WO2002070408A1

WO2002070408A1 - Praseodymium oxide with high specific surface and methods for preparing same

Info

Publication number: WO2002070408A1
Application number: PCT/FR2002/000822
Authority: WO
Inventors: Catherine Hedouin
Original assignee: Rhodia Electronics And Catalysis
Priority date: 2001-03-08
Filing date: 2002-03-07
Publication date: 2002-09-12
Also published as: FR2821836A1

Abstract

The invention concerns a praseodymium oxide characterised in that it has a specific surface of at least 20 m2/g after calcination at a temperature of 750 °C for 4 hours and a compacted bulk specific gravity of at least 0.5 g/cm3. The invention also concerns a praseodymium oxide of similar surface capable of containing zirconium oxide and/or cerium oxide and obtainable by a method which consists in reacting a praseodymium salt, and optionally a zirconium salt and/or a cerium salt with a base to obtain a precipitate, then in ripening the precipitate in a medium with a basic pH and finally in calcining it. Finally, the invention concerns an oxide of the same type obtainable by a method which consists in contacting a cerium sol and/or a zirconium sol and a praseodymium salt; adding a base to the resulting mixture to obtain a precipitate and in calcining the resulting precipitate.

Description

PRASEODYM OXIDE WITH HIGH SPECIFIC SURFACE AND PREPARATION METHODS

The present invention relates to a praseodymium oxide with a high specific surface and its methods of preparation.

Praseodymium oxide can be used as a catalyst support. However, the known processes for preparing it lead to products whose specific surface is not stable. This indeed decreases markedly when the products are subjected to high temperatures, for example greater than 600 ° C. and it can reach values which make these products unsuitable for their use at high temperature as catalyst support. In addition, the known preparation methods are generally not very suitable for industrial implementation.

An object of the invention is therefore to provide a praseodymium oxide whose specific surface remains at a high value even after calcination at high temperature.

Another object of the invention is the development of methods which are more suitable for industrial implementation.

For this purpose and according to a first embodiment, the praseodymium oxide of the invention is characterized in that it has a specific surface of at least 20m ² / g after calcination at a temperature of 750 ° C for 4 hours and a packed bulk density of at least 0.5 g / cm ³ , more particularly at least 0.8 g / cm ³ .

According to a second embodiment, the praseodymium oxide of the invention may optionally contain zirconium oxide and / or cerium oxide, and it is characterized in that it has a specific surface of at least 20m ² / g after calcination at a temperature of 750 ° C for 4 hours and in that it is capable of being obtained by a process which comprises the following stages:

- reacting a praseodymium salt, and optionally a zirconium salt and / or a cerium salt with a base, whereby a precipitate is obtained;

- the precipitate is ripened in a medium at basic pH; - The precipitate thus obtained is calcined.

Furthermore, according to a third embodiment, the praseodymium oxide of the invention contains zirconium oxide and / or cerium oxide, and it is characterized in that it has a specific surface d 'at minus 20m ² / g after calcination at a temperature of 750 ° C for 4 hours and in that it is capable of being obtained by a process which comprises the following stages:

- contacting a cerium and / or zirconium sol and a praseodymium salt;

- Adding a base to the mixture thus formed, whereby a precipitate is obtained;

- Optionally, the precipitate is ripened in a medium at basic pH;

- The precipitate thus obtained is calcined. Other characteristics, details and advantages of the invention will appear even more completely on reading the description which follows, as well as

concrete but not limiting intended to illustrate it. The oxide of the invention will first be described.

It corresponds to the formula PrOy, the oxides generally obtained being PrβOn, Pr0 ₂ and Pr ₂ O ₃ according to the conditions of synthesis and calcination, the oxide Pr ₆ On being most generally obtained.

This oxide is first characterized by its specific surface. By specific surface is meant the specific surface B.E.T. determined by nitrogen adsorption in accordance with standard ASTM D 3663-78 established from the BRUNAUER - EMMETT-TELLER method described in the periodical "The

Journal of the American Chemical Society, 60, 309 (1938) ".

As indicated above, the oxide of the invention according to the three above-mentioned embodiments has an area of at least 20m ² / g after calcination at a temperature of 750 ° C for 4 hours. This surface can be at least 25m ² / g after calcination under the same conditions of temperature and duration.

This same oxide can maintain even higher surface values at higher temperatures. Thus, it can have a specific surface of at least 10m ² / g after calcination at a temperature of 800 ° C for 4 hours.

