JP2594879C - - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a lanthanum-stabilized alumina which retains a high specific surface area at high temperatures, a novel lanthanum-stabilized alumina, a process for its production and its use About the law. BACKGROUND OF THE INVENTION It is well known to use catalysts whose support is based on alumina, in particular for the treatment of exhaust gases of internal combustion engines. [0003] It is also well known that in general, the higher the efficiency of a catalyst, the larger the contact surface between the catalyst and the reactant. In order to achieve this, it is necessary to keep the catalyst as finely divided as possible, that is, to make the solid catalyst particles constituting the catalyst smaller and smaller. Thus, the basic role of the support is to keep the catalyst particles or microcrystals in contact with the reactants as finely divided as possible. In the harsh conditions that exhaust gas treatment catalysts undergo, their supports, on which noble metals can be deposited, must exhibit excellent thermal stability, ie, in particular at 1200 ° C. High specific surface area at high temperatures up to [0005] Alumina commonly used as a support for such catalysts is high temperature (particularly 1,000
C. or higher) undergoes an irreversible phase transition to α-alumina,
The effect is that the specific surface area is reduced to 10 m ² / g or less to cause sintering of the catalyst phase. Thus, the catalyst is degraded and thus loses most of its efficiency. It has already been proposed to add various compounds intended to improve the heat aging properties of alumina. Thus, a preformed and calcined alumina-based catalyst support is coated on a rare earth metal nitrate solution (US Pat. No. 4,061,591).
No. 4) or impregnating with a solution of a silicon compound is known. However,
The resulting alumina still shows an insufficient specific surface area at high temperatures, and the value of this area decreases more significantly as the duration of the heat treatment increases. SUMMARY OF THE INVENTION In the present invention, there is provided a lanthanum-stabilized alumina which retains a high specific surface area at high temperatures, the lanthanum being advantageously distributed evenly in the alumina. Lanthanum stabilized alumina, as well as methods for producing such stabilized alumina, have been developed. [0008] Accordingly, the present invention relates to a novel alumina stabilized with lanthanum. Preferably, the lanthanum is present in the form of an oxide in the alumina. DETAILED DESCRIPTION OF THE INVENTION The lanthanum content in the stabilized alumina according to the invention is advantageously between 1.5 and 15%, preferably expressed as the weight of lanthanum oxide, based on the stabilized alumina. 2.5
-11% and more preferably 3.0-7.5%. [0010] 1,200 ° C. for 4 hours after firing (in an ambient atmosphere), alumina according to the invention, 40 m ² / g or more (e.g. 40~75m ² / g), preferably 45 m ² /
g (for example, 45 to 65 m ² / g), more preferably 50 m ² / g or more (for example, 50 to 60 m ² / g) (measured by the BET method). Advantageously, the alumina according to the invention has a value (according to the BET method) measured after calcining at 1200 ° C. for 24 hours (in an ambient atmosphere), generally at 1200 ° C. for 4 hours ( 50 (measured by the BET method) after firing (in the ambient atmosphere)
%, Preferably greater than 55% and more preferably greater than 60%. Thus, the aluminas according to the invention show very good thermal stability. Note that, according to a variant of the invention, lanthanum can be used with neodymium. In this case, the neodymium is preferably distributed homogeneously in the alumina and is generally present therein in the form of an oxide. Thus, the amount of neodymium present in the stabilized alumina according to the invention, expressed as the weight of neodymium oxide, based on the weight of lanthanum oxide, is usually 8 to 40%, for example 12 to 3%.
It is between 0%. It is another object of the present invention to provide a process for producing the above alumina wherein the stabilizer is introduced at a particular stage. Therefore, still another object of the present invention is to provide an alumina powder produced by rapid dehydration of aluminum hydroxide or oxyhydroxide with at least one lanthanum compound (
(Optionally mixed with a neodymium compound) in the presence of at least one stabilizer comprising the steps of aging and subsequent heat treatment to produce said stabilized alumina from said alumina powder. . The aging operation consists of rehydrating the used alumina powder to develop a so-called boehmite crystal phase. This rehydration is a dissolution-reprecipitation process. that is,
It is preferably carried out by suspending the alumina powder in water at a concentration of usually 50 to 600 g / 1, for example, 200 to 350 g / 1. According to one of the main features of the method according to the invention, this rehydration is carried out in the presence of at least one stabilizer consisting of a lanthanum compound optionally mixed with a neodymium compound. Will be As the lanthanum compound, a lanthanum salt, particularly lanthanum chloride, lanthanum nitrate and / or lanthanum acetate can be used in the form of an aqueous solution. The use of lanthanum nitrate is preferred. The amount of lanthanum compound added is advantageously 1.5 to 15%, preferably 2%, as lanthanum oxide, based on the total weight of the final stabilized alumina.
