation process according to the invention and also act as binders in stages c) and d). ) of heat treatments of said preparation process. The presence of aluminum species makes it possible to improve the adhesion between the micrometric particles of alumina and the binder constituted by aluminum oxide formed at the end of step d) of calcination. Consequently, this makes it possible, after step d) of calcination, to increase the cohesion of the hierarchical organization of the small and large micrometric particles of alumina and thus of the material obtained by the preparation process according to the invention. 'invention. In the case where the shaping of the mixture resulting from step a) is carried out by extrusion in step b), an organic binder may optionally be added in step a) of mixing. The presence of said organic binder facilitates shaping by extrusion. Said organic binder can advantageously be chosen from methylcellulose, hydroxypropylmethylcellulose, hydroxyethylcellulose, carboxymethylcellulose and polyvinyl alcohol. The proportion of said organic binder is advantageously between 0 and 5% by weight, preferably between 0 and 3% by weight and preferably between 0 and 1% by weight, relative to the total amount of solid particles.

At the end of step d) of calcination of the preparation process according to the invention, said organic binder is decomposed, that is to say that at the end of step d) calcination, said Organic binder is no longer present within the material according to the invention. The mixture of step a) of the preparation process according to the invention is advantageously carried out in a single step or in two steps. Indeed, the implementation of said mixing step a) differs according to the shaping technique used in step b) of the preparation process according to the invention. In the case where the shaping step b) is carried out by extrusion, the mixing step a) is preferably carried out in a single step. The micrometric alumina particles, the solid precursor of alumina, the acid in solution and optionally an organic binder are mixed all at once using a batch mixer, preferably with a cam or an arm. Z, or using a twin-screw mixer-mixer. The mixing conditions are adjusted in a manner known to those skilled in the art and aim to obtain a homogeneous and extrudable paste. The extrudability of the paste can advantageously be adjusted with the addition of water to the acid in solution, in order to obtain a paste suitable for carrying out the extrusion forming step b). In the case where the shaping step b) is carried out by wet granulation, the mixing step a) is preferably carried out in two steps. In a first step, the small and large micrometric particles of alumina are advantageously dry mixed for a period of between 1 and 5 min and preferably between 2 and 4 min. The mixture is preferably made using a mixer such as, for example, a bezel or a rotating drum. The solid precursor of alumina and the acid in solution are then advantageously introduced into said dry mixture of micrometric alumina particles, in the form of a binder solution formed with the solid precursor of alumina dispersed in said acidic solution. The acid in solution and the solid precursor of alumina dispersed in said acid solution are advantageously introduced in a second step, using a spray nozzle, to ensure the contact between the components of the mixture. and allow the progressive agglomeration of micrometric alumina particles during rotation of the bezel or rotating drum. The addition in a second step of the solid precursor of alumina dispersed in the acid in solution makes it possible to ensure the shaping by wet granular growth during the granulation shaping step b). The shaping conditions are adjusted in a manner known to those skilled in the art and are aimed at obtaining spherical granules of desired size.

The mixture resulting from step a) is then shaped according to step b) of the process for preparing the material according to the invention. Step b) shaping makes it possible to obtain a green material. By raw material is meant the material shaped and having not undergone any heat treatment steps. Said b) shaping step can advantageously be carried out by wet granulation or by extrusion. Preferably, said shaping step b) is carried out by extrusion in a manner known to those skilled in the art.

In this case, the mixture resulting from stage a), that is to say the paste resulting from stage a) advantageously passes through a die, for example using a piston or a continuous twin-screw or single-screw extruder. The diameter of the die is advantageously variable and is between 1 and 5 mm. The shape of the die, and therefore the form of the millimeter solid obtained in extruded form, is advantageously cylindrical, trilobal, quadrilobed or multilobed.

