EP4065269A1

EP4065269A1 - Alumina having a particular pore profile

Info

Publication number: EP4065269A1
Application number: EP20810988.4A
Authority: EP
Inventors: Julien Hernandez; Virginie Harle; Oguzhan Yaglidere; Andrew D Allerton
Original assignee: Rhodia Operations SAS
Current assignee: Rhodia Operations SAS
Priority date: 2019-11-29
Filing date: 2020-11-25
Publication date: 2022-10-05
Also published as: CN114761123A; JP2023504129A; CA3159463A1; US20220410124A1; WO2021105252A1; KR20220101722A

Abstract

The present invention relates to an alumina having a particular pore profile and good thermal stability. This alumina is also characterised by the fact that it has a high apparent density. The alumina has, after calcination in air at 1100°C for 5 hours: - a pore volume, in the pore size range of between 5 nm and 100 nm, that is between 0.50 and 0.75 ml/g, more particularly between 0.50 and 0.70 ml/g; and - a pore volume, in the pore size range of between 100 nm and 1000 nm, that is less than or equal to 0.20 ml/g, more particularly less than or equal to 0.15 ml/g, or even less than or equal to 0.10 ml/g.

Description

Alumina with a particular porous profile

The present application claims the priority of European patent applications Nos. 19315153.7 and 19315155.2 and both filed on November 29, 2019 and the contents of which are fully incorporated by reference. In the event of an inconsistency between the text of the present application and the text of the French patent application which would affect the clarity of a term or an expression, reference will be made to the present application only.

The present invention relates to an alumina exhibiting a particular porous profile and good thermal stability. This alumina is also characterized by the fact that it has a high bulk density.

Technical area

It is known to use an alumina for the preparation of an automobile pollution control catalyst to convert the pollutants emitted by gasoline or diesel heat engines. Alumina is used as a support for precious metals, in particular platinum, palladium and / or rhodium. It can also be associated with other components of the catalyst, components which will depend on the catalyst and the intended application (diesel or gasoline pollution control). Among the other usual components present in the catalyst, mention may be made of oxides based on rare earths such as cerium oxides or mixed oxides of cerium and zirconium used as oxygen mobility materials for gasoline engine catalysts ( so-called three-way catalyst (TWC in English) or gasoline particulate filters (GPF)). The alumina can also be combined with a zeolite used for example as a hydrocarbon trap for diesel catalysts or else with a zeolite exchanged with copper and / or iron for catalysts for the catalytic reduction of nitrogen oxides with ammonia ( SCR) for the reduction of NO _x emitted by diesel engines.

Technical problem

For all these automotive pollution control applications, the thermal stability of the alumina is required to be high because this makes it possible to maintain the efficiency of the catalyst over time, that is to say to maintain a good conversion of gaseous pollutants. . The term thermal stability is understood to mean the fact of maintaining a high specific surface area after heat treatments at high temperature. A simple and common way to characterize the thermal stability of an alumina is to measure its specific surface area after heat treatment at high temperature, for example at 1200 ° C for 5 hours in air.

The preparation of an automotive pollution control catalyst generally involves the deposition or coating of an alumina-based suspension on a substrate or on a monolith. The alumina of the invention is suitable for the preparation of a suspension having a low viscosity, which makes it possible to prepare a suspension having a high proportion of alumina. Furthermore, the high density of the alumina of the invention facilitates the handling of the alumina powder.

The thermal stability of aluminas is generally partly linked to the pore volume of the alumina. By increasing this pore volume, thermal stability is generally increased. This increase in pore volume, however, leads to a lowering significant in the density of the alumina and an increase in the viscosity of the alumina slurry during the catalyst preparation process.

In order to solve this problem, it has been observed that the specific porosity of the alumina of the invention makes it possible to obtain both high thermal stability as well as high bulk density.

Technical background

US 4,154,812 describes a process for preparing an alumina. This process does not include step (e).

Figures

Fig. 1/1 represents a diffractogram of the alumina of the example of the invention. It can be seen that this alumina exhibits the peaks characteristic of a crystallized alumina.

Brief description of the invention

The invention relates to an alumina as defined in one of claims 1 to 41.

This is characterized by at least one of the following two porosity profiles:

^■ 1 ^st profile: a pore volume in the area of the pores, the size of which is between 5 nm and 100 nm which is between 0.60 and 0.85 mL / g, more particularly between 0.60 and 0.80 mL / g; and a pore volume in the area of the pores, the size of which is between 100 nm and 1000 nm which is less than or equal to 0.20 ml / g, more particularly less than or equal to 0.15 ml / g, or even less than or equal at 0.10 ml / g, or even less than or equal to 0.05 ml / g; and or

^■ 2 ^nd profile: after calcination in air at 1100 ° C for 5 hours: a pore volume in the area of pores having a size between 5 nm and 100 nm is between 0.50 and 0.75 mL / g , more particularly between 0.50 and 0.70 mL / g; and a pore volume in the area of the pores, the size of which is between 100 nm and 1000 nm which is less than or equal to 0.20 ml / g, more particularly less than or equal to 0.15 ml / g, or even less than or equal at 0.10 ml / g, or even less than or equal to 0.05 ml / g; these pore volumes being determined using the mercury porosimetry technique.

This alumina is generally represented by the general formula Al ₂ O ₃ . It can include sodium and sulfate as well as impurities.

The invention also relates to a catalytic composition as described in claim 42 as well as to the use of alumina as described in claim 43.

The invention also relates to a process for preparing alumina as described in one of claims 44 to 51. All these objects will be described in more detail.

Description of the invention

In the present application, it is specified for the remainder of the description that, unless otherwise indicated, in the ranges of values which are given, the values at the limits are included. In addition, the concentrations of the solutions or the proportions in alumina are given in% by weight in oxide equivalents. The formula AI ₂ O _{3 is used} for the calculations of these concentrations or proportions. For example, an aqueous solution of aluminum sulphate having an aluminum concentration of 2.0% by weight corresponds to a solution containing 2.0% by weight in Al ₂ O ₃ equivalent.