For calcinations at 750 ° C for 4 hours surfaces of at least about 45m ² / g can be obtained and for calcinations at 800 ° C for 4 hours surfaces of at least about 35m ² / g can be obtained in in the case of the first embodiment of the invention, the praseodymium oxide also has a packed bulk density of at least 0.5 g / crn ³ . This density can in particular be at least 0.6 g / cm ³ , more particularly at least 0.8 g / cm ³ and even more particularly at least minus 1g / cm ³ . The praseodymium oxides according to the second and third embodiment of the invention can optionally have these density values. This density is measured according to standard NF T 30-042.

According to a variant of the invention, praseodymium oxide can be combined with zirconium oxide or cerium oxide or even with both. In this case, the content of zirconium oxide and / or cerium oxide is at most 50% by weight, this content being expressed relative to the whole of the composition, that is to say that it corresponds to the ratio zirconium oxide and / or cerium oxide / praseodymium oxide + zirconium oxide and / or cerium oxide.

The invention particularly relates to ternary oxides praseodymium oxide, zirconium oxide, cerium oxide.

The content of zirconium oxide and / or cerium oxide may more particularly be at most 40% or at most 30% by weight, it may moreover be at least 10% or even at least 20 % in weight.

For ternary oxides, the respective contents of cerium and zirconium can vary within wide proportions ranging from the case where zirconium is predominant compared to cerium to the opposite case where cerium is predominant compared to zirconium. More particularly, the proportion by weight of zirconium oxide / cerium oxide can vary in a ratio of between approximately 1/3 and 2/3.

The presence of zirconium oxide and / or cerium oxide generally makes it possible to obtain a product with an even higher specific surface area, in particular of at least 30 m ² / g after calcination at a temperature of 750 ° C. for 4 hours and at least 25m ² / g after calcination at a temperature of 800 ° C for 4 hours.

The praseodymium oxides according to the second and the third embodiment of the invention have, by their preparation process, a porosity and a specific pore distribution. The processes for preparing the oxide of the invention will now be described. What will be mentioned below in the description of the processes applies to the definition of the oxides for the second and the third embodiment of the invention.

There are two types of process. According to a first type, and as indicated above, the method comprises a step of reacting a praseodymium salt with a base then a step of ripening followed by a step of calcination. As praseodymium salts which can be used in the process of the invention, mention may, for example, be made of the salts of inorganic or organic acids, for example of the sulfate, nitrate, chloride or acetate type. Note that nitrate is particularly suitable. In the case of the preparation of oxides further containing zirconium oxide and / or cerium oxide, in addition with the praseodymium salt, a zirconium and / or cerium salt of the same type as is used those given above.

Products of the hydroxide, carbonate or hydroxycarbonate type, for example, can be used as base or basic solution. Mention may be made of alkali or alkaline-earth hydroxides, secondary, tertiary or quaternary amines. However, amines and ammonia may be preferred insofar as they reduce the risks of pollution by alkaline or alkaline-earth cations. Mention may also be made of urea. The normality of the basic solution used is not a critical factor according to the invention, and it can thus vary within wide limits, for example between 0.1 and 11 N but it is still preferable to use to solutions whose concentration varies between 1 and 5 N.

It is possible to carry out the reaction of the praseodymium salt and optionally the zirconium salt and / or the cerium salt with the base in the presence of an acid such as acetic acid in an acid / rare earth molar ratio comprised in particular between 0.2 and 0.6.

It is preferable to introduce the praseodymium salt with, if necessary, the zirconium and / or cerium salt in the basic solution. The reaction is usually carried out at room temperature (18 ° C - 25 ° C).

At the end of the reaction, a precipitate is obtained.

This precipitate then undergoes ripening. This ripening can be carried out by leaving the precipitate in the reaction medium where it was obtained. It is however possible, at the end of the reaction and before the ripening step, to separate the precipitate from the reaction medium by any conventional technique of solid-liquid separation such as for example filtration, decantation, spinning or centrifugation, then wash it and finally resuspend it in water to carry out the ripening step. Washing can be done with water or with a solution at basic pH, for example with an ammonia solution.

The ripening takes place in a medium with a basic pH, that is to say greater than 7. Preferably, the pH is at least 10, even more particularly at least 13. The temperature at which the ripening can range, for example, from ambient (18 ° C - 25 ° C) to around 160 ° C and it can more particularly be between 70 ° C and 160 ° C. The duration of the ripening is usually at least 30 minutes and more particularly at least 1 hour and it can be for example between 2 and 8 hours. The ripening can be done at atmospheric pressure or possibly at a higher pressure.