The green material from step b) can therefore be in the form of granules or extrudates and preferably in the form of extrudates. The raw material shaped according to step b) of the preparation process according to the invention is then subjected to the heat treatments of steps c) and d). According to step c) of the preparation process according to the invention, the raw material obtained at the end of step b) undergoes a drying step. Said c) drying step is advantageously carried out according to the techniques known to those skilled in the art to evaporate in a controlled manner the solvent from the acid solution present in the shaped green material obtained at the end of step b), and imparting a first solidification to said material. The drying step c) may advantageously be carried out under vacuum or under air or in moist air, in one or more substeps of determined conditions of temperature, vacuum or gas. Said c) drying step is advantageously carried out at a temperature between 40 and 200 ° C, preferably between 50 and 150 ° C, and preferably between 70 and 100 ° C. The drying step c) is advantageously carried out for a period of between 12 and 72 hours, and preferably between 12 and 24 hours. Said drying step c) is advantageously carried out under ambient air or in moist air, with a water vapor content of between 20 and 100% by volume.

According to step d) of the preparation process according to the invention, the dried material obtained at the end of the drying step c) undergoes a calcination step. Said step d) of calcination is advantageously carried out according to the techniques known to those skilled in the art. Said step d) can be carried out under air or under air with steam, in one or more substeps of determined conditions of temperature or gas. Said d) calcination step is advantageously carried out at a temperature between 400 and 1000 ° C, preferably between 450 and 850 ° C, and preferably between 500 and 700 ° C. The d) calcination step is advantageously carried out for a period of between 1 to 6 hours and preferably between 2 and 3 hours, under ambient air or under air and steam. Said step d) of calcination is advantageously carried out under ambient air. Said step d) of calcination makes it possible to generate the binder constituted by aluminum oxide and to ensure the cohesion of the hierarchical organization of small and large micrometric particles of alumina and therefore of the material obtained by the preparation process according to the invention. invention thanks, in particular, to the transformation of the solid alumina precursor into a transition alumina, and the presence of aluminum species in the medium, originating from the partial dissolution of said solid precursor of alumina and in a minority, micrometric particles of alumina in the presence of a high acid ratio.

At the end of step d) of calcination of the preparation process according to the invention, the material in the form of millimetric object according to the invention is obtained. By material in the form of a millimeter object is meant a material having dimensions of the order of one millimeter.

The material in the form of millimetric object, preferably in extruded form, prepared according to the sequence of steps a), b), c) and d) of the preparation process according to the invention can be characterized according to the following techniques : nitrogen adsorption for the determination of the specific surface according to the BET method; mercury porosimetry for mesoporous and macroporous volume measurements and pore size distribution; scanning electron microscopy (SEM) for the microstructure analysis to observe the hierarchical organization and dimensions of the small and large micrometric alumina particles assembled by the aluminum oxide binder; a grain-to-grain crushing test for evaluating the mechanical strength of the material.

The preparation method of the invention ensures the formation of said material but also: a good sliding of the micrometric particles of alumina with respect to each other during step a) of mixing which limits firstly, the degradation of the particles and thus makes it possible to maintain a high porosity of the material and which also limits the generation of micrometric defects within the mixture and thus makes it possible to improve its mechanical strength; a good flow of the mixture obtained in step a) during step b) which limits the generation of geometric defects and micrometric defects on the surface of the raw material thus improving the appearance and strength of the material as a millimetric object obtained according to the invention; the formation of defects related to the densification of the binder during steps c) and d) is minimized because of its location in the small space between the micrometric particles of alumina which improves the mechanical strength of the material in form millimetric object obtained according to the invention. The following examples illustrate the invention without, however, limiting its scope.

Example 1 (according to the invention): preparation of an alumina-based material, in the form of a millimetric, porous obiet, with a hierarchical organization of two classes of micrometric particles assembled by a binder consisting of aluminum oxide and prepared at a high acid ratio of 61% by weight. The large particle population consists of alumina Puralox Scfa-140 (Sasol).