The term “particle” is understood to mean an agglomerate formed from primary particles. Particle size is determined from a volume distribution of particle sizes obtained using a laser particle sizer. The particle size distribution is characterized using parameters D10, D50 and D90. These parameters have the usual meaning in the field of measurements by laser diffraction. Thus, Dx is denoted the value which is determined on the volume distribution of the sizes of the particles for which x% of the particles have a size less than or equal to this value Dx. D50 therefore corresponds to the median value of the distribution. D90 corresponds to the size for which 90% of the particles have a size which is less than D90. D10 corresponds to the size for which 10% of the particles have a size which is less than D10. The measurement is generally made on a dispersion of the particles in water.

The porosity data are obtained by the mercury porosimetry technique. This technique makes it possible to define the pore volume (V) as a function of the pore diameter (D). A Micromeritics Autopore 9520 device fitted with a powder penetrometer can be used, complying with the instructions recommended by the manufacturer. The ASTM D 4284-07 procedure can be followed.

The term “specific surface area” means the BET specific surface area determined by nitrogen adsorption determined using the Brunauer-Emmett-Teller method. This method has been described in the periodical "The Journal of the American Chemical Society, 60, 309 (1938)". The recommendations of the standard ASTM D3663 - 03 can be complied with. The calcinations for a given temperature and time correspond, unless otherwise indicated, to calcinations in air at a temperature plateau over the indicated time.

The alumina of the invention can be represented by the general formula Al ₂ O ₃ .

The alumina can include residual sodium. The residual sodium level may be less than or equal to 0.50% by weight, or even less than or equal to 0.15% by weight. The sodium level can be greater than or equal to 50 ppm. This rate can be between 50 and 900 ppm, or even between 100 and 800 ppm. This rate is expressed as the weight of Na2O relative to the total weight of the alumina. Thus, for an alumina having a residual sodium level of 0.15%, it is considered that there is per 100 g of alumina, 0.15 g of Na ₂ 0. The method for determining the sodium level in this range of concentrations is known to those skilled in the art. For example, the technique of inductively coupled plasma spectroscopy can be used. The alumina can include residual sulfate. The level of residual sulphate may be less than or equal to 1.00% by weight, or even less than or equal to 0.20% by weight, or even less than or equal to 0.10% by weight. The sulphate level can be greater than or equal to 50 ppm. This rate can be between 100 and 1500 ppm, or even between 400 and 1000 ppm. This rate is expressed in weight of sulphate relative to the total weight of alumina. Thus, for an alumina having a residual sulphate level of 0.50%, it is considered that there is per 100 g of alumina, 0.50 g of SO ₄ . The method for determining the level of sulphate in this range of concentrations is known to those skilled in the art such as, for example, the technique of inductively coupled plasma spectroscopy. Microanalysis techniques can also be used. A Horiba EMIA 320-V2 type microanalysis device could be suitable.

Alumina can also contain impurities other than sodium and sulphate, for example impurities based on silicon, titanium or iron. The proportion of each impurity is generally less than 0.07% by weight (£ 0.07%), or even less than 0.05% by weight (£ 0.05%).

The alumina of the invention is characterized by a particular porosity. Thus, this alumina has at least one of the following two porosity profiles:

1 ^st profile: a pore volume in the area of the pores, the size of which is between 5 nm and 100 nm which is between 0.60 and 0.85 mL / g, more particularly between 0.60 and 0.80 mL / g; and a pore volume in the area of the pores, the size of which is between 100 nm and 1000 nm which is less than or equal to 0.20 mL / g, more particularly less than or equal to 0.15 mL / g, or even less than or equal at 0.10 mL / g, or even less than or equal to 0.05 mL / g.

2 ^nd profile: after calcination in air at 1100 ° C for 5 hours: a pore volume in the area of the pores the size of which is between 5 nm and 100 nm which is between 0.50 and 0.75 mL / g, more particularly between 0.50 and 0.70 mL / g; and a pore volume in the area of the pores, the size of which is between 100 nm and 1000 nm which is less than or equal to 0.20 mL / g, more particularly less than or equal to 0.15 mL / g, or even less than or equal at 0.10 mL / g, or even less than or equal to 0.05 mL / g.

Alumina can also be defined by at least one of the following two porosity profiles:

^■ 1 ^st profile: a pore volume in the region of the pores, the size of which is between 5 nm and 100 nm which is between 0.60 and 0.85 mL / g; and a pore volume in the area of the pores the size of which is between 100 nm and 1000 nm which is less than or equal to 0.20 mL / g; and or

^■ 2 ^nd profile: after calcination in air at 1100 ° C for 5 hours: a pore volume in the area of pores having a size between 5 nm and 100 nm is between 0.50 and 0.75 mL / g ; and a pore volume in the region of the pores the size of which is between 100 nm and 1000 nm which is less than or equal to 0.20 mL / g.

The alumina which is described in the present application can have at least one of the two aforementioned profiles, it being understood that it can have both profiles at the same time.

Furthermore, the alumina can have a high specific surface. It may have a BET specific surface area of between 100 and 200 m ² / g, more particularly between 150 and 200 m ² / g. This specific surface can be greater than or equal to 120 m ² / g, preferably greater than or equal to 140 m ² / g. This specific surface can also be between 100 and 140 m ² / g, or even between 100 and 120 m ² / g.

The alumina also has high thermal stability. It can have a BET specific surface after calcination in air at 1100 ° C. for 5 hours of between 70 and 100 m ² / g.

The alumina generally has a total pore volume which is generally strictly greater than 1.05 mL / g. This total pore volume can advantageously be at least 1.10 mL / g, or even at least 1.20 mL / g, or even at least 1.30 mL / g or at least 1.40 mL / g or at least 1.50 mL / g. This total pore volume is generally at most 2.40 mL / g.