. It corresponds to an amount of 5 to 11% and more preferably 3.5 to 7.5%. Similarly, as the neodymium compound, neodymium salts, especially neodymium chloride, neodymium nitrate and / or neodymium acetate can be used in the form of an aqueous solution. The use of neodymium nitrate is preferred. A mixture of a lanthanum salt and a neodymium salt can be used in the form of an aqueous solution. The addition amount of the neodymium compound is generally equivalent to an amount of 8 to 40%, for example, 12 to 30%, expressed as the weight of neodymium oxide based on the weight of lanthanum oxide. After the introduction of the stabilizer, the pH of the suspension is controlled, preferably by addition of any desired base, generally a weak base such as ammonia, to a value of from 8 to 12, for example from 9 to 11. The aging operation is beneficially at 70-110 ° C. and preferably at about 100 ° C. (eg 9 ° C.)
5-100 ° C.). The aging time is usually 1 to 48 hours. When the aging operation is performed at a temperature not exceeding 100 ° C., the period is, for example, 10 to 48 hours, preferably 20 to 30 hours. When it is carried out at a temperature above 100 ° C., an autoclave is generally used, and the aging period may generally be from 1 to 6 hours, preferably from 1 to 4 hours. The resulting alumina is at least partially in the form of boehmite (or pseudo boehmite). Thus, the content of boehmite (or pseudo boehmite) is usually 5 to 35% by weight, preferably 10 to 30% by weight. This alumina is
It is subsequently separated by known techniques for liquid / solid separation, for example by filtration. The liquid / solid separation operation may, if necessary, efficiently remove sodium hydroxide (which may be present in the alumina starting powder) and / or other impurities, particularly those which may result from the starting material used. Purification can be included to allow for efficient removal. Thus, for the intended application, it is possible to produce stabilized alumina of high purity directly from the product containing impurities, which is a problem with the prior art methods of stabilizing alumina. Generally impossible. The heat treatment is typically performed at a temperature of 100 to 700 ° C. for a time sufficient to remove water that is not chemically bonded to the alumina, for example, 2 to 32 hours. This heat treatment can consist of drying followed by optional baking. Drying is generally carried out at a temperature of 100-250 ° C., preferably 100-200 ° C.
Performed for 4 hours. The firing is generally performed at 250 to 700 ° C, preferably 350 to
It is carried out at a temperature of 600 ° C., usually for 1 to 8 hours. The alumina starting powder used in the process according to the invention is at least one aluminum hydroxide such as bayerite, hydrargillite or gibbsite, nordstrandite and / or at least one aluminum hydroxide such as boehmite and diasporite. Manufactured by rapid dehydration of aluminum oxyhydroxide. This dehydration
It is carried out in any suitable device with the help of a flow of hot gas which allows the evaporated water to be removed and carried off very quickly. The inlet temperature at which the gas enters the device is:
Generally, it is about 400 to 1200C, for example about 800 to 1200C. The contact time between the hydroxide (oxyhydroxide) and the hot gas is usually on the order of a fraction of a second to 4 or 5 seconds. The alumina produced by this dehydration can optionally undergo a treatment to remove at least a portion of the alkali present. The specific surface area, measured by the BET method, of the alumina produced by the rapid dehydration of aluminum hydroxide and / or aluminum oxyhydroxide is generally about 50
４５０450 m ² / g, and the diameter of the particles is usually 0.1-30 μm, preferably 1-120 μm. This alumina is usually 0.10 to 0.50 cm ³
/ G of pore volume. The pores generally have dimensions of 50 nm or less. According to a particular embodiment of the present invention, the starting alumina is a rapidly converted form of Bayer Hydrate (Hydragillite), which is an easily available and very inexpensive industrial aluminum hydroxide. Produced from dehydration. Such aluminas are well known to those skilled in the art and are described in particular in French Patent 1,108,011. Thus, the process according to the invention makes it possible to obtain alumina having an excellent thermal stability, ie retaining a high specific surface area at high temperatures. 4 at 1,200 ° C
