FR2772018A1 - A new process for the preparation of alumina of controlled porosity - Google Patents

Abstract

The process enables the production of an alumina of controlled porosity, 0.6 - 20 nm, using an organic surfactant as a structuring agent, which is subsequently eliminated by calcination. The process is effected in three stages comprises: (a) preparation of a precursor by hydrolysis of inorganic Al sources in the presence of at least one surfactant. The Al sources comprise a cation-monomer and cation-oligomer (alone or mixed). The reaction takes place in a mainly aqueous medium of pH 4.5 - 10; (b) the precipitate obtained is air dried at 40 - 110 deg C for 2 - 30 hrs; and (c) the dried precipitate is calcined at 400 - 600 deg C. An Independent claim is also included for the alumina product obtained.

Description

The invention relates to the field of inorganic porous solids and more particularly to the field of controlled porosity aluminas.

These materials are characterized by their porous volume and specific surface area. These materials have a large adsorbent capacity, they are most often used as catalyst supports, adsorbents, separation means.

The preparation of oxides with controlled porosity is based on the fact that organic molecules classified as surfactants can associate with each other, in particular when they are placed under the conditions of synthesis of oxides with controlled porosity. They thus form micelles which have varying electrical charges and geometry depending on the conditions of the synthesis medium.

These micelles can in turn structure around them polymeric species of mineral oxides such as silica, alumina or other metals such as titanium, and thus generate a mesoporous solid network.

In WO 96/39357, a complementary route using nonionic surfactants such as polyethylene oxides and neutral precursors of inorganic oxides as reagent sources is described. This synthesis is carried out in an organic solvent. This method also allows easy removal of the surfactant. Porous inorganic oxides are prepared by this route from nonionic surfactants.

In the work which has as its basic principle the controlled hydrolysis of Al (OR ') 3 organic aluminum precursors or R' is a linear or branched alkyl group containing from 1 to 5 carbon atoms, there is a case where the structurant is of the quaternary ammonium type, the medium is then of mixed water-alcohol type and the case where the structurant is of the micellar type, the reaction is then carried out in an organic solvent. Vaudry and
Davis in Chem. Mater., 1996, p.1451 use fatty acid surfactants.

S. Bagshaw, T. J. Pinnavaia, in Mesoporous alumina molecular sieves, Angew. Chem. Intl.

Ed. Engl 1996,35; 10, p.1102-1105 describe the obtaining of lamellar phases.

When the structurant is of the quaternary ammonium type RaN ', a single diffraction diffuse under 2 O- = 5 (CuKa) is generally observed on the X-ray diffraction diagram indicating that it is in the presence of a mesostructure with a periodic arrangement at With micellar surfactants, the X-ray diffraction pattern generally has a line corresponding to an inter-sectional distance strictly greater than 3 nanometers indicating a periodic arrangement at a greater distance.

A mesoporous alumina of hexagonal structure was. synthesized from AI species in the presence of sodium dodecyl sulphate, this synthesis is described in the document: M. Yada,
M. Machida, T. Kijima, "Synthesis and deorganization of an aluminum-based dodecyl sulphate mesophase with a hexagonal structure", Chem. Commun., 1996, 769-770. This method requires the use of large amounts of urea whose decomposition leads to a slow increase in pH during prolonged heating. Al3 cations are thus hydrolysed, which allows their polycondensation in a lamellar phase and then in a hexagonal phase when the pH is greater than 7. This structure is preserved after the calcination of the unwashed product but no data on the existence of a controlled porosity is mentioned.

AS Stein, B. Holland, in Aluminum containing mesostructural materials, J. Pgrous
Materials, 1996, 3, 83-92 synthesize an alumina from Algin-based Keggin type polycations in the presence of dodecyl sulphate. But after the heat treatment to remove the surfactant, the solid obtained has a specific surface area obtained by the very low BET method of 18 m 2 / g.

In the present description, use the abbreviation "nm" for nanometer that is to say 109 meters.