The alumina retains a large total pore volume even after calcination at 1100 ° C. for 5 hours. Thus, after calcination at 1100 ° C. for 5 hours, the alumina generally exhibits a total pore volume which is at least 0.90 mL / g. This total pore volume is preferably at least 1.00 mL / g, or even at least 1.10 mL / g, or even more advantageously at least 1.20 mL / g. This total pore volume is generally at most 1.80 mL / g.

It will also be noted that the alumina is crystallized. This can be demonstrated using an X-ray diffractogram. The alumina can comprise a delta phase, a theta phase, a gamma phase or a mixture of at least two of these phases.

The alumina can have an apparent density of between 0.25 g / cm ³ and 0.55 g / cm ³ , more particularly between 0.40 g / cm ³ and 0.55 g / cm ³ . This bulk density of the alumina powder corresponds to the weight of a certain quantity of powder relative to the volume occupied by this powder: bulk density in g / mL = (mass of the powder (g)) / (volume of the powder ( mL))

This bulk density can be determined by the method described below. First, the volume of a cylindrical-shaped test tube of approximately 25 mL is precisely determined. To do this, the empty specimen is weighed (tare T). Distilled water is then poured into the test tube to the edge but without going over the edge (no meniscus). Weigh the test tube filled with distilled water (M). The mass of water contained in the test tube is therefore:

E = M - T The calibrated volume of the test piece is equal to V test _tube = E / (density of water at the measurement temperature). The density of the water is for example equal to 0.99983 g / mL for a measurement temperature of 20 ° C. In the empty and dry test tube, the alumina powder is gently poured using a funnel until it reaches the edge of the test tube. The excess powder is leveled off using a spatula. The powder should not be compacted or packed when filling. The test tube containing the powder is then weighed. apparent density (g / mL) = (mass of the test piece containing the alumina powder - Tare

T (9)) / (V-tube (mL))

Alumina can have an D50 between 2.0 µm and 80.0 µm. It may have a D90 less than or equal to 150.0 μm, more particularly less than or equal to 100.0 μm. It may have a D10 greater than or equal to 1.0 μm.

First embodiment

According to a first embodiment, the alumina has a D50 of between 2.0 and 15.0 μm, or even between 4.0 and 12.0 μm. The D90 can be between 20.0 pm and 60.0 pm, or even between 25.0 pm and 50.0 pm.

According to the first embodiment, when the D50 is between 2.0 and 15.0 pm,

- the apparent density is between 0.25 and 0.40 g / cm ³ ; and / or - the total pore volume is between 1.40 and 2.40 mL / g.

This total pore volume can be more advantageously between 1.50 and 2.40 mL / g. Second embodiment According to a second embodiment, the alumina has a D50 of between 15.0 and 80.0 μm, or even between 20.0 and 60.0 μm. The D90 can be between 40.0 pm and 150.0 pm, or even between 50.0 pm and 100.0 pm.

According to the second embodiment, when D50 is between 15.0 and 80.0 μm, - the bulk density can be between 0.40 and 0.55 g / cm ³ ; and or

- the total pore volume is between 1.05 (excluded value) and 1.80 mL / g.

This total pore volume can be more advantageously between 1, 20 and 1, 80 ml / g.

Special alumina

The alumina of the invention may more particularly exhibit at least one of the following two porosity profiles:

1 ^st profile: - a pore volume in the area of the pores, the size of which is between 5 nm and 100 nm which is between 0.60 and 0.85 mL / g, more particularly between 0.60 and 0.80 mL / g; and a pore volume in the region of the pores the size of which is between 100 nm and 1000 nm which is less than or equal to 0.05 mL / g; and or

2 ^nd profile: after calcination in air at 1100 ° C for 5 hours: a pore volume in the area of the pores the size of which is between 5 nm and 100 nm which is between 0.50 and 0.75 mL / g, more particularly between 0.50 and 0.70 mL / g; and a pore volume in the area of the pores the size of which is between 100 nm and 1000 nm which is less than or equal to 0.05 mL / g.

As previously, this particular alumina can have at least one of the two aforementioned profiles, it being understood that it can have both profiles at the same time. It can also have the following characteristics:

- a BET specific surface area of between 150 and 200 m ² / g; and

a BET specific surface area after calcination in air at 1100 ° C. for 5 hours of between 70 and 100 m ² / g; and

- an apparent density of between 0.40 g / cm ³ and 0.55 g / cm ³ ; and

- a D50 of between 15.0 and 80.0 pm; and

- a D90 of between 40.0 pm and 150.0 pm.

Moreover, this particular alumina can also exhibit the characteristics of total pore volume as described above. Thus, it generally has a total pore volume which is generally strictly greater than 1.05 mL / g. This total pore volume can advantageously be at least 1.10 mL / g, or even at least 1. 20 mL / g, or even at least 1.30 mL / g or at least 1.40 mL / g or at least 1.50 mL / g. This total pore volume is generally at most 2.40 mL / g.

This particular alumina retains a large total pore volume even after calcination at 1100 ° C. for 5 hours. Thus, after calcination at 1100 ° C. for 5 hours, the alumina generally exhibits a total pore volume which is at least 0.90 mL / g. This total pore volume is preferably at least 1.00 mL / g, or even at least 1.10 mL / g, or even more advantageously at least 1.20 mL / g. This total pore volume is generally at most 1.80 mL / g.

The sodium and sulfate levels are for that particular alumina as previously described.

