JP7507037B2 - Alpha alumina molding and its manufacturing method - Google Patents

Description

The present invention relates to a molded body of alpha alumina and a method for producing the same.

Ceramic molded bodies are used as materials for insulators, abrasives, dental agents, catalyst carriers, etc. Molded bodies of alumina are often used as catalyst carriers. Molded bodies of gamma alumina in particular are suitable for use in catalysts due to their large specific surface area and pore volume. Furthermore, gamma alumina has a large amount of acid, so it is used in decomposition reactions that require a large amount of acid. On the other hand, alpha alumina has a small amount of acid, so it is suitable when it is desired to suppress decomposition reactions.

However, alpha alumina has a smaller specific surface area and pore volume than gamma alumina and other materials, and research and development is being conducted to improve these (see Patent Document 1). In Patent Document 1, a molded body is prepared using two types of alpha alumina particles with different average particle sizes, resulting in a large pore volume for the molded body. In Patent Document 2, a raw material that burns is used, and when the alumina molded body is fired, the burned-out parts become pores. This results in a large pore volume for the alumina.

JP 2018-069116 A JP 2019-048741 A

In Patent Document 1, two types of alpha-alumina particles with different average particle sizes are mixed, molded, and fired, resulting in a molded body with a large pore volume. However, because the alpha-alumina particles with a smaller average particle size enter the gaps between the alpha-alumina particles with a larger average particle size, the pore size of the molded body tends to become smaller.

In Patent Document 2, the size of the aluminum colloid is adjusted using carboxylic acid. This adjusts the pore volume of the alumina molded body. However, simply adjusting the size of the aluminum colloid does not increase the pore size of the molded body. The areas where the carboxylic acid is burned off become vacant, but large pores cannot be formed in the molded body. Because only small pores are formed, it is difficult to react large molecules when the molded body is used as a catalyst support.

The object of the present invention is to provide a molded body of alpha alumina with a large pore volume and pore diameter, and a method for producing the same.

In order to achieve the above-mentioned object, the molded body according to the present invention contains 80% or more of α-alumina. Furthermore, when the total pore volume obtained by integrating the pore volume from a pore diameter of 5000 nm to a pore diameter of 5 nm by mercury intrusion porosimetry is taken as 100%, the pore diameter ratio D10/D90 between the pore diameter D10 at which the integrated pore volume is 10% of the total pore volume and the pore diameter D90 at which the integrated pore volume is 90% of the total pore volume is more than 2.0. Furthermore, the pore diameter D10 is greater than 200 nm. Such a molded body of α-alumina can obtain a large total pore volume of 0.3 mL/g or more and 0.5 mL/g or less.

The present invention provides a molded alpha alumina body with a large pore volume and pore diameter, and a method for producing the same.

1 is a graph showing the relationship between pore diameter measured by mercury intrusion porosimetry and integrated pore volume (Example 1).

The molded body of the present invention has a pore size ratio D10/D90 of more than 2.0 and a pore size D10 of more than 200 nm. Therefore, large pore sizes exist in a wide pore distribution in the molded body. In other words, large voids of different sizes exist inside the molded body. As a result, even if the molded body contains 80% or more of alpha alumina, the molded body can have a large total pore volume of 0.3 mL/g or more and 0.5 mL/g or less.

The proportion of α-alumina can be calculated from the results of X-ray diffraction measurement. The proportion of α-alumina in the molded body is expressed as a percentage of the value obtained from the formula B/(A+B) expressed by the highest peak intensity A among the peaks of crystals other than α-alumina obtained when the molded body is subjected to X-ray diffraction measurement, and the intensity B of the Miller index (104) of α-alumina obtained by X-ray diffraction measurement. The peak of the crystals other than α-alumina does not have to be the peak of the alumina crystal. The X-ray diffraction measurement can be performed using MiniFlex600 manufactured by Rigaku Corporation. The measurement conditions are described in the Examples. The counting unit of the intensity of the X-ray diffraction measurement is expressed in cps (counts per second), and the unit of the formula B/(A+B) is expressed in cps/cps.
The molded body must contain 80% or more of alpha alumina (the unit is % expressed as a percentage of cps/cps). According to the present invention, even if the alpha alumina in the molded body is 80% or more, a large total pore volume can be achieved. The molded body preferably contains 90% or more of alpha alumina, more preferably 95% or more, and even more preferably 99% or more. If the proportion of alpha alumina in the molded body is high, the acid amount of the molded body is reduced. Therefore, when the molded body is used as a catalyst carrier, the reactants in the catalytic reaction are less likely to be decomposed by the acid.