The invention relates to a process for the synthesis of a controlled porosity alumina between 0.6 nm and 20 nm. This process is carried out in several stages among which at least one step a) in which a precursor of alumina is prepared by hydrolysis of at least one inorganic source of aluminum with a base, in the presence of at least one surfactant. Inorganic sources of aluminum are cation-monomers and / or cationsoligomers. Step a) of the process is carried out in a substantially aqueous medium having a pH of about 4.5 to 10.5. The process according to the invention also comprises at least one step b) in which the precipitate obtained is air-dried at a temperature of about 40 ° C. to 110 ° C. for a duration of about 2 to 30 hours, and at least one step c) in which the dried precipitate is calcined at a temperature sufficient to remove the surfactant molecules present in the precursor.

The invention also relates to controlled porosity alumina between 0.6 and 2 nm obtained by this method.

Among the advantages of the process according to the invention, it may be noted that: the process is carried out with inexpensive and easy-to-use inorganic aluminum sources, in a low-polluting and inexpensive environment where the majority solvent, and often unique, is water.

The porosity of the materials can be controlled by the following main parameters: the nature of the inorganic aluminum source, the nature of the surfactant (s) and the nature of any organic compounds used as additives - the role of these materials additives being then modifying the surfactants-, the pH of the medium where the precursor is formed, the calcination procedure used to remove the surfactants. These main parameters are not the only factors involved, the temperature, the duration of formation of the precursor can also affect the characteristics of the final porosity.

According to a step a) of the synthesis process according to the invention, at least one inorganic source of aluminum is reacted in the form of cations-monomers and / or cationsoligomers with at least one surfactant. The inorganic source of aluminum is introduced into the mixture in cationic form 1 +, the surfactants are of anionic type S and / or nonionic S. If the solution contains at least anionic surfactants S, it is optionally possible to add cationic surfactants S +, the amount of cationic surfactants added is such that the total charge of all the surfactants is negative, the charge of the inorganic surface remaining positive, it follows a mechanism of type SI However, depending on the pH of the solution, especially when the pH is greater than 9, the negative charges on the surface of the aluminum species I may become the majority, these micelles can then produce S-type interactions M1 where M is the surfactant compensation cation. -active.

According to a particular embodiment of the process according to the invention, the inorganic aluminum source is in the form of cations-monomers and / or cations-oligomers, a cation-monomer may be, for example the ion [Al (H 2 O) 6] 3, a cation-oligomer may be, for example, the Keggin ™ Al1304 (OH) 24 (H20) 12 type structure ion] 7
Without wishing to be bound by any theory, it is possible to think that surfactants control the polycondensation of inorganic precursors by multiple interactions such as covalent interactions denoted by S1, ionic interactions of S 1 hydrogen bridge type, and interactions between charged species. SI and S I. These interactions depend on the nature of the surfactants and the pH of the aqueous medium. The interactions are always of the type 5.1 if anionic surfactants are used, to these electrostatic interactions can be added interactions S1 or S1 if one associates nonionic surfactants with the preceding micelles. It is also possible to control the polycondensation by interactions of only S 1 (or Sl) type using nonionic surfactants. In the case where cationic surfactants S are added, S-type interactions are also present.

The process according to the invention is carried out in a predominantly aqueous medium whose pH preferably favors the formation on the surface of the inorganic material of positive charges in a majority manner with respect to the negative charges, these positively charged surfaces are preferably obtained by presence of micelles globally negative, the sum of the interactions between the charged species SI being most often greater than the sum of the interactions between the charged species S I. The pH of the reaction medium is generally 4.5 to 10.5, preferably from 5 to 10.

In a step c), after having dried in a step b) the precipitate obtained in step a), calcination is carried out at a temperature sufficient to remove the surfactant molecules present in the precursor. The calcination step can be carried out under an inert atmosphere, for example nitrogen, by gradually increasing the temperature for a period of about 2 to 6 hours and then maintaining the calcination temperature for a period of 1 to 4 hours, the calcination temperature being about 400 ° C to 600 ° C. Calcination can also be carried out by flash calcination at a temperature of about 400 ° C to 600 ° C in a medium that contains oxygen, for example, air.

The product obtained by the process according to the invention has the following characteristics: a controlled porosity between 0.6 nm and 20 nm, preferably between 0.6 nm and 15 nm, even more preferably between 0.6 and 10 nm, a specific surface area measured by the BET method generally from about 100 to 1000 m 2 / g and a total pore volume generally from about 0.05 to 1.00 cm 3 / g.

