FR2780716A1 - A new dispersible aluminium hydrate compound for use in the preparation of alumina for catalysts or catalysts supports - Google Patents

Abstract

The aluminium hydrate is obtained with a high dispersibiltiy in water which enabled the subsequent preparation of alumina with a monomodal pore distribution and small pore diameter. The aluminium hydrate has a dispersibiltiy of at least 80% wt. for a diameter less than or equal to 0.3 mu . Independent claims are also included for its preparation and uses.

Description

NEW DISPERSIBLE ALUMINUM HYDRATE,

ITS PREPARATION PROCESS

  AND ITS USE FOR THE PREPARATION OF CATALYSTS

  The present invention relates to a new aluminum hydrate, a process for the preparation of this hydrate and its use for the preparation of catalysts or

catalyst supports.

  Aluminum hydrates are used for the preparation of catalysts or catalyst supports. The preparation usually consists of shaping the aluminum hydrate, then calcining it to transform it into alumina. The properties of aluminum hydrate influence the characteristics of the alumina obtained and, by

  Therefore, the application properties of catalysts and catalyst supports.

  Thus, a high dispersibility in water of aluminum hydrates is a sought-after property because it makes it possible to prepare catalysts or catalyst supports with advantageous properties: - first of all, a dispersible aluminum hydrate leads to catalysts not containing agglomerates or alumina aggregates. So we get

homogeneous supports.

  - then, with a dispersible aluminum hydrate, the shaping conditions are milder (for example, the acid content is lower) which allows

  maintain a large total pore volume.

  - Finally, the catalysts or catalyst supports from a dispersible aluminum hydrate have a monomodal porous distribution without meso- and / or macropores (by meso and / or macropores is meant any pore larger than the median diameter). In certain applications such as hydrotreatment in petroleum refineries, the absence of these meso and / or macropores is sought: in fact, if the pore diameter is small (100 A), the metals do not enter the support which avoids its deactivation. In addition, if the porous distribution is narrow, only certain molecules enter the support, which makes it possible to carry out molecular sieving. Furthermore, the effects of cockage are limited and the absence of macropores

  increases the mechanical resistance of the support.

  An object of the present invention is therefore to propose an aluminum hydrate

  with high dispersibility in water.

  Another object is to propose a process for the preparation of this aluminum hydrate. For these purposes, the invention relates first of all to an aluminum hydrate having a dispersibility in water of at least 80% by weight for a diameter

less than or equal to 0.3 pm.

  The invention also relates to the process for the preparation of such an aluminum hydrate by implementing the following steps: 1- mixing an acid source of aluminum and a base, or a basic source of aluminum and an acid, so as to precipitate an aluminum monohydrate, 2-ripening, 3-filtration, 4-washing, and - drying,

  the mixing of step 1 being carried out without back-mixing.

  Finally, the invention relates to the use of this aluminum hydrate for the

  preparation of catalysts or catalyst supports.

  Figure 1 shows the particle size distributions of an aluminum hydrate

  according to the invention and an aluminum hydrate according to the prior art.

  The invention therefore relates first of all to an aluminum hydrate having a

  dispersibility of at least 80% by weight for a diameter less than or equal to 0.3 μm.

  The aluminum hydrate according to the invention is mainly of crystalline structure

  aluminum monohydrate (AIOOH, nH20) of the boehmite and / or pseudoboehmite type.

  Mostly means that the level of aluminum monohydrate of the aluminum hydrate according to the invention is greater than 30% by weight, preferably greater than%. The crystallization rate and the nature of the crystalline phase are measured by X-ray diffraction. The supplement to aluminum monohydrate is generally an amorphous aluminum hydrate or a very fine aluminum hydrate which cannot be analyzed by

X-ray diffraction.

  This aluminum hydrate is characterized by its high dispersibility, which is at least 80% by weight for a diameter less than or equal to 0.3 μm, and preferably at least 90% by weight for a diameter less than or equal to 0.3 pm. The aluminum hydrate according to the invention is in particular dispersible in water at a pH different from the point

  isoelectric (PIE) of aluminum hydrate.

  The dispersibility of aluminum hydrate is evaluated from. of its particle size analysis giving the cumulative weight as a function of the equivalent sphere diameter. This analysis is obtained by gravity sedimentation of a suspension

  of the hydrate to be analyzed in a Sedigraph apparatus by following the following protocol.

  The aluminum hydrate is kneaded for 30 minutes using a kneader of the Brabender type with nitric acid, at a rate of 6% by weight relative to the aluminum hydrate, and with l 'water, the amount of water being such that the mixture leads to a paste having a loss on ignition at 1000 C of about 58% by weight. Then, the paste obtained is diluted to 100 g / I with deionized water and the dispersion obtained is homogenized with ultrasound of moderate power (2 minutes with a 5 mm probe at the power of 100 Watts). Finally, the dispersion is analyzed using a Sedigraph using the following sedimentation conditions: - density of alumina: 3.25 g / cm3 - carrier liquid: water - density of liquid: 0.9957 g / cm3 - viscosity of the medium: 0.8007 mPa.s

- test temperature: 30 C.