In a final stage of the process, the precipitate from the ripening stage is then calcined after optionally drying, which can be done in particular by atomization after resuspension in water of the precipitate.

The calcination is carried out at a temperature generally between 200 and 800 ° C and preferably between 300 and 800 ° C. This calcination temperature must be sufficient to transform the precipitate into oxide, and it is also chosen as a function of the temperature of subsequent use of the oxide and taking into account that the specific surface of the product is all the smaller. that the calcination temperature used is higher. The duration of the calcination can, for its part, vary within wide limits, for example between 1 and 24 hours, preferably between 4 and 10 hours. Calcination is generally carried out in air, but calcination carried out for example under inert gas is obviously not excluded.

The second type of process of the invention will now be described. This embodiment applies to the preparation of oxides further comprising zirconium and / or cerium.

This process is characterized in that it comprises the following stages: - a cerium and / or zirconium sol is brought into contact with a praseodymium salt;

- Adding a base to the mixture thus formed, whereby a precipitate is obtained;

- Optionally, the precipitate is ripened in a medium at basic pH; - The precipitate thus obtained is calcined.

The first step of the process uses a praseodymium salt with one or more soils.

The praseodymium salt can be of the type described above in the case of the first process. The terms cerium sol and zirconium sol denote any system consisting of fine solid particles of colloidal dimensions based on oxide and / or hydrated oxide (hydroxide) of cerium or zirconium, in suspension in an aqueous liquid phase, said furthermore, optionally, contain residual amounts of bound or adsorbed ions such as for example nitrates, acetates or ammoniums. It will be noted that in such soils, the cerium or the zirconium may be found either completely in the form of colloids, or simultaneously in the form of ions and in the form of colloids, but, in a preferred manner, without however that the proportion represented by the ionic form does not exceed approximately 10% of the total of the species in the soil. According to the invention, soils are preferably used in which the cerium and zirconium are completely in colloidal form. Cerium soils can be obtained by any suitable technique, in particular, but not limited to, according to the methods described in patent applications FR-A- 2,583,735, FR-A- 2,583,736, FR-A- 2,583 737, FR-A- 2,596,380, FR-A- 2,596,382, FR-A- 2,621,576 and FR-A- 2,655,972, the description of which may be referred to. Cerium soils can be used, the average size of the colloids, as determined by quasi-elastic light scattering, can vary from 3 nm to 100 nm, preferably between 5 and 50 nm.

It is possible to use zirconium soils obtained for example by hot hydrolysis of a sulfuric solution of zirconium, in a nitric medium or in a hydrochloric medium at a temperature between 80 ° C and 200 ° C, the molar ratio SO ₃ / ZrO ₂ of the zirconium sulfuric solution preferably being between 0.34 and 1 and its concentration preferably varying from 0.1 to 2 mole / l expressed as ZrO ₂ . The basic zirconium sulfate thus obtained is then neutralized with a base, preferably ammonia, until a pH of approximately 8 is obtained; then washing and then dispersing the gel obtained by adding a nitric acid solution, the pH of the dispersion medium then preferably being between 0.5 and 5.

It is possible to use zirconium soles having an average size of the colloids of between 5 nm and 500 nm, and advantageously of between 10 and 200 nm.

It will be noted that the initial pHs, the concentrations, as well as the order of introduction, of the praseodymium salt and of the starting soil (s) are chosen and adjusted so that the resulting mixture has a stable and homogeneous character. To this end, more or less extensive agitation operations may be made necessary.

The initial mixture being thus obtained, one then proceeds, in accordance with the second step of the method according to the invention, to the addition of a base in this mixture. This addition is carried out until complete precipitation of the species. The same bases can be used as those which have been mentioned above for the process of the first type.

In a practical manner, the amount of base to be added is determined so that the final pH of the medium at the end of precipitation is preferably greater than or equal to 7.

The addition can be carried out at once, gradually or continuously, and it is preferably carried out with stirring. This operation is generally carried out at room temperature (18 - 25 ° C).

At the end of this step, it is possible to carry out a ripening of the precipitate under the same conditions as those which have been given for the first type of process.

The precipitate can also be washed as described above.

In the last stage of the process, the precipitate from the ripening stage is then calcined after optionally drying. It should be noted that drying can be done by atomization.

The calcination conditions are the same as those described above.

Examples will now be given.