Stage a) of the synthesis process according to the invention in which, in the presence of a base, at least one inorganic source of aluminum is reacted in the form of cation-monomers and / or cation-oligomers with a surfactant. active can also be carried out in the presence of one or more organic compounds. More particularly, when using a surfactant in micellar form, these organic compounds have the role of modifying the characteristics of the micelles, their shape, their size. They are essentially non-polar or slightly polar molecules whose insertion in the hydrophobic part of the micelles or in the region separating the hydrophilic part of the hydrophobic part of the micelles modifies their diameter and their radius of curvature.

In general, the method according to the invention is carried out as follows: an aqueous solution A containing an inorganic source of aluminum and a solution B containing at least one anionic surfactant and / or at least one nonionic surfactant. When the solution contains at least one anionic surfactant, it is optionally possible to add at least one cationic surfactant. Depending on the species involved, the pH of the solution
A can be very acidic, solution B for its part contains a base which is added in such a quantity that the final mixture has a pH of about 4.5 to 10.5 and preferably about 5 to 10. The solution B also optionally contains an organic compound intended to modify the characteristics of the micelles (their shape, their size) as well as the solubility of the micelles in the aqueous medium. Solution B is poured all at once with stirring to solution A. The precipitate which results from these mixtures is stirred for a period of about 6 to 48 hours, preferably about 10 to 30 hours. During this time, the reaction medium is maintained under the conditions under which the mixing has been carried out.

As regards the temperature, it is about 15 ° C to 90 ° C, preferably about 15 ° C to 30 ° C.

It is not beyond the scope of the present invention to add a basic solution prepared separately in the form of a solution C and added to solution A all at once with vigorous stirring at the same time as solution B or after the addition of this solution B - solution B does not contain any basic compound. This variant of the process according to the invention may be useful when it is not possible to obtain complete dissolution of the surfactant (s) in solution B in the presence of a base. Other variants of the present invention are used to increase the solubility of the surfactant (s), these variants consist of heating the solution B at a temperature of about 30 ° C to 90 ° C or to replace in part the water contained in solution B by at least one organic compound miscible with water and wherein the surfactant or surfactants are more soluble. In all cases, the solvent medium remains predominantly aqueous, the amount of organic compounds being about 0.001 to 20% by weight relative to the total mass of solvent.

After reaction, the precipitate is isolated by appropriate means of filtration or centrifugation, the precipitate is then washed with water and dried under air at a temperature of about 40 ° C to 110 ° C until a constant mass of the precipitate, most often the drying time is from about 2 hours to 30 hours and preferably from about 10 hours to 20 hours.

At the end of these treatments, a precursor of the controlled porosity alumina is obtained.

This precursor of the alumina is then calcined at a temperature sufficient to remove the structuring agents present in the porous space.

More particularly, the solutions A containing aluminum in the form of cationic monomers such as [Al (H 2 O) 6] 3 are prepared by dissolving the aluminum salts corresponding to strong acids (chlorides, nitrates, sulphates, perchlorates). ), their pH is generally about 1 to 3. The aluminum-containing solutions A in the form of cationsoligomers such as [A11304 (OH) 24 (H 2 O) 127+ with a Keggin-type structure are prepared according to the following methods described in US Pat. literature: JW Akitt et al, J. Chem. Soc. Dalton Trans., 1988, p. 1347; JW Akitt et al., J. Chem. Soc. Dalton Trans 1981, p. 1617; JW Akitt et al,
J. Chem. Soc. Dalton Trans., 1981, p. 1624; A. Schutz et al., Clays and Clay Mineras, 1987, 35 (4), 251. The aluminum concentration of solution A is generally between 0.05 mol / l and 5 mol / l and preferably between 0.1 mol / l and 1 mol / l. The pH of these solutions is generally about 3 to 5 and the species are characterized by a degree of hydrolysis defined by the molar ratio OH / Al which is about 1.3 to 2.7. For the [A11304 (OH) 24 (H20) 1217C ion with structure of type
In Keggin, the OHVAI molar ratio is preferably about 2.3 to 2.7.

The anionic surfactants optionally present in the solution B used in the present invention are generally selected from the group consisting of alkyl carboxylates, alkyl sulphates, alkyl sulphonates and alkyl phenyl sulphonates.

The nonionic surfactants optionally present in the solution B used in the present invention are generally chosen from the group formed by the alkylpolyethylene oxides, the alkylphenylpolyethylene oxides, the alkylphosphine oxides and the oxides. alkyl amine.