  From the particle size curve given by the Sedigraph apparatus, the dispersibility of the aluminum hydrate is read, which corresponds to the percentage by mass of particles whose size is less than 0.23 μm. The higher this percentage,

the better the dispersibility.

  The aluminum hydrate according to the invention is usually in the form of a powder. The invention also relates to the process for the preparation of such an aluminum hydrate, in which the following steps are carried out: 1- mixing an acid source of aluminum and a base, or a source basic aluminum and an acid, so as to precipitate an aluminum monohydrate, 2-ripening, 3-filtration, 4-washing, and drying,

  the mixing of step 1 being carried out without back-mixing.

  This process uses the usual steps for the preparation of an aluminum hydrate by acid-base precipitation, namely precipitation of the hydrate from acid and / or basic aluminum salts, then ripening, filtration, washing and drying

of the hydrate obtained.

  In step 1, the acid source of aluminum can be, for example, chosen from at least one of the following compounds: aluminum chloride, aluminum sulfate, aluminum nitrate. The basic source of aluminum can be chosen from the salts

  aluminum basics such as sodium aluminate, potassium aluminate.

  Depending on the acidic or basic nature of the aluminum-based starting compound, the aluminum hydrate is precipitated using a base or an acid chosen, for example, from: hydrochloric acid, l sulfuric acid, sodium hydroxide or a basic or acidic aluminum compound as mentioned above. The two reagents can, for example, be aluminum sulfate and sodium aluminate.

  Reagents are usually used in the form of aqueous solutions.

  The precipitation of aluminum hydrate is obtained by controlling the pH. The pH is chosen so as to obtain the precipitation of aluminum monohydrate; in general, this pH is between 6 and 10 for a temperature of 60 C, preferably between 8 and 10. Control is usually carried out by the choice of flow rates and

  concentrations of reagents introduced into the reactor.

  Upon precipitation, a suspension of aluminum hydrate is formed.

  The essential characteristic of the present process is due to the nature of the reactor used in this step 1. According to the invention, the mixing of the reagents of step 1 must be carried out so that the back-mixing phenomenon is minimized, and preferably completely avoid. By retromixing is meant the phenomenon by which the product resulting from the reaction, that is to say aluminum hydrate, comes into contact with the starting reagents which continue to be introduced into the reactor and with l hydrate which was previously formed in the reactor. The reactor used must therefore, for example, be such that the reaction product is removed from the place of introduction of the

starting reagents.

  Such a mixture can be produced in a piston flow reactor and,

  in particular, in a static mixer. Mention may be made of Lightnin mixers.

  Steps 2 to 5 are similar to those commonly used in the

aluminum hydrate synthesis.

  In step 2, the aluminum hydrate obtained, in dispersion in water, is matured. It can be simply stored in a tank, it can also be heated or put

under pressure in an autoclave.

  In step 3, the suspension of matured aluminum hydrate is filtered.

  In step 4, the filtered aluminum hydrate agglomerate is washed with water to

  remove unwanted impurities.

  In step 5, the washed aluminum hydrate is dried: an aluminum hydrate powder is obtained. This drying is carried out at a temperature which avoids elimination

bound water.

  Finally, the invention relates to the use of the preceding aluminum hydrate or resulting from the preceding process for the preparation of catalysts or catalyst supports

based on alumina.

  This preparation is known to those skilled in the art and generally consists in

  form the aluminum hydrate, then calcine it.

  The shaping can be carried out by any known method, such as for example: kneading / extrusion, shaping in drops (oil-drop), granulation, compacting, atomization. The hydrate formed is then calcined at a temperature which can vary between

  300 and 1000 C, preferably between 500 and 800 C.

  The major advantage of the aluminum hydrate according to the invention in the preparation of catalysts or catalyst supports is that its use makes it possible to control the level of macropores in the catalysts or catalyst supports. The use of this hydrate in fact makes it possible to avoid the presence of meso- or macropores in the alumina of the catalysts or catalyst supports. By macropores is meant all the pores whose diameter is greater than the value of the median diameter, preferably than the value of the median diameter plus 30 A. The median diameter is defined, on the other hand, from the porous distribution curve, obtained by mercury porosity measurement: it corresponds to the inflection point of this curve (i.e. the diameter for which the

  derived from the curve volume = f (diameter) goes through a maximum).

  The following examples illustrate the invention without, however, limiting its scope.

EXAMPLES

  Example 1 according to the invention Etaoe 1: precipitation

  A static Lightnin DN25 diameter mixer is used.