EXAMPLE 1

This example concerns the synthesis of PrβOn. The raw materials used are:

Pr (N0 ₃ ) ₃ in 99.99% solution concentration: 2.996mol / l density: 1.8g / cm ³ NH ₄ OH at 20% (Prolabo)

CH3COOH in 100% density solution: 1.05 g / cm ³ Prolabo. The first step consists in preparing 250 ml of a mixture of a solution containing 0.25 mol of Pr (NO ₃ ) ₃ and 0.125 mol of CH ₃ COOH.

500 ml of NH4OH solution are introduced into a 1 liter capacity reactor equipped with an anchor stirrer (v = 300 revolutions / min)

(concentration: 2mol / l). This solution is added with a flow rate of 4.2 ml / min

250ml of the mixture of praseodymium nitrate and acetic acid prepared previously. The pH at the end of the addition is equal to 9.2.

The precipitate is filtered through a No. 4 frit and then washed with 500 ml of ammonia water at pH = 9.

The cake thus obtained is resuspended in 750 ml of ammonia water (pH final = 10.5). The suspension is matured with stirring (v = 300rpm) 3h at 95 ° C. After cooling, the whole is centrifuged (10 minutes v = 4500 rpm), then resuspended and centrifuged again.

The cake obtained is resuspended at a concentration of 10% expressed as rare earth oxide and then atomized with buchi ^® . The air inlet and outlet temperatures of buchi ^® are respectively equal to 220 ° C and 110 ° C.

The dried solid is then calcined in a 4 hour muffle furnace at 750 ° C.

The product obtained has a BET surface area of 20 m ² / g and a packed density of 1.14 g / cm ³ .

EXAMPLE 2

This example relates to the synthesis of an oxide at 90% by weight of Pr ₆ On and at 10% by weight of ZrO ₂ .

The raw materials used are: Pr (N0 ₃ ) ₃ in 99.99% solution concentration: 2.996mol / l density:

1.8g / cm ³

ZrO (NO ₃ ) ₂ in 19.4% solution in ZrO ₂

NH ₄ OH at 20% (Prolabo)

CH ₃ COOH at 100% density: 1.05 g / cm ³ Prolabo. The first step consists in preparing 250 ml of a mixture of a solution containing 0.217 mol of Pr (NO ₃ ) ₃ with a solution containing 0.0247 mol of ZrO (N0 ₃ ) ₂ and 0.125 mol of CH ₃ COOH.

500 ml of the NH4OH solution (concentration: 2 mol / l) are introduced into a reactor of 1 liter capacity equipped with an anchor stirrer (v = 300 rpm). This solution is added with a flow rate of 4.2 ml / min

250ml of the mixture of praseodymium nitrate, zirconium nitrate and acetic acid prepared previously. The pH at the end of the addition is equal to 9.

The precipitate is filtered through a No. 4 frit and then washed with 500 ml of ammonia water at pH = 9. The cake thus obtained is resuspended in 750 ml of ammonia water (pH = 10.5). The suspension is matured with stirring (v = 300 rpm) 3 h at 95 ° C. After cooling, the whole is centrifuged (10 minutes v = 4500 rpm), then resuspended and centrifuged again.

The dried solid is then calcined in a 4 hour muffle furnace at 750 ° C. The product obtained has a BET surface area of 29 m ² / g and a packed density of ^1.2 g / cm ³ .

EXAMPLE 3 This example relates to the preparation of a ternary oxide

Pr6θn / Ce0 ₂ / Zr0 ₂ in the respective proportions by weight 75/15/10. The raw materials used are:

Pr (N0 ₃ ) ₃ in 99.99% solution concentration: 2.996mol / l density: 1.8g / cm ³ Ce0 ₂ soil RHODIA 67.6% CeO ₂

Sol of Zr0 ₂ Nyacol ^® at 20.12% in ZrO ₂ NH ₄ OH at 20% (Prolabo).

250 ml of a solution containing 0.08812 mole of Pr (N0 ₃ ) _3> 0.0174mole of Ce0 ₂ and 0 are introduced into a 500 ml capacity reactor fitted with an anchor stirrer (v = 300 rpm). 0.0162 mole of ZrO ₂ . The pH of this mixture is equal to 1.33.

Then add the concentrated ammonia. The pH at the end of the addition is equal to 7.5.

The suspension obtained is centrifuged then resuspended (pH = 7.5) and again centrifuged.