The cationic surfactants optionally present in the solution B used in the present invention are generally chosen from the group formed by the alkylammonium salts, the alkylphosphonium salts and the alkylsulfonium salts.

It would not be outside the scope of the present invention to use surfactants in which several functions coexist, namely amphoteric surfactants where the anionic and cationic functions coexist, or surfactants where the anionic and nonionic functions coexist, such as C-betaine, N-betaine, alkylpolyoxyethylene sulfate or alkylpolyoxyethylene sulfonate.

The organic compounds which may optionally be added to solution B are generally non-polar or slightly polar molecules, these organic compounds are chosen from: alkanes whose chain contains between 5 and 12 carbon atoms per molecule, preferably the nonane or decane, the aromatic hydrocarbons substituted with one or more hydrocarbon groups, preferably one will use mesitylene, the aliphatic alcohols whose chain contains between 5 and 12 carbon atoms per molecule, preferably one will use the n-octanol or the aliphatic amines whose chain contains between 5 and 12 carbon atoms per molecule, preferably hexylamines will be used.

The concentration of the surfactant (s) in the B solutions depends on their nature, ie their solubility and their critical micellar concentration. When solution B is an aqueous solution, the surfactant concentration of solution B is generally about 0.1 to 0.5 mol / l. When solution B is a mixture of water and an organic compound, the surfactant concentration of solution B is generally about 0.20 to 1.5 mol / l.

The ratio of the number of moles of surfactants to the number of moles of aluminum in the A + B mixture is often from about 0.05 to 1.5 and preferably from about 0.10 to 0.80.

The base added to solution B is preferably chosen from the group formed by sodium hydroxide (NaOH), potassium hydroxide (KOH), ammonium hydroxide (NH 4 OH), methylamine (CH 3 NH 2) or an alkali metal aluminate. preferably potassium aluminate or sodium aluminate.

This base is chosen according to the nature of the aluminum source in solution A, that is to say, the degree of hydrolysis of this aluminum source and the desired pH.

The aluminas obtained are characterized by a nitrogen adsorption and desorption isotherm, an X-ray diffraction pattern and a nuclear magnetic resonance (NMR) spectrum of 27Al aluminum with magic angle rotation.

The nitrogen adsorption and desorption isotherm makes it possible to calculate the specific surface area by the BET method described in J. Am. Chem. Soc. 73, 373-380 (1951). Total pore volume, average pore diameter, and pore volume between 1.7 and 20 nm can also be determined by the BJH method described in J. Am. Chem. Soc 60 p. 309-319 (1938).

By the nuclear magnetic resonance (NMR) spectrum of aluminum 27AI with magic angle rotation, the coordination of aluminum in the structure can be known. The NMR spectra are obtained by taking as reference an aqueous solution of Li (AlNO 3) 3.

X-ray diffraction patterns make it possible to determine the intercross-distance "d" of synthesized compounds. The X-ray diffractometer used is of the type
Phillips PW 1130 and the nuclear magnetic resonance spectrometer is of the Bruker type
DSX 400.

The examples which follow illustrate the invention without limiting its scope.

Exemole 1
A solution A is obtained by dissolving 7.5 g of Al (NO 3) 3, 9 H 2 O in 190 ml of water.
A solution B is obtained by dispersing 2.23 g of sodium palmitate (C, 6H3, O2Na) and 0.73 g of hexadecyltrimethylammonium bromide in 40 ml of water by heating at 40 ° C.

A solution C is obtained by dissolving 2.4 g of sodium hydroxide (NaOH) in 20 ml of water.

Solution B is poured at once into solution A with vigorous stirring and then added immediately after solution C. The pH stabilizes towards 7.5. The mixture is stirred at room temperature for 14 hours. The product obtained is filtered, washed with water and then dried for 14 hours at 600C, under air at atmospheric pressure.

3.75 g of precursor are obtained. 1 g of this precursor is then calcined under nitrogen, the gradual increase of the temperature to 450 ° C. is carried out over a period of 3 hours, and then this temperature is maintained for 2 hours, thus obtaining a light-gray powder. .

On the X-ray diffraction pattern of this compound, there is the presence of a diffuse central line (Figure la). The NMR spectrum is reproduced in FIG. 1b and shows the presence of. octahedral aluminum sites (d = 5.9 ppm) and tetrahedral aluminum sites (d = 67.6 ppm).