Use of alumina

The alumina of the invention can be used in the field of catalysis for the depollution of the exhaust gases of gasoline or diesel heat engines. A catalytic composition comprising the alumina of the invention and at least one oxide based on cerium and optionally at least one rare earth other than cerium is used in this field. This oxide can be, for example, cerium oxide (generally represented by the formula Ce0 ₂ ), a mixed oxide based on cerium, on zirconium and optionally on at least one rare earth other than cerium. The rare earth other than cerium can be chosen from the group formed by yttrium, praseodymium or neodymium. Preparation process

The invention also relates to a process for preparing an alumina, in particular alumina as described above or as described in one of claims 1 to 39, comprising the following steps:

(a) in a tank initially containing an acidic aqueous solution the pH of which is between 0.5 and 4.0, or even between 0.5 and 3.5, the following are introduced with stirring:

(a1) - either an aqueous solution of sodium aluminate until a pH of the reaction mixture is obtained between 8.0 and 10.0, or even between 8.5 and 9.5;

(a2) - either simultaneously (i) an aqueous solution of aluminum sulphate and (ii) an aqueous solution of sodium aluminate until a pH of the reaction mixture is obtained between 6.5 and 10, or even between 7, 0 and 8.0 or between 8.5 and 9.5; so that at the end of step (a), the aluminum concentration of the reaction mixture is between 0.50% and 3.0% by weight;

(b) then an aqueous solution of aluminum sulphate and an aqueous solution of sodium aluminate are then introduced simultaneously, the introduction rates of which are such that the average pH of the reaction mixture is maintained in the pH range referred to in 1 'step (a); the temperature of the reaction mixture for steps (a) and (b) being at least 60 ° C;

(c) at the end of step (b), the pH of the reaction mixture is optionally adjusted to a value between 7.5 and 10.5, or even between 8.0 and 9.0 or between 9.0 and 10.0;

(d) the reaction mixture is then filtered and the solid recovered is washed;

(e) a dispersion in water of the solid recovered at the end of step (d) undergoes mechanical or ultrasonic treatment so as to reduce the size of the particles of the dispersion;

(f) the dispersion obtained at the end of step (e) is dried;

(g) the solid resulting from step (f) is then calcined in air. step (a)

In step (a), the following are introduced with stirring into a tank initially containing an acidic aqueous solution, the pH of which is between 0.5 and 4.0, or even between 0.5 and 3.5:

(a2) - either simultaneously (i) an aqueous solution of aluminum sulphate and (ii) an aqueous solution of sodium aluminate until a pH of the reaction mixture is obtained between 6.5 and 10.0, or even between 7.0 and 8.0 or between 8.5 and 9.5; so that at the end of step (a), the aluminum concentration of the reaction mixture is between 0.50% and 3.0% by weight.

The acidic aqueous solution initially contained in the tank has a pH of between 0.5 and 4.0, or even between 0.5 and 3.5. This solution may consist of a dilute aqueous solution of a mineral acid such as, for example, sulfuric acid, hydrochloric acid or nitric acid.

The acidic aqueous solution can also consist of an aqueous solution of an acidic aluminum salt such as aluminum nitrate, chloride or sulfate. Preferably, the aluminum concentration of this solution is between 0.01% and 2.0% by weight, or even between 0.01% and 1.0% by weight, or even between 0.10% and 1.0% by weight. . Preferably the acidic aqueous solution is an aqueous solution of aluminum sulfate. This solution is prepared by dissolving aluminum sulphate in water or else by diluting preformed aqueous solution (s) in water. The pH of the aqueous solution developed by the presence of aluminum sulfate is generally between 0.5 and 4.0, or even between 0.5 and 3.5.

Step (a) is implemented according to two embodiments (a1) or (a2). According to embodiment (a1), an aqueous solution of sodium aluminate is introduced with stirring. According to embodiment (a2), (i) an aqueous solution of aluminum sulphate and (ii) an aqueous solution of sodium aluminate are simultaneously introduced with stirring.

Preferably, the aqueous solution of sodium aluminate does not have precipitated alumina. The sodium aluminate preferably has an Na ₂ 0 / Al ₂ C> ₃ ratio greater than or equal to 1. 20, for example between 1. 20 and 1. 40.

The aqueous solution of sodium aluminate may have an aluminum concentration of between 15.0% and 35.0% by weight, more particularly between 15.0% and 30.0% by weight, or even between 20.0% and 30.0% by weight. , 0%. The aqueous solution of aluminum sulphate can have an aluminum concentration of between 1.0% and 15.0% by weight, more particularly between 5.0% and 10.0% by weight.

At the end of step (a), the aluminum concentration of the reaction mixture is between 0.50% and 3.0% by weight.

In this step (a), the duration of introduction of the solution (s) is generally between 2 min and 30 min.

In step (a), the introduction of the aqueous solution of sodium aluminate has the effect of increasing the pH of the reaction mixture.

In particular for embodiment (a1), the aqueous solution of sodium aluminate can be introduced directly into the reaction medium, for example by means of at least one introduction pipe. In particular for embodiment (a2), the two solutions can be introduced directly into the reaction medium, for example by means of at least two introduction pipes. For these two embodiments (a1) and (a2), the solution (s) is / are preferably introduced into a well-stirred zone of the reactor, for example in a zone close to the stirring wheel, so as to obtain an effective mixture of the solution (s) introduced into the reaction mixture. For embodiment (a2), when the solutions are introduced via at least two introduction pipes, the injection points through which the two solutions are introduced into the reaction mixture are distributed so as to that the solutions dilute effectively in said mixture. Thus, for example, it is possible to place two rods in the tank so that the points of injection of the solutions into the reaction mixture are diametrically opposed. step (b) In step (b), an aqueous solution of aluminum sulphate and an aqueous solution of sodium aluminate are introduced simultaneously, the introduction rates of which are regulated so as to maintain the average pH of the reaction mixture in the range of pH referred to in step (a). Thus, the target value of the mean pH is between: between 8.0 and 10.0, or even between 8.5 and 9.5, for the case where the embodiment (a1) has been followed in step (a ); or else between 6.5 and 10.0, or even between 7.0 and 8.0 or between 8.5 and 9.5, for the case where the embodiment (a2) has been followed in step (a ).

The term “mean pH” is understood to mean the arithmetic mean of the pH values of the reaction mixture which are recorded continuously during step (b).