The cake thus obtained is resuspended in 250ml of ammonia water (pH = 10.5). The suspension is matured with stirring (v = 300 rpm) 3 h at 95 ° C. After cooling, the whole is centrifuged (10 minutes v = 4500 rpm), then resuspended and centrifuged again. The cake obtained is resuspended at a concentration of 10% expressed as rare earth oxide and then atomized with buchi ^® . The air inlet and outlet temperatures of buchi ^® are respectively equal to 220 ° C and 110 ° C.

The dried solid is then calcined in a 4 hour muffle furnace at 750 ° C. The product obtained has a BET surface area of 30m ² / g.

EXAMPLE 4

The procedure is as in Example 3 but by modifying the amount of reagents to obtain a ternary oxide Pr ₆ θn / Ce0 ₂ / ZrO ₂ in the respective proportions by weight 50/35/15.

The product obtained has a BET surface area of 40m ² / g after 4h calcination at 750 ° C and 31m ² / g after 4h calcination at 800 ° C.

Claims

1- Praseodymium oxide, characterized in that it has a specific surface of at least 20m ² / g after calcination at a temperature of 750 ° C for 4 hours and an apparent bulk density of at least 0.5g / cm ³ , more particularly at least 0.8 g / cm ³ , even more particularly at least 1 g / cm ³ .

2- Praseodymium oxide optionally containing zirconium oxide and / or cerium oxide, characterized in that it has a specific surface of at least 20m ² / g after calcination at a temperature of 750 ° C. for 4 hours and in that it is capable of being obtained by a process which comprises the following steps: - reacting a praseodymium salt, and optionally a zirconium salt and / or a cerium salt with a base, whereby a precipitate is obtained;

- the precipitate is ripened in a medium at basic pH;

- The precipitate thus obtained is calcined.

3- praseodymium oxide according to claim 2, characterized in that it is capable of being obtained by a process in which the praseodymium salt, and optionally the zirconium salt and / or the cerium salt is reacted with a base in the presence of an acid.

4- Praseodymium oxide containing zirconium oxide and / or cerium oxide, characterized in that it has a specific surface of at least 20m ² / g after calcination at a temperature of 750 ° C for 4 hours and in that it is capable of being obtained by a process which comprises the following stages: - a cerium and / or zirconium sol is brought into contact with a praseodymium salt;

- Adding a base to the mixture thus formed, whereby a precipitate is obtained;

- Optionally, the precipitate is ripened in a medium at basic pH; - The precipitate thus obtained is calcined. 5- Praseodymium oxide according to claim 2, 3 or 4, characterized in that it is capable of being obtained by a process in which a ripening is carried out at a temperature between 70 ° C and 160 ° C.

6- Oxide according to one of the preceding claims, characterized in that it has a specific surface of at least 25m ² / g after calcination at a temperature of 750 ° C for 4 hours.

7- Oxide according to one of the preceding claims, characterized in that it has a specific surface of at least 10m ² / g after calcination at a temperature of 800 ° C for 4 hours.

8- Oxide according to one of the preceding claims, characterized in that it contains zirconium oxide and / or cerium oxide in a proportion by weight of at most 50%.

9- Oxide according to one of the preceding claims, characterized in that it has a specific surface of at least 30m ² / g after calcination at a temperature of 750 ° C for 4 hours.

10- Oxide according to one of claims 2 to 9, characterized in that it has a packed bulk density of at least 0.5 g / cm ³ , more particularly at least 0.8 g / cm ³ , even more particularly at least 1g / cm ³ .

11- Process for preparing an oxide according to one of the preceding claims, characterized in that it comprises the following steps:

- reacting a praseodymium salt, and optionally a zirconium salt and / or a cerium salt with a base, whereby a precipitate is obtained; - the precipitate is ripened in a medium at basic pH;

- The precipitate thus obtained is calcined.

12- A method according to claim 11, characterized in that after reaction with the base, the precipitate is separated from the reaction medium, it is washed and it is resuspended in water for ripening. 13- A method according to claim 11 or 12, characterized in that reacts the praseodymium salt, and optionally the zirconium salt and / or the cerium salt with a base in the presence of an acid.

14- Process for the preparation of an oxide according to one of claims 2 to 10, characterized in that it comprises the following steps:

- contacting a cerium and / or zirconium sol and a praseodymium salt;

- Adding a base to the mixture thus formed, whereby a precipitate is obtained;

- Optionally, the precipitate is ripened in a medium at basic pH;

- The precipitate thus obtained is calcined.

15- Method according to one of claims 11 to 14, characterized in that the ripening is carried out at a temperature between 70 ° C and 160 ° C.