  The following reagents are introduced into the reactor at the concentrations and flow rates as indicated below: aluminum sulphate 22.75 g / I 500 I / h sodium aluminate 290 g / I 86 Vh

  The temperature is maintained at 60 C.

  At the outlet of the static mixer, the pH is 9 and the hydrate concentration

of aluminum is 60 g / I.

  Step 2- ripening The hydrate obtained is introduced into a stirred reactor, the agitator of which is a propeller, the stirring power being 500 kW / m3. The reactor volume is

liters.

  Sodium aluminate at 290 g / l is reintroduced batchwise to maintain the pH at 9.

  The ripening temperature is 90 C. The duration is 3 hours.

  tes 3. 4. 5 The hydrate is then filtered and washed with water. The amount of washing water is

  about 30 40 Vkg of aluminum hydrate.

  Then, the hydrate is spray-dried, the outlet temperature is 11 5 C.

  Result The hydrate obtained has the following characteristics: majority crystalline nature: boehmite, - boehmite content: 75% by weight, - size of the crystallites: 45 A,

  - loss on ignition at 1000 C: 23% by weight.

  The dispersibility of the hydrate is measured using a Sedigraph 5000 ET according to the protocol defined above. More specifically, 46.4 g of aluminum hydrate are kneaded for 30 minutes with a solution containing 3.15 g of nitric acid and 38.64 g of deionized water. The dough obtained has a loss on ignition at 1000 C of 59.5% by weight. This paste is diluted to 100 g / I with deionized water and the dispersion obtained is homogenized with ultrasound of moderate power (2 minutes with a 5 mm probe at the power of 100 Watts). Finally, the dispersion is analyzed at

  Sedigraph by retaining the sedimentation conditions defined above.

  The particle size analysis appears in FIG. 1. It can be seen that the percentage by mass of particles whose size determined by Sedigraph is

  less than 0.23 µm is 92%. This aluminum hydrate is highly dispersible.

  Comparative Example 2 EtaPe i1: Drécioitation A stirred reactor is used, the agitator of which is a propeller, the stirring power being 500 kW / m3. The reactor volume is 12 liters, it operates in

  continuous using level control.

  The following reagents are introduced into the reactor at the concentrations and flow rates as indicated below: aluminum sulphate 22.75 g / I 52.5 Vh sodium aluminate 290 g / I 8.75 I / h

  The temperature is maintained at 60 C.

  The pH is maintained at 9 by controlling the flow rates of the reagents, and the final concentration of aluminum hydrate is of the order of 60 g / I. Step 2- mOrissement

  This step is identical to that of Example 1.

Etaes 3. 4.5

  The aluminum hydrate is then filtered, washed and dried as in Example 1.

  Results The aluminum hydrate obtained has the following characteristics: - majority crystalline nature: boehmite, - boehmite content: 75% by weight, - size of the crystallites: 45 A. The dispersibility of the hydrate is measured using of a Sedigraph 5000 ET according to the protocol defined above. More specifically, the aluminum hydrate is kneaded for 30 minutes with nitric acid, at a rate of 6% by weight relative to the hydrate, and deionized water so as to obtain a paste having a loss to fire at 1000 C of 57% by weight. This paste is diluted to 100 g / I with deionized water and the dispersion obtained is homogenized with ultrasound of moderate power (2 minutes with a 5 mm probe at the power of 100 Watts). Finally, the dispersion is analyzed using a Sedigraph, retaining the sedimentation conditions defined above.

Claims (8)

  1. Aluminum hydrate, characterized in that it has a dispersibility of at least 80% by weight for a diameter less than or equal to 0.3 μm. 2. Aluminum hydrate according to claim 1, characterized in that it has a   dispersibility of at least 90% by weight for a diameter less than or equal to 0.3 Pom.   3. Aluminum hydrate according to claim 1 or 2, characterized in that it is   mainly based on aluminum monohydrate.   4. Method for preparing an aluminum hydrate according to one of claims 1 to   3, in which the following steps are implemented: 1- mixing an acid source of aluminum and a base or a basic source of aluminum and an acid so as to precipitate a monohydrate of alumina, 2 ripening, 3- filtration, 4- washing, and 5- drying,   characterized in that the mixing of step 1 is carried out without back-mixing.   5. Method according to claim 4, characterized in that the mixture of step 1 is   produced in a piston flow reactor.   6. Method according to claim 4 or 5, characterized in that the mixture of step 1   is carried out in a static mixer.   7. Use of the aluminum hydrate according to one of claims 1 to 3 or from   process according to any one of claims 4 to 6, for the preparation of   alumina catalysts or catalyst supports.   8. Use according to claim 7 for controlling the level of macropores of alumina   in catalysts or catalyst supports.