These particles have a BET specific surface area of 144 m 2 / g determined by nitrogen adsorption (ASAP 2420 - Micromeritics), a mesoporous volume of 0.45 ml / g and a median mesopore diameter of 11.5 nm determined by porosimetry at mercury (Autopore 9500 - Micromeritics). The median diameter, i.e., the Dv50 of said large particles is about 19, the Dv10 is about 7 μm, and the Dv90 is about 38 μm, determined by laser diffraction particle size (Mastersizer 2000 - Malvern). The small micrometric alumina particles are obtained by spray drying of a boehmite sol prepared from the Pural powder SB3 (Sasol). The sol is prepared so as to obtain a mass ratio (mass of boehmite / mass of water) × 100 of 3.4% by weight and a level of acid (mass of nitric acid / mass of alumina formed from boehmite after calcination) of 4.6% by weight. The powder Pural SB3 referenced (Sasol) is introduced into the acid solution, and the mixture is subjected to strong stirring for 2 hours. After the mixing time, the soil is centrifuged for 20 min at a speed of 4000 rpm to remove sediment of undispersed boehmite. The boehmite sol is then spray-dried using a laboratory spray dryer (Büchi B-290). Atomization conditions such as feed rate, compressed air flow, inlet and outlet temperatures, and suction flow are optimized to produce small spherical particles in the separation cyclone. the median diameter is about 2 μm. The particles obtained by spray drying are then calcined in a muffle furnace at 600 ° C. for 2 hours in order to ensure the conversion of boehmite to gamma-alumina, and thus to obtain the small micrometric and spherical particles of alumina. These particles have, after calcination, a BET surface area of 212 m 2 / g determined by nitrogen adsorption (ASAP 2420 - Micromeritics), a mesoporous volume of 0.35 ml / g and a median diameter of the mesopores of 7.5 nm. determined by mercury porosimetry (Autopore 9500 - Micromeritics).

The median diameter, i.e., the Dv50 of said small particles is about 1.7 μm, the Dv10 is about 0.75 μm, and the Dv90 is about 3.4 μm, determined by laser diffraction granulometry (Mastersizer 2000 - Malvern).

The solid precursor of alumina, necessary for the formation of the binder consisting of aluminum oxide, is the powder referenced Pural SB3 (Sasol), and it is a highly dispersible boehmite in acid solution. The large micrometric particles (Puralox Scfa-140 - Sasol), the small micrometric particles obtained by spray drying of a boehmite sol, and the solid alumina precursor referenced Pural SB3 (Sasol), are dry blended in such a way that to obtain, respectively, the proportions of 80% and 20% by weight of large and small particles, and 19% by weight of solid precursor of alumina. The proportions of large and small particles are expressed in relation to the total amount of micrometric particles, and the proportion of solid precursor of alumina is expressed in relation to the total amount of solid. The dry blending is carried out using a cam blender (Brabender 50 EHT) at an arm rotation speed of 10 rpm for 2 minutes. Then, a solution of nitric acid is introduced using a syringe pump, so as to obtain an acid ratio (mass of introduced nitric acid / solid precursor mass of introduced alumina) x100 of 61% in weight. The water is added with the acid solution so as to obtain a cohesive, homogeneous and extrudable paste. The kneading is continued at a rotation speed of the arms of 16 revolutions / min for 30 minutes, after the end of the addition of the acid solution. The nitric acid solution associated with the mechanical energy provided during the kneading, make it possible to disperse the micrometric crystallite agglomerates of the solid precursor of alumina, and at the end of the mixing process, the formed dough consists of coarse and micrometric small particles of alumina bound by a binder whose particles have an average size of the order of a hundred nanometers. In addition, the high acid ratio promotes the partial dissolution of the solid alumina precursor and in a minor way, the micrometric alumina particles, and therefore promotes the formation of alumina species which will help form the binder. The paste obtained is shaped using a piston extruder (MTS), equipped with a cylindrical die 3 mm in diameter and 6 mm in length, to form the millimeter object. The extrudates are then dried in a ventilated oven at 80 ° C for 16 hours in ambient air, in order to evaporate the solvent from the acid solution. The dried extrudates are then calcined in a muffle furnace at 600 ° C. for 2 hours in ambient air, in order to ensure the solidification of the material thanks to the conversion of the aluminum precursor, in this example, the boehmite to gamma alumina, and thus forming the binder consisting of aluminum oxide.