The pore size can be measured by mercury intrusion. The total pore volume is the pore volume integrated from a pore size of 5000 nm to a pore size of 5 nm by mercury intrusion. When integrating the pore volume from a pore size of 5000 nm to a pore size of 5 nm by mercury intrusion, the pore size at which the integrated pore volume is 10% of the total pore volume is defined as pore size D10. In other words, the pore volume integrated from a pore size of 5000 nm to a pore size of D10 is 10% of the total pore volume. The pore size at which the integrated pore volume is 90% of the total pore volume is defined as pore size D90. For reference, a graph of the pore size and integrated pore volume obtained by mercury intrusion in Example 1, as well as the positions of pore size D10 and pore size D90 are shown in FIG. 1. The pore size D is expressed by the formula D=-4σcosθ/P. σ is the surface tension of mercury, θ is the contact angle of mercury, and P is the pressure when mercury is injected into the sample, and the pressure P during measurement is converted to pore size. The measurement conditions for the mercury intrusion method are described in the examples below.

The pore diameter ratio D10/D90 of the pore diameter D10 to the pore diameter D90 must be greater than 2.0, but is preferably greater than 2.5. When the pore diameter ratio D10/D90 is greater than 2.5, the pore volume of the molded body is large. It is more preferable that the pore diameter ratio D10/D90 is greater than 5.0. This increases the specific surface area of the molded body. It is even more preferable that the pore diameter ratio D10/D90 is 7.0 or more. It is preferable that the pore diameter ratio D10/D90 is lower than 15. When the pore diameter ratio D10/D90 is lower than 15, the strength of the molded body is high. Furthermore, the number of small pore diameters in the molded body is reduced, so the pore volume is large. It is preferable that the pore diameter ratio D10/D90 is lower than 10.

The pore diameter D10 must be greater than 200 nm, but is preferably 300 nm or greater. If the pore diameter ratio D10/D90 of the molded body is greater than 2.0 and the pore diameter D10 is 300 nm or greater, the pore volume tends to be large. The pore diameter D10 is more preferably 500 nm or greater, and even more preferably 600 nm or greater. If the pore diameter D10 is 600 nm or greater and the pore diameter ratio D10/D90 is greater than 5, the specific surface area becomes large. In addition, the pore diameter D10 is preferably 1000 nm or less. This reduces the pore volume of pores with diameters greater than 1000 nm. This results in a broader pore distribution.

The ratio of the pore volume of 1000 nm or less to the total pore volume (unit: volume %, hereinafter sometimes referred to as "%") is preferably 95% or more and 100% or less. When it is in this range, the pore distribution becomes broad. The pore volume of 1000 nm or less can be obtained by integrating the pore volume from 1000 nm to a pore diameter of 5 nm using the mercury intrusion method. The ratio of the pore volume of 1000 nm or less to the total pore volume is expressed as a percentage by dividing the pore volume of 1000 nm or less by the total pore volume. It is more preferable that the ratio of the pore volume of 1000 nm or less to the total pore volume is 97% or more and 100% or less. When it is in this range, the pore size distribution becomes broad.

Mercury intrusion can be measured using a pore size distribution analyzer (Quanta Chrome PM-33GT1LP). Measurement conditions are described in the examples.