Preferably, the aqueous solution of sodium aluminate is introduced at the same time as the aqueous solution of aluminum sulphate at a rate which is regulated so that the average pH of the reaction mixture is equal to the target value. The flow rate of the aqueous sodium aluminate solution used to regulate the pH may fluctuate during step (b).

The duration of introduction of the two solutions may be between 10 minutes and 2 hours, or even between 30 minutes and 90 minutes. The rate of introduction of the or both solutions can be constant.

It is necessary that the temperature of the reaction mixture for steps (a) and (b) is at least 60 ° C. This temperature can be between 60 ° C and 95 ° C. To do this, the solution initially contained in the tank in step (a) may have been preheated before the start of the introduction of the solution (s). It is also possible to preheat the solutions which are introduced into the tank in steps (a) and (b) beforehand. step (cl

In step (c), the pH of the reaction mixture is optionally adjusted to a value between 7.5 and 10.5, or even between 8.0 and 9.0 or between 9.0 and 10.0, by l addition of a basic or acidic aqueous solution.

The acidic aqueous solution which can be used to adjust the pH may consist of an aqueous solution of a mineral acid such as, for example, sulfuric acid, hydrochloric acid or nitric acid. The aqueous acidic solution can also consist of an aqueous solution of an acidic aluminum salt such as aluminum nitrate, chloride or sulfate.

The basic aqueous solution which can be used to adjust the pH may consist of an aqueous solution of a mineral base such as, for example, soda, potassium hydroxide, ammonia. The basic aqueous solution can also consist of an aqueous solution of a basic aluminum salt such as sodium aluminate. Preferably, an aqueous solution of sodium aluminate is used.

Preferably, the pH is adjusted by stopping:

(c1) - the introduction of the aqueous sulphate solution and the introduction of the aqueous sodium aluminate solution is continued until the target pH is reached; or (c2) - the introduction of the aqueous solution of sodium aluminate and the introduction of the aqueous solution of aluminum sulphate is continued until the target pH is reached.

According to one embodiment, the introduction of the aqueous solution of aluminum sulphate is stopped and the aqueous solution of sodium aluminate is continued to be introduced until a target pH of between 8.0 and 10.5 is reached. , preferably between 9.0 and 10.0. The duration of step (c) can be variable. This duration can be between 5 min and 30 min. step (d)

In step (d), the reaction mixture is filtered. The reaction mixture is generally in the form of a slurry. The solid collected on the filter can be washed with water. To do this, you can use hot water with a temperature of at least 50 ° C. step (e)

In step (e), a dispersion in water of the solid recovered at the end of step (d) undergoes mechanical or ultrasonic treatment so as to reduce the size of the particles of the dispersion. The pH of this dispersion before grinding can optionally be adjusted between 5.0 and 8.0. You can use a nitric acid solution, for example.

The D50 of the particles of the dispersion before the mechanical or ultrasonic treatment is generally between 10.0 pm and 40.0 pm, or even between 10.0 pm and 30.0 pm. The D50 of the particles of the solid after the mechanical or ultrasonic treatment is preferably between 1.0 μm and 15.0 μm, or even between 2.0 μm and 10.0 μm.

Mechanical treatment involves applying mechanical stress or shear forces to the dispersion so as to split the particles. The mechanical treatment can, for example, be carried out using a ball mill, a high pressure homogenizer or a grinding system comprising a rotor and a stator. On a laboratory scale, it is possible to use a Microcer ball mill or a Labstar Zêta, both marketed by the company Netzsch (for more details, see: https://www.netzsch-qrindinq.com/ en / products-solutions / wet-brovaqe / mini-series-laboratory-grinders / j. A grinding system as described in the examples can be used. In the case of a ball mill, it is possible to use example of beads of zirconium oxide stabilized with yttrium It is possible, for example, to use 0.2 mm ZetaBeads Plus beads.

Ultrasound treatment consists of applying a sound wave to the dispersion. The sound wave which propagates in the liquid medium induces a phenomenon of cavitation allowing the particles to be split. On a laboratory scale, it is possible to use an ultrasound system with a Vibracell VC750 type sound generator equipped with a 13 mm probe. The duration and the power applied are adjusted so as to reach the target D50.

The mechanical or ultrasonic treatment can be carried out in batch mode or else continuously. step (f)

In step (f), the dispersion from step (e) is dried, preferably by atomization.

Spray drying has the advantage of resulting in particles with a controlled particle size distribution. This drying method also has good productivity. It involves spraying the dispersion into a cloud of droplets in a stream of hot gas (eg a stream of hot air) circulating in an enclosure. The quality of the spray controls the size distribution of the droplets and hence the size distribution of the dried particles. Spraying can be carried out using any sprayer known per se. There are two main types of spray devices: turbines and nozzles. On the various spraying techniques that may be used in the present process, reference may be made in particular to the basic work by MASTERS entitled “SPRAY-DRYING” (second edition, 1976, Editions George Godwin London). operating parameters on which a person skilled in the art can act are in particular the following: the flow rate and the temperature of the dispersion entering the sprayer; the flow rate, the pressure, the humidity and the temperature of the hot gas. of the gas is generally between 100 ° C. and 800 ° C. The gas outlet temperature is generally between 80 ° C. and 150 ° C.

The D50 of the powder recovered at the end of step (g) is generally between 2.0 µm and 80.0 µm. This size is linked to the size distribution of the droplets at the outlet of the sprayer. The vaporizer capacity of the atomizer is generally related to the size of the enclosure. Thus, at the laboratory scale (Büchi B 290), the D50 can be between 2.0 and 15.0 µm. On a larger scale, the D50 can be between 15.0 and 80.0 µm. step (g)

In step (g), the solid resulting from step (f) is calcined in air. The calcination temperature is generally between 500 ° C and 1000 ° C, more particularly between 800 ° C and 1000 ° C. The duration of the calcination is generally between 1 and 10 h. It is possible to use the calcination conditions given in the examples.