The extrudates obtained are characterized according to the following techniques: nitrogen adsorption, for the determination of the BET specific surface area; mercury porosimetry for mesoporous and macroporous volume measurements and mesopore diameter distribution; by scanning electron microscopy (JEOL JSM 6340F) for observing the hierarchical organization of small and large micrometric alumina particles and in particular the distance from one interface to another between said large particles (Dgrosses) and an interface to another between said small particles (Dsmites) and the surface percentage of small (Spetites) and large particles (Sgrosses) relative to the total surface of the SEM image; and the grain-to-grain (GGE) crush test to determine the lateral crushing strength of the material.

The characteristics of the material obtained according to the formulation of Example 1, are summarized in Table 1. It is observed that the material according to the invention obtained according to Example 1 has good mechanical strength due to the hierarchical organization of small and large particles assembled by a binder consisting of aluminum oxide. The distances Dgrosses and Dpetites and the surface percentages Sgrosses and Spetites are essential criteria for obtaining the hierarchical organization of said small and large particles and are consistent with those of the material of the invention. The textural properties of the material obtained are related to the textural properties of the micrometric alumina particles and are consistent with the expected properties of the material according to the invention. The hierarchical organization makes it possible to increase the mesoporous volume of the material, by maximizing the proportion of micrometric particles with respect to the binder content. The hierarchical organization also makes it possible to have good sliding of the small and large micrometric alumina particles relative to each other, which results in a low mixing torque of the order of 7 Nm. EXAMPLE 2 invention): preparation of an alumina-based material, in the form of millimetric object, highly porous, with a hierarchical organization of two classes of micrometric particles assembled by a binder made of aluminum oxide and prepared at a high acid ratio of 61% by weight. The large micrometric and porous alumina particles are obtained from calcination in a muffle furnace at 600 ° C. for 2 hours, under ambient air, with the Pural TH100 powder (Sasol), in order to transform the starting boehmite into alumina. gamma. The micrometric particles have, after calcination, a specific surface area of 142 m 2 / g determined by nitrogen adsorption (ASAP 2420 - Micromeritics), a mesoporous volume of 0.87 ml / g and a median mesopore diameter of 22.5 nm. determined by mercury porosimetry (Autopore 9500 - Micromeritics). The median diameter, i.e., the Dv50 of said coarse particles is about 40 the Dv10 is about 15prn, and the Dv90 is about 77 determined by laser diffraction particle size (Mastersizer 2000 - Malvern) . The small micrometric particles are obtained by spray drying of a boehmite sol prepared from the powder Pural TH100 (Sasol). Said boehmite sol is prepared according to the same conditions described in Example 1; and the atomization parameters are optimized to produce in the separation cyclone small spherical particles whose median diameter is about 3 μm. The particles obtained by spray drying are subjected to the same heat treatment as Example 1, in order to ensure the conversion of boehmite to gamma-alumina, and thus to obtain small micrometric and spherical particles of alumina. These particles have, after calcination, a BET specific surface area of 130 m 2 / g determined by nitrogen adsorption (ASAP 2420 - Micromeritics), a mesoporous volume of 0.78 ml / g and a median mesopore diameter of 23 nm, determined by mercury porosimetry (Autopore 9500 - Micromeritics). The median diameter, i.e., the Dv50 of said small particles is about 2.7 μm, the Dv10 is about 1.7 μm, and the Dv90 is about 8 μm, determined by particle size distribution. laser diffraction (Mastersizer 2000 - Malvern). The solid precursor of alumina, necessary for the formation of the binder consisting of aluminum oxide, is the powder referenced Pural TH100 (Sasol). The large micrometric particles obtained after calcination of the powder referenced Pural TH100 (Sasol), the small micrometric particles obtained by spray drying of a boehmite sol, and the solid precursor of alumina referenced Pural TH100 (Sasol), are mixed with in the same proportions and under the same conditions described in Example 1. Then, a solution of nitric acid is introduced in the same way as that described in Example 1, and in order to obtain an acid ratio. (Mass of introduced nitric acid / mass of introduced alumina solid precursor) x100 of 61% by weight. The kneading is continued under the same conditions as in Example 1.

The water is added with the acid solution so as to obtain a cohesive, homogeneous and extrudable paste. The high acid ratio associated with the mechanical energy contributed to the kneading leads to the same technical effects described in Example 1. The paste obtained is then shaped according to the same conditions as in Example 1.