From the nitrogen adsorption and desorption isotherm (FIG. 1c), the specific surface area can be determined by the BET method, the total pore volume, the average pore diameter and the pore volume between 1.7. nm and 20 nm, these characteristics are recorded in Table 1.

TABLE 1

<tb> Surface <SEP> BET <SEP> 530 <SEP> m2 / g
<tb> Volume <SEP> porous <SEP> total <SEP> 0.88 <SEP> cm3 / g
<tb> Average <SEP> diameter <SEP> of <SEP> pores <SEP> between <SEP> 1.7 <SEP> and <SEP> 20 <SEP> nm <SEP> 6.5 <SEP> nm
<tb> Porous volume <SEP> porous <SEP> between <SEP> 1.7 <SEP> and <SEP> 20 <SEP> nm <SEP> 0.80 <SEP> cm3 / g
<Tb>
Example 2
A solution A is obtained by dissolving 7.5 g of Al (NO 3) 3, 9H 2 O in 190 ml of water.

A solution B is obtained by dissolving 7.64 g of aqueous solution of N, N-dimethyldodecylamine-N-oxide at 30% by weight and 2.4 g of sodium hydroxide (NaOH) in 40 ml of water.

Solution B is poured at once into solution A with vigorous stirring. The pH stabilizes at around 7.5. The mixture is stirred at room temperature for 14 hours. The product obtained is filtered, washed with water and then dried for 14 hours at 60 ° C. in air at atmospheric pressure. 1.9 g of precursor are obtained. 1 g of this precursor is then calcined under nitrogen, the gradual increase in temperature to 450 ° C. is carried out over a period of 3 hours, and then this temperature is then maintained for 2 hours. A dark gray powder is thus obtained.

On the X-ray diffraction pattern of this compound, there is the presence of a diffuse central line (Figure 2a). The NMR spectrum is reproduced in FIG. 2b and shows the presence of octahedral aluminum sites (d = 4.9) and tetrahedral aluminum sites (d = 66.3 ppm).

From the nitrogen adsorption and desorption isotherm (FIG. 2c), the specific surface area can be determined by the BET method, the total pore volume, the average pore diameter and the pore volume between 1.7. nm and 20 nm, these characteristics are recorded in Table 2.

TABLE 2

<tb><SEP> Surface <SEP> BET <SEP> 485 <SEP> m2 / <SEP>
<tb><SEP> Volume <SEP> porous <SEP> total <SEP> 0.68 <SEP> cm3 / g
<tb><SEP> Average <SEP> diameter <SEP> of <SEP> pores <SEP> between <SEP> 1.7 <SEP> and <SEP> 20 <SEP> nm <SEP> 5.5 <SEP> nm
<tb> Volume <SEP> porous <SEP> between <SEP> 1.7 <SEP> and <SEP> 20 <SEP> nm <SEP> | <SEP> 0.63 <SEP> cm3 / <SEP>
<Tb>
Example 3
A solution A is obtained by dissolving 7.5 g of Al (NO 3) 3,9H 2 O in 190 ml of water.

A solution B is obtained by dissolving 2.0 g of lauric acid (C12H2402) and 2.56 g of
NaOH in 40 ml of water while heating at 50 ° C.

Solution B is poured at once into solution A with vigorous stirring. The pH stabilizes between 5.5 and 6. The mixture is left stirring at room temperature for 14 hours.

The product obtained is filtered, washed with water and then dried for 14 hours at 60 ° C. in air at atmospheric pressure to give 3.4 g of precursor, and 1 g of this precursor is then calcined under nitrogen. Gradual increase of the temperature to 450 ° C. is carried out over a period of 3 hours and then this temperature is then maintained for 2 hours. A gray powder is thus obtained.

On the X-ray diffraction pattern of this compound, there is the presence of a very broad and poorly defined line whose position corresponds to an inter-sectional distance "d" of 5.3 nm (FIG. 3a). The NMR spectrum is reproduced in FIG. 3b, the presence of an intense peak at 4.6 ppm corresponding to an octahedral coordination and two more diffuse peaks: one at 64 ppm corresponds to a tetrahedral coordination, another at 34 ppm corresponds to pentavalent coordination.

