JPH0445452B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to a method for producing a composite oxide consisting of lanthanum and alumina, and particularly to a method for producing a heat-resistant carrier that can be stably used in a temperature range of 1500° C. or lower. [Background of the Invention] Automobile exhaust gas purification, high-temperature steam reforming, and the like are known as methods for carrying out reactions at high temperatures using catalysts. Recently, there has been a movement to apply catalytic combustion technology to boilers, gas turbines, etc.
In these methods, the reaction temperature reaches 1000°C or higher, and depending on the conditions, it reaches 1400-1500°C. There is a desire to develop a catalyst that can be used stably in such a high temperature range, but for this purpose a heat-resistant support that has a high specific surface area even at high temperatures is required. Conventionally, the carrier used at relatively high temperatures of 700°C to 800°C is mainly activated alumina. However, activated alumina is thermally unstable above 800°C, especially above 1000°C, and undergoes a phase change to α-alumina through various transition-type aluminas. Along with this, crystal growth also progresses, and the specific surface area decreases significantly. A decrease in the specific surface area of the carrier causes agglomeration of the active ingredients dispersedly supported on the carrier, leading to a decrease in catalyst performance. As a method to improve such drawbacks of activated alumina, a method using magnesia/alumina spinel prepared by firing a mixture of alumina powder and magnesia powder at high temperature as a carrier (Japanese Patent Publication No. 3419/1983);
A carrier made by adding chromium, tungsten, cerium, etc. to alumina (Japanese Unexamined Patent Publication No. 1988-99988), alumina with higher alkaline earth metals and molybdenum trioxide, zirconia, silica, tin oxide, lanthanum oxide and silica,
Carrier containing lanthanum oxide and tin oxide (Unexamined Japanese Patent Publication No. 1983
-117387) are known. Although each of the above alumina modification methods has advantages, they are not sufficient in terms of heat resistance. [Object of the Invention] An object of the present invention is to improve the above-mentioned drawbacks of the prior art and to provide a method for producing a heat-resistant carrier material that has a high specific surface area even under high-temperature reaction conditions. [Summary of the invention] Activated alumina generally has a high specific surface area and is often used as a carrier or coating material.
At temperatures above 800°C, especially above 1000°C, the specific surface area decreases due to phase transition to α-alumina and growth of crystal particle size, which in turn causes a decrease in catalytic activity. As a result of intensive research to improve the above-mentioned thermal instability of alumina, the present inventors discovered lanthanum-β-alumina (La ₂ O ₃・_{11〜14Al2O3 )}
It has been found that the specific surface area of the carrier mainly composed of is less reduced even at high temperatures. Furthermore, as a result of various studies on manufacturing methods, the present invention has been completed. A method of adding rare earth metal oxides such as cerium and lanthanum to activated alumina to prevent alumina crystal phase transition at high temperatures is known as described in Japanese Patent Application Laid-Open No. 14600/1983. However, these conventional methods cannot sufficiently satisfy the performance required for a heat-resistant carrier used at high temperatures of 1000° C. or higher. That is, it is not possible to maintain a high specific surface area at high temperatures. One of the reasons for this is that in the method of impregnating an activated alumina support with a water-soluble salt such as lanthanum nitrate, alumina and lanthanum oxide react during firing, and lanthanum aluminate (LaAlO ₃ ), which has a perovskite structure, is mainly produced. to be generated. It was confirmed that crystal growth of this compound progresses easily and the specific surface area also decreases. The present inventors previously discovered that a carrier whose main component is lanthanum-β-alumina (La ₂ O ₃ 11-14Al ₂ O ₃ ), which is a type of composite oxide of lanthanum oxide and alumina, has a high specific surface area even at high temperatures. It was revealed that he had.
The present invention was arrived at as a result of detailed studies on the method for producing this compound. Specifically, an aqueous solution of aluminum salt and lanthanum salt is prepared with an atomic ratio of lanthanum/alumina of 2/98 to 20/