The specific surface area of the molded body is preferably 5 m ² /g or more and 15 m ² /g or less. When the molded body is used as a catalyst carrier within this range, the reaction rate of the catalytic reaction is high. The specific surface area of the molded body is more preferably more than 6 m ² /g and 15 m ² /g or less, and further preferably 8 m ² /g or more and 15 m ² /g or less. The specific surface area can be measured by the BET adsorption method. The measurement conditions are described in the examples below.

The shape of the molded body is not particularly limited, but any known shape can be selected. Examples include a spherical shape, a cylindrical shape, or a shape similar to these. The shape of the molded body is preferably a cylindrical shape or a shape similar to these. This cylindrical shape also includes shapes such as a cylindrical shape, a trilobal shape, and a four-leaf clover shape. It is preferable that the diameter is in the range of 0.5 mm to 5 mm, and the length is in the range of 1 mm to 10 mm. There is also no particular limit to the size of the molded body.

The composition of the molded body is preferably 90% or more of alumina, more preferably 95% or more, and even more preferably 98% or more (the unit is mass %, but hereinafter, it may be referred to as "%" in some cases). Since the molded body contains 80% or more of α-alumina, which has a low acid content, the higher the ratio of the alumina composition, the lower the acid content of the molded body. The composition of the molded body can be measured by MagiXPRO manufactured by PHLPIPS. As components other than alumina, the molded body may contain sulfate radicals, sodium salts, inorganic oxides other than alumina, and the like. The sulfate radicals are preferably 2% or less in terms of SO ₃ , more preferably 0.5% or less, and more preferably 0.1% or less. The sodium salts are preferably 0.2% or less in terms of Na ₂ O, more preferably 0.1% or less, and even more preferably 0.05% or less. Ionic substances such as sulfate radicals and sodium salts may be catalyst poisons, but they may be contained in small amounts to block the acid sites of alumina. The inorganic oxide may be contained in the molded body as required.

The manufacturing method of the molded body of the present invention is described in detail below.

First, two or more types of aluminum hydroxides or oxides having different crystal structures (hereinafter, the aluminum hydroxides or oxides are referred to as aluminum compositions) are mixed to prepare a mixture of aluminum compositions (first step). When the aluminum compositions have different crystal structures, the volume (amount of shrinkage) that decreases during firing also differs. This amount of shrinkage is thought to be due to the phase change of the crystals and the decrease in water of crystallization that accompanies firing. When the amount of shrinkage differs, the pore size distribution of the molded body becomes wider, and the pore volume increases. Another reason for the increase in pore volume is thought to be that in the mixture of aluminum compositions, different crystal structures are difficult to sinter together during firing. It is presumed that if the aluminum composition shrinks in the parts of the molded body that are difficult to sinter, voids are more likely to occur inside the molded body. As a result, the pore volume of the molded body is thought to increase.

At least one of the aluminum compositions is preferably aluminum hydroxide. When aluminum hydroxide is mixed in the first step, the aluminum hydroxide, which has a large amount of shrinkage during firing, shrinks, and the pore volume of the molded body becomes large. In the first step, it is preferable to mix at least pseudo-boehmite or alumina having a boehmite crystal structure with aluminum hydroxide. This increases the pore size ratio D10/D90 of the molded body. When aluminum hydroxide is added in the first step, it is preferable that the aluminum hydroxide has a gibbsite structure. When producing a molded body, it is preferable to mix aluminum compositions having two to three different crystal structures. This increases productivity. When producing a molded body, it is more preferable to mix aluminum compositions having two different crystal structures. In this case, the crystal structures of the aluminum compositions having two different crystal structures are preferably pseudo-boehmite and gibbsite structures.

The crystalline structure of the aluminum composition includes, but is not limited to, amorphous, gibbsite, bayerite, hydrargillite, pseudoboehmite, boehmite, nordstrandite, diaspore, tordite, chi-alumina, kappa-alumina, delta-alumina, p-alumina, eta-alumina, theta-alumina, and alpha-alumina. The aluminum composition may be amorphous aluminum hydroxide. Candidate aluminum compositions include various oxide, hydroxide, and hydrate forms of aluminum.