From the nitrogen adsorption and desorption isotherm (FIG. 3c), the specific surface area can be determined by the BET method, the total pore volume, the average pore diameter and the pore volume between 1.7. nm and 20 nm. These characteristics are shown in Table 3. It has also been determined that the pore volume of less than 1 nm is equal to 0.06 cm 3 / g.

TABLE 3

<tb> Surface <SEP> BET <SEP> 245 <SEP> m2 / g
<tb> Volume <SEP> porous <SEP> total <SEP> 0.28 <SEP> cm3 / g
<tb> Average <SEP> diameter <SEP> of <SEP> pores <SEP> between <SEP> 1.7 <SEP> and <SEP> 20 <SEP> nm <SEP> 4.5 <SEP> nm
<tb> Volume <SEP> porous <SEP> between <SEP> 1.7 <SEP> and <SEP> 20 <SEP> nm <SEP> 0.21 <SEP> cm3 / g <SEP>
<Tb>
The solutions A used in the following examples are obtained from a solution of Keggin polycations prepared according to the following procedure.

A solution of Keggin aluminum polycations [A11304 (OH) 24 (H20) 12] 7+ is prepared by dropwise pouring, with the aid of a dropping funnel, a solution containing 8 g of sodium hydroxide (NaOH) in 360 ml of water in a very strongly stirred solution containing 30 g of Al (NO 3) 3,9H 2 O in 400 ml of water. The final solution has a pH close to 4.5, it is perfectly clear.

To obtain the solutions A of the following examples, 190 ml of the Keggin polycation solution are taken.

Example 4
Solution B is obtained by dissolving 2.0 g of lauric acid and 0.4 g of NaOH in 40 ml of water while heating to 40 ° C.

Solution B is poured at once into solution A with vigorous stirring. The pH stabilizes at 9.5. The mixture is stirred at room temperature for 14 hours. The product obtained is filtered, washed with water and then dried for 14 hours at 600C, under air at atmospheric pressure. 2.5 g of precursor are obtained. 1 g of this precursor is then calcined under nitrogen, the gradual increase in temperature to 450 ° C. is carried out over a period of 3 hours, and then this temperature is then maintained for 2 hours. A beige powder is thus obtained. On the X-ray diffraction pattern of this compound, a broad and weak line is observed whose position corresponds to an inter-sectional distance "d" of 4.2 nm (FIG. 4a). The NMR spectrum is reproduced in FIG. 4b, the presence of an intense peak at 4.6 ppm corresponding to an octahedral coordination and two more diffuse peaks: one at 64 ppm corresponds to a tetrahedral coordination, another at 34 ppm corresponds to pentavalent coordination.

From this precursor is then calcined under nitrogen: the gradual increase in temperature up to 450 OC is carried out over a period of 3 hours and then this temperature is maintained for 2 hours. A beige powder is thus obtained.

On the X-ray diffraction pattern of this compound, only a diffuse central line is observed (Figure 5a). The NMR spectrum is reproduced in FIG. 5b and shows the presence of octahedral coordination (d = 6.9 ppm) and tetrahedral coordination (d = 69 ppm).

From the nitrogen adsorption and desorption isotherm (FIG. 5c), the specific surface area can be determined by the BET method, the total pore volume, the average pore diameter and a pore volume between 1.7. at 20 nm (BJH method), these characteristics are recorded in Table 5.

TABLE 5

<tb> Surface <SEP> BET <SEP> 122 <SEP> m2 / g
<tb> Volume <SEP> porous <SEP> total <SEP> 1 <SEP> 0, <SEP> 0.09cm3 / g <SEP>
<tb> Average <SEP> diameter <SEP> of <SEP> pores <SEP> between <SEP> 1.7 <SEP> and <SEP> 20 <SEP> nm <SEP> 3.0 <SEP> nm
<tb> Volume <SEP> porous <SEP> between <SEP> 1.7 <SEP> and <SEP> 20 <SEP> nm <SEP> 0.07 <SEP> cm3 / g
<Tb>
Exemole 6
A solution B is obtained by dissolving 2.23 g of sodium palmitate (C16H3102Na) with 0.73 g of hexadecyltrimethylammonium bromide and 2.4 g of 1,3,5-trimethylbenzene in 40 ml of water while heating at room temperature. 60 "C.