It is possible to carry out the two steps (f) and (g) in the same equipment in which the dispersion resulting from step (e) undergoes a heat treatment allowing both drying and calcination to be carried out.

Preferably, the alumina which is recovered at the end of step (g) (that is to say at the end of the calcination) has a D50 generally between 2.0 μm and 80.0. pm. It generally has a D90 less than or equal to 150.0 μm, more particularly less than or equal to 100.0 μm.

According to a first embodiment, at the end of step (g), the D50 can be between 2.0 and 15.0 pm, or even between 4.0 and 12.0 pm. The D90 can be between 20.0 pm and 60.0 pm, or even between 25.0 pm and 50.0 pm. This embodiment can instead be implemented when step (f) is carried out on a laboratory scale using, for example, a Büchi B 290 atomizer.

According to a second embodiment, at the end of step (g), the D50 can be between 15.0 and 80.0 pm, or even between 20.0 and 60.0 pm. The D90 can be between 40.0 pm and 150.0 pm, or even between 50.0 pm and 100.0 pm. This embodiment can rather be implemented when step (f) is carried out on a larger scale.

The process can also include a final step whereby the solid obtained in the previous step undergoes grinding in order to adjust the particle size of the solid. You can use a knife, air jet, hammer or ball mill. Preferably, the ground product has an OD generally between 2.0 µm and 15.0 µm. The D90 can be between 20.0 pm and 60.0 pm, or even between 25.0 pm and 50.0 pm.

The alumina of the invention is in the form of a powder.

More details for the preparation of the alumina of the invention will be found in the following illustrative example.

Example

Measurement of the specific surface:

For the remainder of the description, the term “specific surface area” means the specific surface area B.E.T. determined by nitrogen adsorption in accordance with the ASTM D 3663-03 standard established from the BRUNAUER-EMMETT-TELLER method described in the periodical "The Journal of the American Chemical Society, 60, 309 (1938)". The specific surface is determined automatically using an apparatus, for example of the T ristar II 3020 type from Micromeritics, in accordance with the indications recommended by the manufacturer. The samples are pretreated at 250 ° C. for 90 min under vacuum (for example to reach a pressure of 50 mm of mercury). This treatment makes it possible to eliminate volatile species that are physisorbed on the surface (such as for example H2O, ...).

Measurement of mercury porosity

The measurement is carried out using a mercury porosimetry measuring device. In our case, a Micromeritics Autopore IV 9520 device fitted with a powder penetrometer was used, complying with the instructions recommended by the manufacturer. The following parameters were used: penetrometer used: 3.2 ml (Micromeritics reference: type No. 8 penetrometer); capillary volume: 0.412 ml; max pressure ("head pressure"): 4.68 psi; contact angle: 130 °; surface tension of mercury: 485 dynes / cm; density of mercury: 13.5335 g / ml. At the start of the measurement, a vacuum of 50 mm Hg is applied to the sample for 5 min. The equilibrium times are as follows: low pressure range (1, 3-30 psi): 20 s - high pressure range (30-60,000 psi): 20 s. Prior to the measurement, the samples are treated at 200 ° C. for 120 min to eliminate the volatile species physisorbed on the surface (such as for example H2O, etc.).

Particle size measurement (D10, D50, D90)

To carry out the particle size measurements, a MALVERN Mastersizer 2000 or 3000 laser diffraction particle size analyzer is used (more details on this device given here: https://www.malvernpanalvtical.com/fr/products/product-ranqe/mastersizer - ranqe / mastersizer-3000). The laser diffraction technique used consists in measuring the intensity of the light scattered during the passage of a laser beam through a sample of dispersed particles. The laser beam passes through the sample and the intensity of the scattered light is measured as a function of the angle. The diffracted intensities are then analyzed to calculate the particle size using the Mie diffusion theory. The measurement makes it possible to obtain a volume distribution of sizes from which the parameters D10, D50 and D90 are deduced.

Example: preparation of an aluminum oxide according to the invention

157 kg of deionized water are introduced into a tank stirred using a mobile stirrer with inclined blades, fitted with a pH probe located in the upper part of the liquid, which is heated to 85 ° C. This temperature will be maintained throughout steps (a) to (c). 13.8 kg of an aluminum sulphate solution with a concentration of 8.3% by weight of alumina (AI2O3) are introduced via an introduction rod close to the stirring unit, at a flow rate of 920 g of solution. / min. At the end of the introduction, the pH of the starter is close to 2.6 and the aluminum concentration is 0.7% by weight. The introduction of the aluminum sulphate solution is then stopped. step (a): using a second introduction rod close to the stirring unit, a sodium aluminate solution with a concentration of 24.9% by weight of alumina (AI2O3) and a molar ratio Na ₂ 0 / Al ₂ C> ₃ from 1.27 at a flow rate of 690 g of solution / min until a pH of 9.0 is reached. The introduction is then stopped. The aluminum concentration of the reaction mixture is then 2.10%.

In step (b), the introduction of the aluminum sulphate solution is again started at a flow rate of 570 g of solution / min and the sodium aluminate solution is simultaneously introduced into the stirred reactor at a flow rate regulated so as to maintain the pH at a value of 9.0. This stage lasts 45 minutes,

In step (c), the introduction of the aluminum sulphate solution is stopped and the sodium aluminate solution is continued to be added with a flow rate of 320 g of solution / min until a pH of 9.5. The addition of the sodium aluminate solution is stopped.

In step (d), the reaction slurry is poured onto a vacuum filter. After filtration, the cake is washed with deionized water at 65 ° C.

In step (e), the cake is redispersed in deionized water to obtain a suspension with a concentration of around 10% by weight of oxide (Al2O3). A nitric acid solution with a concentration of 69% by weight is added to the suspension so as to obtain a pH close to 6. The suspension is passed through a ball mill of the LME20 brand from the manufacturer Netzsch. The operating conditions of the mill are adjusted so as to obtain a D50 of 3.5 microns.