The aluminum composition may be in the form of a powder, gel, sol, or the like, but is not particularly limited. Of the aluminum compositions having two or more different crystal structures, it is preferable that at least one of them is in the form of a sol or gel. This allows the mixture of aluminum compositions to be mixed uniformly. When mixed uniformly, different crystal structures are more likely to come into contact with each other, making it difficult for sintering to occur inside the molded body during firing. This results in a larger pore volume.

It is preferable that at least one of the particles in the mixture mixed in the first step has a particle size of 5 μm or more and 30 μm or less. When it is in this range, it is easy to mix two or more types of aluminum compositions.

It is preferable that at least one of the mixtures mixed in the first step is contained in the aluminum composition mixture at 40% by weight or more and 60% by weight or less. When it is in this range, the pore size distribution of the molded body becomes wider. Therefore, the pore volume of the molded body becomes larger. It is preferable that the aluminum composition mixture contains aluminum hydroxide at 40% by weight or more and 60% by weight or less. This increases the amount of aluminum hydroxide that shrinks inside the molded body during the firing step, and therefore the pore size distribution of the molded body becomes wider.

The mixing method in the first step is not particularly limited. Examples of mixing methods include a wet method and a kneading method. In particular, mixing by kneading is preferred. This simplifies the manufacturing process.

After preparing the mixture of aluminum compositions, the mixture is molded to form an unsintered molded body (second step). The molding method is not particularly limited, and any known molding method can be used. Examples include spray granulation, extrusion molding, spherical granulation, tablet molding, and extrusion granulation. In particular, extrusion molding is preferred. This improves the productivity of the molded body.

After preparing the green molded body, the green molded body is fired to form a molded body (third step). The firing temperature may be any temperature at which α-alumina is produced. The firing temperature is preferably 1100°C or higher and 1500°C or lower, more preferably 1150°C or higher and 1300°C or lower, and even more preferably 1150°C or higher and 1250°C or lower. The higher the firing temperature, the higher the crystallinity of the α-alumina. The lower the firing temperature, the higher the specific surface area. In particular, since the raw material is an aluminum composition having two or more different crystal structures, the specific surface area is likely to be high when fired at 1150°C or higher and 1250°C or lower.
The firing time is not particularly limited, but is preferably from 2 hours to 5 hours.

Examples of the present invention will be described in detail below. Table 1 shows the outline of the preparation conditions of the molded body.
[Example 1]
<Preparation of gel with pseudo-boehmite structure>
40 kg of sodium _aluminate with a concentration of 5% in terms of _Al2O3 was placed in a tank with a capacity of 100 L and equipped with a steam jacket, and heated to 60°C. 40 kg of _aluminum sulfate with a concentration of 2.5% in terms of _Al2O3 was added over 10 minutes to obtain a mixed slurry. The mixed slurry was dehydrated and washed with a 0.3 wt% aqueous solution of ammonium hydrogen carbonate. Then, the slurry was diluted with _ion -exchanged water to a concentration of 15 wt% in terms of _Al2O3 to obtain a slurry. 15 wt% aqueous ammonia was added to adjust the pH to 10.3, and the mixture was stirred at 95°C for 10 hours to obtain a matured slurry. The matured slurry was dehydrated to obtain a gel with a pseudo-boehmite structure.

<Preparation of Molded Body>
As raw materials for aluminum hydroxide or oxide, gel with a pseudo-boehmite structure and aluminum hydroxide with a _gibbsite structure (manufactured by Nippon Light Metals Co., Ltd.: B103) were charged into a twin-arm kneader equipped with a steam jacket so that the weight ratio calculated based on _Al2O3 was 1:1. The mixture was mixed and heated while kneading in the kneader to obtain an aluminum composition mixture. This mixture was molded into a cylindrical shape with a diameter of 1.8 mm using an extrusion molding machine to obtain an unfired molded body. The unfired molded body was dried at 120°C for 16 hours and then cut to a length of 4 to 5 mm. It was then fired at 1300°C for 3 hours to obtain a molded body.