The extrudates are then dried and calcined according to the conditions described in Example 1; and characterized according to the same techniques described in the same example. The characteristics of the material obtained according to the formulation of Example 2 are collated in Table 1.

It is observed that the material according to the invention obtained according to Example 1 has a good mechanical strength due to the hierarchical organization of small and large particles assembled by a binder consisting of aluminum oxide. The distances Dgrosses and Dpetites and the surface percentages Sgrosses and Spetites are essential criteria for obtaining the hierarchical organization of said small and large particles and are consistent with those of the material of the invention. The textural properties of the material obtained are related to the textural properties of the micrometric alumina particles and are consistent with the expected properties of the material according to the invention. The hierarchical organization makes it possible to increase the mesoporous volume of the material, by maximizing the proportion of micrometric particles with respect to the binder content. The hierarchical organization also makes it possible to have good sliding of the small and large micrometric particles of alumina with respect to each other, which results in a low mixing torque of the order of 9 Nm.

Example 3 (according to the invention): preparation of an alumina-based material, in the form of a millimetric, porous object, with a hierarchical organization of three classes of micrometric particles assembled by a binder consisting of aluminum oxide and prepared at a high acid ratio of 61% by weight.

The large particles of alumina are composed of two populations of particles whose median diameter is distinct, and which are identified as: a population of so-called lower fat particles and referenced Puralox Scfa-140 (Sasol) and a so-called larger population referenced Puralox SCCa 150/200 (Sasol). The characteristics of the alumina powder referenced Puralox Scfa-140 are described in Example 1. The alumina powder referenced Puralox SCCa 150/200 has a BET specific surface area of 199 m2 / g determined by nitrogen adsorption (ASAP 2420 - Micromeritics), a mesoporous volume of 0.43 mL / g and a median mesopore diameter of 8.7 nm determined by mercury porosimetry (Autopore 9500 - Micromeritics). The median diameter, i.e., the Dv50 of said larger fat particles is about 160 μm, the Dv10 is about 104 μm, and the Dv90 is about 247 μm, determined by laser diffraction particle size ( Mastersizer 2000 - Malvern). The small particles are obtained in the same manner as described in Example 1 and their characteristics are also described in the same example. The solid precursor of alumina is the boehmite referenced Pural SB3 (Sasol), also used in Example 1.

The conditions for mixing the alumina particles with the solid alumina precursor and the acid solution, the mixing conditions, the extrusion shaping and the heat treatments, are identical to those described in Example 1, but with 4 powders instead of 3 powders. The proportions of coarse particles, that is to say of the whole of the coarse lower and upper coarse particles, and small micrometric particles of alumina are respectively 76% and 24% by weight, expressed relative to the total quantity of particles. micrometric, and the proportion of solid precursor of alumina is 13% by weight relative to the total amount of solid. The proportion of so-called smaller particles relative to the totality of large particles is 40% by weight. The ratio of acid (mass of introduced nitric acid) / (mass of solid precursor of alumina) × 100 used is 61%. The characteristics of the material obtained according to the formulation of Example 3, and determined according to the techniques described in Example 1, are summarized in Table 1. It is observed that the material according to the invention obtained according to Example 3 shows a good mechanical resistance due to the hierarchical organization of small and large particles assembled by a binder consisting of aluminum oxide. The distances Dgrosses and Dpetites and the surface percentages Sgrosses and Spetites are essential criteria for obtaining the hierarchical organization of said small and large particles and are consistent with those of the material of the invention. The textural properties of the material obtained are related to the textural properties of the micrometric alumina particles and are consistent with the expected properties of the material according to the invention. The hierarchical organization makes it possible to increase the mesoporous volume of the material, by maximizing the proportion of micrometric particles with respect to the binder content. The hierarchical organization also makes it possible to have good sliding of the small and large micrometric alumina particles relative to each other, which results in a low mixing torque of the order of 12 Nm.

Example 4 (comparative): preparation of an alumina-based material, in the form of a millimetric, porous object, with a single class of micrometric particles assembled by a binder consisting of aluminum oxide and prepared at a ratio of high acid of 61% by weight.