80, neutralize this solution to form a precipitate, and from the precipitate, lanthanum and β-
This method produces a composite oxide whose main component is alumina. Specifically, the precipitate is washed, dried, calcined, and then molded to produce a composite oxide of lanthanum and alumina whose main components are lanthanum and β-alumina. A feature of this method is that it includes a step of producing precipitates of aluminum and lanthanum hydroxides, basic carbonates, etc. by neutralization from an aqueous solution of aluminum salt and lanthanum salt. Especially when generating this precipitate,
Rather than precipitating the hydroxides of aluminum and lanthanum separately and mixing them later, it is preferable to use a so-called coprecipitation method in which they are precipitated at the same time. By using the coprecipitation method, aluminum and lanthanum are well mixed on a microscopic scale, so that precursors of lanthanum and β-alumina are easily produced, and lanthanum and β-lumina are easily produced during the firing process. Other than this coprecipitation method, lanthanum aluminate (LaAlO ₃ ) is easily generated, and lanthanum/β-
In order to generate alumina, a high temperature of 1500°C or higher is required, and in this case, it is difficult to form a porous carrier.
As a specific method of coprecipitation, it is good to add aqueous ammonia to a mixed aqueous solution of aluminum salt and lanthanum salt to form a precipitate, or conversely, add aluminum salt to a precipitant such as aqueous ammonia. It is also good to drop an aqueous solution of lanthanum salt and lanthanum salt at the same time to form a precipitate. Alternatively, a precipitant such as aqueous ammonia and an aqueous solution of an aluminum salt and a lanthanum salt may be simultaneously dropped into distilled water to form a precipitate. The pH for forming a precipitate is generally preferably in the range of 7 to 12. If the pH is below 7, precipitation of lanthanum hydroxide will not proceed completely, and if the pH is above 12, aluminum hydroxide may generate aluminate ions and be redissolved. The precipitate obtained by the coprecipitation method is usually filtered and dried after aging and washing steps, but the aging and washing steps may be omitted. When the precipitate of hydroxide of lanthanum and aluminum obtained by the coprecipitation method is calcined at 800°C or higher, preferably 900°C or higher, lanthanum β-alumina is produced. This firing step may be carried out after shaping the precipitate into various shapes, or it may be carried out after the precipitate has been fired to produce lanthanum β-alumina powder. The upper limit of firing temperature is
It is desirable to set it to 1500℃. As aluminum raw materials, soluble nitrate,
Sulfates, chlorides, etc. can be used. On the other hand, soluble nitrates, chlorides, oxalates, acetates, etc. can be used as lanthanum raw materials. The precipitating agent may be any basic substance that can raise the pH of the solution of aluminum salt and lanthanum salt to 7 or higher, such as aqueous ammonia, sodium hydroxide, potassium hydroxide, calcium hydroxide, sodium carbonate, and carbonic acid. Representative examples include potassium, sodium hydrogen carbonate, and potassium hydrogen carbonate. Alternatively, urea, which decomposes to generate ammonia when heated in an aqueous solution, may be used. The composition of lanthanum and aluminum in the carrier is preferably in the range of La/Al=2/98 to 20/80 in terms of the atomic ratio of the metal components. When the amount of lanthanum is less than this range, a large amount of alumina is produced in addition to lanthanum and β-alumina, and the specific surface area decreases at high temperatures. Further, when there is a large amount of lanthanum, a large amount of lanthanum aluminate is produced in addition to lanthanum/β-alumina, and the specific surface area is reduced. The carrier obtained by the present invention is lanthanum β-
The main component is alumina, but it may contain other forms of compounds of lanthanum oxide and alumina as components other than the main component, or it may contain uncombined surplus alumina and/or lanthanum oxide. . Furthermore, silica, magnesium, calcia,
Barrier, beryllia, zirconia, titania, thoria, oxides such as tin oxide, cordierite, mullite, spodiume, aluminum titanate,
It is possible to contain one or more selected from compounds such as silicon carbide and silicon nitride. In addition, rare earth elements other than lanthanum, such as cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium,