Solution B is poured at once into solution A with vigorous stirring. The pH stabilizes at 6.5. The mixture is stirred at room temperature for 14 hours. The product obtained is filtered, washed with water and then dried for 14 hours at 60 ° C. under atmospheric pressure to give 3.9 g of precursor.

1 g of this precursor is then calcined under nitrogen, the gradual increase in temperature to 450 ° C. is carried out over a period of 3 hours, and then this temperature is then maintained for 2 hours. This gives a dark beige color powder.

On the X-ray diffraction pattern of this compound only a strong central scattering corresponding to an angle value of less than 3 is observed.

From the nitrogen adsorption and desorption isotherm (FIG. 6), the specific surface area can be determined by the BET method, the total pore volume, the average pore diameter and a pore volume between 1.7. at 20 nm (BJH method), these characteristics are recorded in Table 6.

TABLE 6

<tb> Surface <SEP> BET <SEP> í <SEP> 815 <SEP> m2 / g
<tb><SEP> Volume <SEP> porous <SEP> total <SEP> 0.61 <SEP> cm3 / g
<tb><SEP> Average <SEP> diameter <SEP> of <SEP> pores <SEP> between <SEP> 1.7 <SEP> and <SEP> 20 <SEP> nm <SEP> 2.8 <SEP> nm
<tb><SEP> Porous <SEP> Volume <SEP> between <SEP> 1.7 <SEP> and <SEP> 20 <SEP> nm <SEP> 0.46 <SEP> cm3 / <SEP>
<Tb>
The products of Examples 7 and 8 were synthesized according to the same general method, only the calcination procedure varies.

A solution B is obtained by dissolving 7.65 g of aqueous solution of N, N-dimethyldodecylamine-N-oxide at 30% by weight, 2.4 g of 1,3,5-trimethylbenzene and 0.4 g of sodium hydroxide ( NaOH) in 40 ml of water at 35 ° C.

Solution B is poured at once into solution A with vigorous stirring. The pH stabilizes at about 7. The mixture is left stirring at room temperature for 14 hours. After 14 hours, the product obtained is filtered, washed with water and then dried at 60 ° C.

2.5 g of precursor are obtained.

Example 7 of the precursor obtained by the method described above is then calcined under nitrogen.
The gradual increase of the temperature up to 450 OC is carried out over a period of 3 hours and then this temperature is then maintained for 2 hours. A dark brown powder is thus obtained.

On the X - ray diffraction pattern of this compound, the presence of a line whose position corresponds to an interplanar spacing "d" of 5.9 nm is noted.

From the nitrogen adsorption and desorption isotherm (FIGS. 7a, 7b, 7c), the specific surface area can be determined by the BET method, the total pore volume, the average pore diameter and the pore volume. between 1.7 nm and 20 nm, these characteristics are recorded in Table 7
TABLE 7

<tb> Surface <SEP> BET <SEP><SEP> i <SEP> 483 <SEP> m2 / g <SEP>
<tb> Volume <SEP> porous <SEP> total <SEP> 0.40 <SEP> cm3 / g <SEP>
<tb> Diameter <SEP> average <SEP> of <SEP> pores <SEP> between <SEP> 1.7 <SEP> and <SEP> 20 <SEP> nm <SEP> I <SEP> 2.9 <SEP > nm
<tb> Volume <SEP> porous <SEP> between <SEP> 1.7 <SEP> and <SEP> 20 <SEP> nm <SEP><SEP> 0.34 <SEP> cm3 / <SEP>
<Tb>
EXAMPLE 8 lg of the precursor obtained by the method described above is calcined by flash calcination under air: the sample is introduced into a preheated at 500 ° C. in air and the sample is kept at this temperature for 30 minutes. gets a white powder.

The X-ray diffraction pattern of this product has a line whose position corresponds to an inter-sectional distance "d" of 5.5 nm.