The micrometric and porous alumina particles come from the Puralox SCfa-140 (Sasol) referenced powder whose characteristics are described in Example 1. The solid alumina precursor is the boehmite referenced Pural SB3 (Sasol), also used in Example 1

The conditions for mixing the alumina particles with the solid alumina precursor and the acid solution, the mixing conditions, the extrusion shaping and the heat treatments, are identical to those described in Example 1, but with 2 powders instead of 3 powders. The proportions of micrometric particles of alumina and alumina solid precursor relative to the total amount of solid are respectively 74 and 26% by weight. The ratio of acid (mass of introduced nitric acid) / (mass of solid precursor of alumina) × 100 used is 61%.

The characteristics of the material obtained according to the formulation of Example 4, and determined according to the techniques described in Example 1, are summarized in Table 1. A lower mechanical resistance is observed with respect to the materials of Examples 1, 2 and 4. 3 according to the invention because of the presence of a single class of micrometric particles of alumina which also leads to a lower mesoporous volume and BET specific surface area compared to the examples according to the invention. A higher macroporous volume is also observed because of the lack of hierarchical organization of small and large particles in this material.

The sliding of the micrometric particles of alumina with respect to each other is less favored which results in a mixing torque of the order of 20 Nm. EXAMPLE 5 (comparative): preparation of an alumina-based material as a millimetric, porous object with a single class of micrometric particles joined by a binder of aluminum oxide and prepared at a low acid ratio of 2.37% by weight. The micrometric and porous alumina particles come from the Puralox SCfa-140 (Sasol) referenced powder whose characteristics are described in Example 1. The solid alumina precursor is the boehmite referenced Pural SB3 (Sasol), also used in Example 1. The conditions for mixing the alumina particles with the solid precursor of alumina and the acidic solution, the mixing conditions, the extrusion shaping and the heat treatments, are identical to those described in FIG. Example 4, but with 2 powders instead of 3 powders. The proportions of micrometric particles of alumina and alumina solid precursor relative to the total amount of solid are respectively 74 and 26% by weight. The ratio of acid (mass of introduced nitric acid) / (mass of solid precursor of alumina) × 100 used is 2.37%. The characteristics of the material obtained according to the formulation of Example 5, and determined according to the techniques described in Example 1, are summarized in Table 1. There is a lower mechanical strength compared to the materials of Examples 1, 2 and 4. 3 according to the invention due to the presence of a single class of micrometric particles of alumina and a binder prepared with a low acid ratio. A higher macroporous volume is also observed because of the lack of hierarchical organization of small and large particles in this material.

The sliding of the micrometric particles of alumina with respect to each other is less favored which results in a mixing torque of the order of 24 Nm. Table 1 SBET Vmesoporous (moult Vmacroporous dmésopore EGG Dgrosses Dsmites Ssposses Spetites (m2 / 0 (mw (nm) b (daN / mm) c (pie (ijm) d (%) d (%) d Example 1 154 0.37 0.10 11 1.8 44 0.9 62 (Invention) Example 2 137 0.87 0.10 22 1.2 4.5 1.1 63 14 (invention) Example 3 160 0.38 0.13 11 0.9 4.8 1.5 74 22 (Invention) Example 4 100 0.25 0.23 11 0.4 12 nde 72 nde (comparative) Example 5 158 0.40 0.23 11.2 0.3 12 nde 70 nde (comparative) 'Nitrogen uptake; bPorosimetry with mercury;' The examples demonstrate that the materials of Examples 1 to 3 according to the invention having both a hierarchical organization of small and large particles of alumina and comprising a binder. consisting of aluminum oxide prepared at high acid ratio, pre feel increased mechanical properties compared to the material of the prior art (Example 5) or low acid (Example 4). The hierarchical organization also makes it possible to increase the mesoporous volume of the material, thanks to a maximization of the proportion of micrometric particles with respect to the binder content (Examples 1 to 3). Even higher textural properties can be achieved from the use of micrometer particles of high porosity alumina.