Time (measured by the BET method) a specific surface area after firing (in an ambient atmosphere) is, 40 m ² / g or more (e.g. 40~75m ² / g), preferably 45 m ² / g or more (e.g. 45～65M ² / G), and more preferably 50 m ² / g or more (for example, 50 to 60 m ² / g). Advantageously, the method according to the invention has a value (according to the BET method) measured after calcining at 1200 ° C. for 24 hours (in an ambient atmosphere), generally at 1200 ° C. for 4 hours ( Obtaining an alumina further having a specific surface area of greater than 50%, preferably greater than 55% and more preferably greater than 60% of the value measured by calcining (in the ambient atmosphere) (by the BET method) Enable. Such a specific surface is not possible with alumina doped by impregnation with a solution of a rare earth metal salt. Thus, alumina prepared according to the present invention has better thermal stability than alumina doped by impregnation with a solution of a rare earth metal salt. Advantageously, the lanthanum is distributed homogeneously in the alumina, in which case it is preferably present in the form of an oxide. Alumina stabilized according to the present invention can be used as a catalyst support. It is used in particular as a carrier for catalysts in the treatment of exhaust gases of internal combustion engines. It can be used either in the form of beads or in the form of a porous coating layer ("washcoat") attached to a monolithic ceramic or metal carrier. The active phase of the catalyst can consist of a noble metal. In particular, the active phases described in U.S. Pat. No. 4,378,307 can be used. Alumina stabilized according to the invention can also be used as a catalyst, in particular as a hydrocarbon conversion catalyst. It is used, for example, as an FCC (fluidized bed catalytic cracking) catalyst. The following examples are illustrative of the present invention, but do not limit the scope of the present invention. Example 1 (Comparative) The starting material used was an alumina powder having a specific surface area of about 390 m ² / g, produced by rapid dehydration of hydrargillite. The alumina powder is impregnated with 5% by weight of lanthanum oxide at ambient temperature with an aqueous lanthanum nitrate solution and then dried at 120 ° C. for 12 hours. Thus obtained alumina (in an ambient atmosphere) for 4 hours at 1,200 ° C. The specific surface area and 1,200 ° C. for 24 hours after firing 35m ² / g (in an ambient atmosphere) just after firing 10 m ² / g. Example 2 (according to the invention) The same starting materials as in Example 1 are used. The alumina powder is aged or rehydrated by suspending it in water at a concentration of 250 g / 1 at 98 ° C. for 24 hours. To this water, an aqueous lanthanum nitrate solution is added at the start of the operation in an amount corresponding to an amount of 5% lanthanum, expressed as the weight of lanthanum oxide, based on the total weight of the final stabilized alumina. The pH is controlled at about 10 by adding ammonia at the start of the operation. At the end of the treatment, a liquid / solid separation is performed and the alumina is dried at 120 ° C. for 12 hours. The alumina thus obtained has a specific surface area of 52 m ² / g after calcining at 1200 ° C. for 4 hours (in ambient atmosphere) and 28 m ² / g after calcining at 1200 ° C. for 24 hours (in ambient atmosphere). Specific surface area. Example 3 (according to the invention) The procedure performed was as in Example 2, except that the amount of lanthanum nitrate aqueous solution added was based on the weight of lanthanum oxide based on the total weight of the final stabilized alumina. The only difference is that it corresponds to the amount of 10% lanthanum expressed. The alumina thus obtained has a specific surface area of 47 m ² / g after calcining at 1200 ° C. for 4 hours (in ambient atmosphere) and 26 m ² / g after calcining at 1200 ° C. for 24 hours (in ambient atmosphere). Specific surface area. Example 4 (according to the invention) The procedure of Example 2 is carried out, except that the starting materials used are prepared by rapid dehydration of hydrargillite having a specific surface area of about 360 m ² / g. The only difference is that the powder is alumina powder. The alumina thus obtained has a specific surface area of 46 m ² / g after calcining at 1200 ° C. for 4 hours (in ambient atmosphere) and 25 m ² / g after calcining at 1200 ° C. for 24 hours (in ambient atmosphere). Specific surface area.

Claims (1)

Claims: 1. A method of producing stabilized alumina from alumina powder by subjecting alumina powder produced by rapid dehydration of aluminum hydroxide or oxyhydroxide to a heat treatment following an aging operation. Wherein the alumina powder is suspended in water in the presence of at least one stabilizer consisting of a lanthanum compound and a neodymium compound as an optional component, the pH of the suspension is adjusted to 8 to 12, and the temperature is adjusted to 70 to 110 ° C. A method for producing stabilized alumina, wherein a ripening operation is carried out. 2. The process according to claim 1, wherein the starting alumina powder is the product of rapid dehydration of hydrargillite. 3. The method according to claim 1, wherein the temperature of the heat treatment is between 100 and 700 ° C.