It is also possible to contain one or more selected from thulium, yzterbium, yttrium, scandium, lutetium, etc. [Examples of the Invention] Hereinafter, the method of the present invention will be explained in more detail with reference to Examples, but the method of the present invention is not limited to these Examples. Example 1 1.88 kg of aluminum nitrate and 114 g of lanthanum nitrate were dissolved in 10 g of distilled water. While stirring this solution
3N ammonia water was added dropwise to neutralize the pH to 8. The resulting coprecipitate of aluminum and lanthanum was filtered, and the precipitate was thoroughly washed with distilled water and then dried at 150°C for one day. Next, it was fired in an electric furnace at a temperature of 800°C for 2 hours, and the resulting powder was ground in a ball mill and then classified into 60 meshes or less. Add 1% graphite to this fine powder and use a press molding machine to make it 3mm in diameter.
After making it into a cylindrical shape with a length of 3 mm, it was finally heated to 3 mm at 1200℃.
It was baked for a period of time to form a carrier. The composition ratio of the carrier is La/Al=5/95 in atomic ratio. The specific surface area of this carrier was measured using the BET method using Na gas adsorption.
It was 58.5m ² /g. Furthermore, as a result of crystal structure analysis by X-ray diffraction, diffraction peaks of lanthanum and β-alumina were observed. Comparative Example 1 Comparative example carrier 1 consisting only of alumina was obtained by preparing in the same manner as in Example 1 except that lanthanum nitrate was not added. The specific surface area of this carrier is 6.0
m ² /g. Further, as a result of X-ray diffraction, only the diffraction peak of α-alumina was observed. Comparative Example 2 Spherical γ-alumina carrier with a diameter of 3 mm (commercial product)
After drying 100g at 150℃ for a day and night, lanthanum nitrate
44.7 g was impregnated in 50 ml of a pre-dissolved solution in distilled water. Next, dry at 150℃ for a day and night, and then continue
It was baked at 1100°C for 5 hours. The composition ratio of this carrier is La/Al=5/95 in atomic ratio. Specific surface area is 18
m ² /g. Furthermore, as a result of X-ray diffraction, a large and sharp diffraction peak of lanthanum aluminate (LaAlO ₃ ) having a perovskite structure was mainly observed, and the diffraction peak of lanthanum/β-alumina was very small. It was found that lanthanum aluminate was mainly produced, and lanthanum/β-alumina was less likely to be produced. Example 2 It was prepared in exactly the same manner as in Example 1 except that the amounts of aluminum nitrate and lanthanum nitrate were changed, and carriers A (La/Al=2/98, atomic ratio, same below), B
(La/Al=3/97), C (La/Al=7/93), D
(La/Al=10/90) and E (La/Al=20/80) were produced. These carriers were incubated at a temperature of 1000°C or 1200°C.
Baked for an hour. The measurement results of specific surface area are shown in the figure.
In addition, the results of examining the crystal structure by X-ray diffraction are shown in the table. As shown in the table, the form of the oxide in the carrier of this example is mainly lanthanum β-alumina (La-β-Al ₂ O ₃ ), and in addition, Al ₂ O ₃ and
It is clear that it contains _LaAlO3 .

【table】

〔Effect of the invention〕

As described above, according to the method of the present invention, lanthanum and β, which have a high specific surface area even at high temperatures.
- A heat-resistant carrier containing alumina as a main component can be produced, and as a result, this carrier can be applied as a catalyst or adsorbent used at high temperatures.

[Brief explanation of drawings]

The figure is a characteristic diagram showing the relationship between the composition of the carrier, the firing temperature, and the specific surface area.

Claims (1)

[Claims] 1. A solution in which aluminum salt and lanthanum salt are mixed so that the lanthanum/aluminum atomic ratio is within the range of 2/98 to 20/80 is neutralized to precipitate lanthanum and aluminum, and the lanthanum and aluminum are precipitated. 1. A method for producing a lanthanum-alumina composite oxide, which comprises producing a composite oxide of lanthanum and aluminum containing lanthanum and β-alumina by calcining a precipitate. 2. The method for producing a lanthanum-alumina composite oxide according to claim 1, characterized in that aqueous ammonia is added to the mixed solution to neutralize it. 3. The method for producing a lanthanum-alumina composite oxide according to claim 1, characterized in that the mixed solution is neutralized to a pH of 7 to 12.