Claims (16)

REVENDICATIONS1. Material, in the form of a millimetric object, consisting of large particles of alumina having a median diameter of between 10 and 200 μm, said large particles being distant, from one interface to another, of less than 10 μm, small alumina particles having a median diameter of between 0.5 and 10 μm, said small particles being spaced apart, from one interface to another, of less than 5 μm and being located in the space between the large particles and d a binder consisting of aluminum oxide, said binder being located in the space between said small and large particles, said material having a mesoporous volume, measured by mercury porosimetry, of between 0.2 and 2 ml / g, a macroporous volume, measured by mercury porosimetry, of between 0.05 and 0.2 mUg and a BET specific surface area of between 90 and 350 m 2 / g. 2. Material according to claim 1 wherein said large particles of alumina have a median diameter of between 10 and 80 pm. 3. Material according to one of claims 1 or 2 wherein said material has a distribution of the particle size of said large particles of alumina in said material such that the diameter D10 is at most 3 times less than the median diameter D50 and such that the diameter D90 is at most 3 times greater than the median diameter D50. 4. Material according to one of claims 1 to 3 wherein said large particles have a median diameter less than 1/10 times the average diameter of the smallest dimension of the material. 5. Material according to one of claims 1 to 4 wherein said coarse particles are spaced from one interface to another, less than 6 pm. 6. Material according to one of claims 1 to 5 wherein said coarse particles comprise a proportion of particles, called lower coarse particles, having a median diameter of between 10 and 60 pm and a proportion of particles, called larger particles greater, 30 having a median diameter of between 60 and 200 μm. 7. The material of claim 6 wherein said material has a distribution of the particle size of said lower coarse particles within said material such that the diameter D10 is at most 3 times less than the median diameter D50 and such that the diameter D90 is at most 3 times greater than the median diameter D50 and a distribution of the particle size of said larger larger particles within said material such that the diameter D10 is at most 3 times smaller than the median diameter D50 and such that the diameter D90 is at most 3 times greater than the diameter median D50. 20 8. Material according to one of claims 1 to 7 wherein wherein said small particles have a median diameter of between 0.5 and 3 pm. 9. Material according to one of claims 1 to 8 wherein said material has a distribution of the particle size of said small particles of alumina within said material such that the diameter D10 is at most 3 times less than the median diameter D50 and such that the diameter D90 is at most 3 times greater than the median diameter D50. 10. Material according to one of claims 1 to 9 wherein said small particles have a median diameter less than 1/5 times the median diameter of large particles, 11. Material according to one of claims 1 to 10 wherein the surface percentage of small particles between 5 and 55% relative to the total surface of the SEM image of said material. 12. Material according to one of claims 1 to 11 wherein the surface percentage of large particles is between 35 and 85% relative to the total surface of the SEM image of said material. 13. Process for preparing the material according to one of claims 1 to 12 comprising at least the following steps: a) the mixture of 10 to 60% by weight of small particles of alumina of median diameter between 0.5 and 10 pm and 40 to 90% by weight of large alumina particles of median diameter between 10 and 200 pm, 5 to 50% by weight of a solid precursor of alumina, the percentages of small and large particles of alumina being expressed relative to the total amount of micrometric particles, and the percentage of solid precursor of alumina being expressed relative to the total amount of solid, and an acid in solution selected from acetic acid, the acid hydrochloric acid, sulfuric acid, formic acid, citric acid and nitric acid, alone or as a mixture, in proportions such as the ratio of the mass of acid introduced into the mass of solid precursor of alumina introduced between 2.5 and 115% in weight, b) the shaping of the mixture resulting from step a), c) the drying of the green material obtained at the end of step b) and, d) the calcination of the dried material resulting from the step vs). 14. The method of claim 13 wherein said solid precursor of alumina is selected from aluminum oxides, aluminum hydroxides and aluminum oxyhydroxides soluble or dispersible in the acid solution. 15. Method according to one of claims 13 or 14 wherein said ratio of the mass of acid introduced on the solid precursor mass of introduced alumina is between 30 and 62% by weight. 16. Method according to one of claims 13 to 15 wherein said step b) shaping is carried out by granulation or extrusion.