From the nitrogen adsorption and desorption isotherm (FIGS. 8a, 8b, 8c), the specific surface area can be determined by the BET method, the total pore volume, the average pore diameter and the pore volume. between 1.7 nm and 20 nm. These characteristics are recorded in the following table:

<tb> Surface <SEP> BET <SEP> 498 <SEP> m2 / g
<tb> Volume <SEP> porous <SEP> total <SEP> 0.50 <SEP> cm3 / g
<tb> Average <SEP> diameter <SEP> of <SEP> pores <SEP> between <SEP> 1.7 <SEP> and <SEP> 20 <SEP> nm <SEP> 3.2 <SEP> nm
<tb> Volume <SEP> porous <SEP> average <SEP> between <SEP> 1.7 <SEP> and <SEP> 20 <SEP> nm <SEP> | <SEP> 0.47 <SEP> cm3 /
<Tb>
The synthesis process according to the invention has made it possible to prepare aluminas having characteristics (BET surface area, total pore volume, average pore diameter between 1.7 and 20 nm, average pore volume between 1.7 and 20 nm) which sweep largely the range of characteristics of the aluminas. BET surfaces range from 122 m2 / g to 815 m2 / g, total pore volumes range from 0.09 cm3 / g to 0.88 cm3 / g, average pore diameters from 1.7 to 20 nm vary from 2 , 8 nm to 6.5 nm and mean pore volumes between 1.7 and 20 nm range from 0.07 cm3 / g to 0.80 cm3 / g.

Claims (11)

CLAIMS 1- Process for the synthesis of alumina with controlled porosity between 0.6 nm and 20 nm, characterized in that it comprises at least one step a) in which a precursor of this alumina is prepared by hydrolysis of inorganic sources of aluminum in the presence of at least one surfactant, these inorganic sources of aluminum being chosen from the group formed by the cation-monomers and the cations-oligomers, these cations being used alone or in mixtures, this step a) being carried out in a predominantly aqueous solvent medium having a pH of about 4.5 to 10.5, at least one step b) in which the precipitate obtained is air-dried at a temperature of about 40.degree. 110 C for a period of about 2 to 30 hours, and at least one step c) wherein the dried precipitate is calcined at a temperature of about 4000C to 600 ° C. 2- alumina synthesis process according to claim 1 characterized in that in step c) the temperature is gradually increased for a period of about 2 to 6 hours to a calcination temperature, then maintained this temperature calcination for a period of 1 to 4 hours, the calcination temperature being about 400 ° C to 600 ° C, this step being carried out under an inert atmosphere. 3- alumina synthesis process according to claim 1 characterized in that in step c) is carried out a flash calcination at a temperature of about 400 "C to 6000C in a medium that contains oxygen. 4- alumina synthesis process according to one of claims 1 to 3 characterized in that the cation-monomer used as reagent is [Al (H 2 O) 6] 5-alumina synthesis process according to one of claims 1 to 3, characterized in that the cation-oligomer used as a reagent is [Al1304 (OH) 24 (H2O) 12] 7 with a Keggin type structure. 6. Process for synthesizing alumina according to one of claims 1 to 5 characterized in that the surfactant or surfactants used in step a) comprises at least one anionic surfactant chosen from the group formed by alkyls. carboxylates, alkyl sulphates, alkyl sulphonates, alkyl phenyl sulphonates. 7-alumina synthesis process according to claim 6, characterized in that the reagents of step a) are added to at least one cationic surfactant chosen from the group formed by the alkylammonium salts, the salts alkylphosphonium salts, the alkylsulfonium salts, the amount of cationic surfactants being such that the total charge of all the surfactants is negative and the inorganic surface is positive. 8- alumina synthesis process according to one of claims 1 to 7 characterized in that the surfactant or surfactants used in step a) comprises at least one nonionic surfactant selected from the group consisting of alkylpolyethylene oxides, alkylphenylpolyethylene oxides, alkylphosphine oxides, alkylphosphine oxides and alkylamine oxides. 9- alumina synthesis process according to one of claims 1 to 8 characterized in that step a) is carried out in the presence of at least one organic compound representing about 0.001 to 20% by weight relative to the mass total solvent. 10. Process for synthesizing alumina according to claim 9, characterized in that the organic compound is a hydrocarbon whose chain contains from 5 to 12 carbon atoms per molecule chosen from the group formed by the alkanes, the aromatic hydrocarbons substituted by a or more hydrocarbon groups, aliphatic alcohols and aliphatic amines. 11-controlled porosity alumina between 0.6 nm and 20 nm obtained by the synthesis method according to one of claims 1 to 10. 12-alumina with controlled porosity according to claim 11 characterized in that its specific surface area is about 100 to 1000 m2 / g. 13- controlled porosity alumina according to claim 1 1 or 12 characterized in that its total pore volume is about 0.05 to 1.00 cm3 / g.