<Measurement method>
The pore volume, pore diameter, specific surface area, and crystal structure were measured by the following methods. From these measurement results, the total pore volume, the ratio of pore volume of 1000 nm or less, the pore diameter D10, and the pore diameter ratio D10/D90 were calculated. These results are shown in Table 1. The firing temperature is also shown in Table 1. Furthermore, the same measurements and calculations were performed for the examples and comparative examples described later, and the results are shown in Table 1.

(Measurement of pore volume and pore diameter)
The pore volume of the molded body from pore diameter 5000 nm to pore diameter 5 nm was measured by mercury intrusion porosimetry (mercury contact angle: 130°, surface tension: 473 dyn/cm). The pore volume from pore diameter 5000 nm to pore diameter 5 nm was integrated to calculate the total pore volume.
The pore volume of pores with diameters from 1000 nm to 5 nm was integrated to calculate the volume of pores with diameters of 1000 nm or less, and then the ratio of the volume of pores with diameters of 1000 nm or less was calculated by dividing the volume of pores with diameters of 1000 nm or less by the total volume of pores.

The pore diameter D10 was calculated when the integrated pore volume was 10% of the total pore volume. The pore diameter D90 was calculated when the integrated pore volume was 90% of the total pore volume.

(Measurement of specific surface area)
Approximately 30 mL of the molded body was placed in a magnetic crucible, and heat-treated at 300° C. for 2 hours, then cooled to room temperature in a desiccator to obtain a measurement sample. Next, 1 g of the measurement sample was taken and the specific surface area (m ² /g) of the sample was measured by the BET method using a fully automatic surface area measuring device (Multisorb 12, manufactured by Yuasa Ionics Co., Ltd.).

(Measurement of crystal structure)
The molded body was collected in a mortar and pulverized using a pestle. X-ray diffraction measurements were performed using MiniFlex600 (manufactured by Rigaku Corporation) (ray source: Cu-Kα radiation, acceleration voltage and current: 40k and 15mA, light receiving slit: open, scan speed: 10°/min, step width: 0.020°, measurement range (2θ): 15° to 90°). Analysis software used was PDXL Version 2 manufactured by Rigaku Corporation. It was confirmed that the crystal structure of the molded body was only α-alumina in all examples and comparative examples.

[Example 2]
A molded body was prepared in the same manner as in Example 1, except that the firing temperature was 1200°C.

[Comparative Example 1]
A molding was prepared in the same manner as in Example 1, except that aluminum hydroxide having a gibbsite structure was not added.

[Comparative Example 2]
A molded product was prepared in the same manner as in Comparative Example 1, except that the firing was carried out at 1500° C. for 10 hours.

Claims (4)

A molded body containing 80% or more of alpha alumina,
When a total pore volume obtained by integrating a pore volume from a pore diameter of 5,000 nm to a pore diameter of 5 nm by mercury intrusion porosimetry is taken as 100%, a pore diameter ratio D10/D90 between a pore diameter D10 at which the integrated pore volume is 10% of the total pore volume and a pore diameter D90 at which the integrated pore volume is 90% of the total pore volume is 2.5 or more ;
The pore diameter D10 is greater than 200 nm,
A molded body, characterized in that the total pore volume is in the range of 0.3 mL/g or more and 0.5 mL/g or less. 2. The molded body according to claim 1, which has a specific surface area of 8 m ² /g or more and 15 m ² /g or less. A first step of mixing alumina having a pseudo-boehmite or boehmite crystal structure with aluminum hydroxide to prepare an aluminum composition mixture;
a second step of molding the mixture of aluminum compositions to form a green body;
and a third step of firing the green body at a temperature at which alpha alumina is produced .
A method for producing a molded body , wherein at least one of the alumina having a pseudo-boehmite or boehmite crystal structure and the aluminum hydroxide is in a gel form . The method for producing a molded body according to claim 3 , characterized in that in the third step, the firing temperature is 1150°C or higher and 1250°C or lower.

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