In step (f), the suspension from step (e) is atomized to obtain a dry powder of aluminum hydrate doped with lanthanum.

In step (g), the atomized powder is calcined at 940 ° C for 2 hours (temperature rise rate of 3 ° C / min). The mass loss observed during this calcination is 26.9%.

An alumina is then obtained which has the following characteristics:

- specific surface: 174 m ² / g;

- specific surface after calcination in air at 1100 ° C / 5 h: 81 m ² / g; - pore volume in the field of pores, the size of which is between 5 nm and 100 nm: 0.69 mL / g;

- pore volume in the field of pores, the size of which is between 100 nm and 1000 nm: 0.02 mL / g; - total pore volume: 1.51 ml / g;

- after calcination in air at 1100 ° C / 5 h, pore volume in the range of pores, the size of which is between 5 nm and 100 nm: 0.55 ml / g;

- after calcination in air at 1100 ° C / 5 h, pore volume in the range of pores whose size is between 100 nm and 1000 nm: 0.01 mL / g: - after calcination in air at 1100 ° C / 5 h, total pore volume: 1.15 mL / g. other characteristics of alumina

Claims

1. Alumina characterized by at least one of the following two porosity profiles:

^■ 1 ^st profile: - a pore volume in the area of the pores, the size of which is between 5 nm and 100 nm which is between 0.60 and 0.85 mL / g; and a pore volume in the area of the pores the size of which is between 100 nm and 1000 nm which is less than or equal to 0.20 mL / g; and / or ■ 2 ^nd profile: after calcination in air at 1100 ° C for 5 hours: a pore volume in the area of pores having a size between 5 nm and 100 nm is between 0.50 and 0.75 mL / g; and a pore volume in the area of the pores the size of which is between 100 nm and 1000 nm which is less than or equal to 0.20 mL / g; these pore volumes being determined using the mercury porosimetry technique.

2. Alumina according to claim 1 characterized in that for the 1 ^st profile, the pore volume in the region of the pores, the size of which is between 5 nm and 100 nm, is between 0.60 and 0.80 mL / g.

3. Alumina according to claim 1 or 2 characterized in that for the 2 ^nd profile, the pore volume in the range of pores whose size is between 5 nm and 100 nm is between 0.50 and 0.70 mL / g.

4. Alumina according to one of the preceding claims, characterized in that for the 1 ^st profile, the pore volume in the region of the pores, the size of which is between 100 nm and 1000 nm, is less than or equal to 0.15 mL / g, or even less than or equal to 0.10 mL / g, or even less than or equal to 0.05 mL / g.

5. Alumina according to one of the preceding claims, characterized in that for the 2 ^nd profile the pore volume in the region of the pores, the size of which is between 100 nm and 1000 nm, is less than or equal to 0.15 mL / g, or even less than or equal to 0.10 mL / g, or even less than or equal to 0.05 mL / g.

6. Alumina according to one of the preceding claims having a BET specific surface area of between 100 and 200 m ² / g, more particularly between 150 and 200 m ² / g.

7. Alumina according to one of the preceding claims having a BET specific surface which is greater than or equal to 120 m ² / g.

8. Alumina according to one of the preceding claims having a BET specific surface which is greater than or equal to 140 m ² / g.

9. Alumina according to one of the preceding claims having a specific surface.

BET after calcination in air at 1100 ° C for 5 hours between 70 and 100 m ² / g.

10. Alumina according to one of the preceding claims having an apparent density of between 0.25 g / cm ³ and 0.55 g / cm ³ , more particularly between 0.40 g / cm ³ and 0.55 g / cm ^3. .

11. Alumina according to one of the preceding claims having a total pore volume which is strictly greater than 1.05 mL / g, this pore volume being determined using the mercury porosimetry technique.

12. Alumina according to one of the preceding claims having a total pore volume which is at least 1.10 mL / g, this pore volume being determined using the mercury porosimetry technique.

13. Alumina according to one of the preceding claims having a total pore volume which is at least 1.20 mL / g, this pore volume being determined using the mercury porosimetry technique.

14. Alumina according to one of the preceding claims having a total pore volume which is at most 2.40 mL / g, this pore volume being determined using the mercury porosimetry technique.

15. Alumina according to one of the preceding claims having, after calcination at 1100 ° C for 5 hours, a total pore volume which is at least 0.90 mL / g, this pore volume being determined using the technique. mercury porosimetry.

16. Alumina according to one of the preceding claims having, after calcination at 1100 ° C for 5 hours, a total pore volume which is at least 1.10 mL / g, this pore volume being determined using the technique. mercury porosimetry.

17. Alumina according to one of the preceding claims having, after calcination at 1100 ° C for 5 hours, a total pore volume which is at least 1.20 mL / g, this pore volume being determined using the technique. mercury porosimetry.

18. Alumina according to one of the preceding claims having, after calcination at 1100 ° C for 5 hours, a total pore volume which is at most 1.80 mL / g, this pore volume being determined using the technique. mercury porosimetry.

19. Alumina according to one of the preceding claims, characterized for the 1 ^st profile, by a pore volume in the region of the pores, the size of which is between 100 nm and 1000 nm, which is less than or equal to 0.05 mL / g.

20. Alumina according to one of the preceding claims, characterized for the 2 ^nd profile, by a pore volume in the region of the pores, the size of which is between 100 nm and 1000 nm which is less than or equal to 0.05 mL / g, this pore volume being determined after calcination in air at 1100 ° C. for 5 hours.

21. Alumina according to one of the preceding claims, characterized by a D50 of between 2.0 μm and 80.0 μm, D50 denoting the median of a volume distribution of the particle sizes obtained using a laser particle size analyzer. .

22. Alumina according to one of the preceding claims, characterized by a D90 less than or equal to 150.0 pm, more particularly less than or equal to 100.0 pm, D90 designating the size for which 90% of the particles have a size which is less. at D90, D90 being determined from a volume distribution of the particle sizes obtained using a laser particle size analyzer.

23. Alumina according to claim 21 characterized by a D50 of between 15.0 and 80.0 pm, or even between 20.0 and 60.0 pm.

24. Alumina according to claim 23 characterized by a D90 of between 40.0 pm and 150.0 pm, or even between 50.0 pm and 100.0 pm, D90 designating the size for which 90% of the particles have a size which is. less than D90, D90 being determined from a volume distribution of the particle sizes obtained using a laser particle size analyzer.

25. Alumina according to claim 23 or 24, characterized by an apparent density of between 0.40 and 0.55 g / cm ³ .

26. Alumina according to one of claims 23 to 25, characterized by a total pore volume of between 1.05 (excluded value) and 1.80 mL / g.

27. Alumina according to claim 26 characterized by a total pore volume of between 1.20 and 1.80 mL / g.

28. Alumina according to claim 21 characterized by a D50 of between 2.0 and 15.0 pm, or even between 4.0 and 12.0 pm.

29. Alumina according to claim 28 characterized by a D90 of between 20.0 pm and 60.0 pm, or even between 25.0 pm and 50.0 pm, D90 designating the size for which 90% of the particles have a size which is less than D90, D90 being determined from a volume distribution of the particle sizes obtained using a laser particle size analyzer.

30. Alumina according to claim 28 or 29, characterized by an apparent density of between 0.25 and 0.40 g / cm ³ .

31. Alumina according to claim 28 to 30, characterized by a total pore volume of between 1.40 and 2.40 mL / g.

32. Alumina according to claim 31, characterized by a total pore volume of between 1.50 and 2.40 mL / g.

33. Alumina according to one of the preceding claims having a sodium level less than or equal to 0.50% by weight, or even less than or equal to 0.15% by weight, this sodium level being expressed by weight of Na ₂ 0 relative to the total weight of the alumina.

34. Alumina according to one of the preceding claims having a sodium level greater than or equal to 50 ppm, this sodium level being expressed by weight of Na ₂ 0 relative to the total weight of the alumina.

35. Alumina according to one of the preceding claims having a sodium level of between 50 and 900 ppm, or even between 100 and 800 ppm, this sodium level being expressed by weight of Na ₂ 0 relative to the total weight of the alumina. .

36. Alumina according to one of the preceding claims having a sulphate content less than or equal to 1.00% by weight, or even less than or equal to 0.20% by weight, or even less than or equal to 0.10% by weight. , this sulphate level being expressed by weight of SO ₄ relative to the total weight of the alumina.

37. Alumina according to one of the preceding claims having a sulphate level greater than or equal to 50 ppm, this sulphate level being expressed by weight of SO ₄ relative to the total weight of the alumina.

38. Alumina according to one of the preceding claims having a sulphate level of between 100 and 1500 ppm, or even between 400 and 1000 ppm, this sulphate level being expressed by weight of SO ₄ relative to the total weight of the alumina.

39. Alumina according to one of the preceding claims, characterized in that it is crystallized.

40. Alumina according to one of the preceding claims of general formula Al ₂ O ₃ .

41. Alumina according to one of the preceding claims exhibiting the 1 ^st and the 2 ^nd porosity profile.

42. Catalytic composition comprising the alumina according to one of claims 1 to 41 and at least one oxide based on cerium and optionally on at least one rare earth other than cerium.

43. Use of an alumina according to one of claims 1 to 41 in the preparation of a catalyst for depolluting the exhaust gases of gasoline or diesel heat engines.

44. Process for preparing an alumina, in particular an alumina as described in one of claims 1 to 41, comprising the following steps:

(a2) - either simultaneously (i) an aqueous solution of aluminum sulphate and (ii) an aqueous solution of sodium aluminate until a pH of the reaction mixture is obtained between 6.5 and 10, or even between 7, 0 and 8.0 or between 8.5 and 9.5; so that at the end of step (a), the aluminum concentration in the reaction mixture is between 0.50% and 3.0% by weight; (b) then an aqueous solution of aluminum sulphate and an aqueous solution of sodium aluminate are then introduced simultaneously, the introduction rates of which are such that the average pH of the reaction mixture is maintained in the pH range referred to in 1 'step (a); the temperature of the reaction mixture for steps (a) and (b) being at least 60 ° C;

(d) the reaction mixture is then filtered and the solid recovered is washed;

(f) the dispersion obtained at the end of step (e) is dried;

(g) the solid resulting from step (f) is then calcined in air.

45. The method of claim 44 characterized in that the acidic aqueous solution initially contained in the tank is an aqueous solution of a mineral acid or an aqueous solution of an acid salt of aluminum.

46. The method of claim 44 or 45 wherein for embodiment (a1), the aqueous solution of sodium aluminate is introduced directly into the reaction mixture, in particular via at least one cane of sodium aluminate. introduction.

47. Method according to one of claims 44 to 46 wherein for embodiment (a2), the two solutions are introduced directly into the reaction mixture, in particular via at least two introduction pipes.

48. Method according to one of claims 44 to 47 wherein the target value referred to in step (b) is: between 8.0 and 10.0, or even between 8.5 and 9.5, for the case where the embodiment (a1) has been followed in step (a); or else between 6.5 and 8.5, or even between 7.0 and 8.0, for the case where the embodiment (a2) has been followed in step (a).

49. Method according to one of claims 44 to 48 wherein the D50 of the particles of the dispersion before the mechanical or ultrasonic treatment is between 10.0 pm and 40.0 pm, or even between 10.0 pm and 30, 0 pm.

50. Method according to one of claims 44 to 49 wherein the particles of the solid after the mechanical or ultrasonic treatment is between 1.0 pm and 15.0 pm, or even between 2.0 pm and 10.0 pm.

51. Method according to one of claims 44 to 50 wherein the mechanical treatment of step (e) is carried out using a ball mill, a high pressure homogenizer or a grinding system. comprising a rotor and a stator.