JPH09165277A - Method for producing porous alumina body - Google Patents

Abstract

(57) 【Abstract】 PROBLEM TO BE SOLVED: High temperature and high pressure are required for supercritical drying, and the restrictions on the equipment are extremely large, and moreover, they are generated at the gas-liquid interface of the solvent penetrating into pores and voids. Due to the influence of surface tension, the fine pore structure and voids are destroyed, and cracks and the like occur. SOLUTION: The alumina precursor gel is held under constant conditions to control the progress of gelation of the alumina precursor gel, and then the solvent remaining in the pores and voids of the gel is treated under supercritical conditions. Substituting with an organic solvent having a higher solubility in the dispersion medium used in step 1 and a lower critical temperature, the gel is dried and calcined under mild supercritical conditions.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to the production of an alumina porous body having controlled pores suitable for various filters, separators, heat insulating materials, soundproofing materials, carriers for functional materials such as catalysts and enzymes, and the like. It is about the method.

[0002]

2. Description of the Related Art Conventionally, porous materials made of various materials such as organic materials have been used for various general filters, separation materials, heat insulation materials, sound insulation materials, and carriers for functional materials such as catalysts and enzymes. However, as various requirements for mechanical, thermal, chemical stability, etc. have increased, various inorganic porous materials have come to the forefront, and their performance is applied to each application. It has been clarified that the material used for the inorganic porous material is greatly influenced by the pore diameter, the pore volume, and the pore diameter distribution.

Therefore, for example, an alumina porous body can be prepared relatively easily by the sol-gel method. Therefore, the aluminum alkoxide is hydrolyzed with a large amount of water heated to 80 ° C. or more and then peptized with an acid to remove water. It has been proposed to prepare a precursor gel of alumina used as a solvent and calcine the precursor gel.

However, the alumina porous body obtained by the above-mentioned manufacturing method has relatively small pores having a diameter of about several nm, and since the pore volume is too small, it has the above-mentioned various uses. Even if it is not applied to, it was not possible to obtain a sufficient permeation rate, and it was extremely difficult to produce an alumina porous body in which the pore diameter and the pore volume were controlled to be arbitrary according to each application.

Further, during the drying process of the alumina precursor gel, due to the effect of the surface tension generated at the gas-liquid interface of the solvent that has penetrated into the pores and voids of the precursor gel, a contracting force acts to cause gelation. There is a possibility that the fine pore structure and voids formed may be destroyed and cracks and the like may occur.

Therefore, in order to solve the problems in the drying process, there has been proposed a method of drying a wet gel-like substance obtained by hydrolysis of alkoxide such as alkoxysilane in a supercritical state of a solvent. (See Japanese Patent Publication No. 7-37323 and Japanese Patent Application Laid-Open No. 6-135712).

[0007]

However, in the case of a gel using water obtained by hydrolyzing the alkoxide as a solvent, in order to extract and dry the solvent in a supercritical state, for example, 37
Since a high temperature of 4 ° C. or higher and a pressure of 22.1 MPa or higher are required, the restrictions on the apparatus are extremely large. Further, the water is previously stored in a solvent having a critical temperature and a critical pressure lower than that of water, such as ethanol. Even if trying to replace it, it is difficult to completely replace the solvent, and as a result, there is a problem that it is difficult to completely prevent the destruction of the fine structure as described above and the occurrence of cracks even when supercritically dried. .

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object thereof is to make it possible to control pore size such as pore size and pore volume by a simple operation procedure, and The pore structure and voids are not destroyed, or cracks do not occur, and have any pore diameter according to each application,
It is an object of the present invention to provide a method for producing an alumina porous body which can efficiently produce an alumina porous body having a large pore volume and a sufficient permeation rate.

[0009]

Means for Solving the Problems As a result of intensive studies on the above problems, the present inventors have found that an alumina precursor gel containing alcohol or the like as a solvent is kept at a temperature not higher than the boiling point of the solvent. , The amount of solvent in the alumina precursor gel is controlled within the range of the wet gel state, and after changing the degree of gelation of the alumina precursor gel, the water remaining in the pores and voids of the gel is changed. Water generated by decomposition, alcohol and other additives, etc., having a high solubility with the dispersion medium used under supercritical conditions, replaced by an organic solvent having a lower critical temperature, preferably solvent-substituted gel The alumina porous body having a large pore volume can be set by drying and calcining this gel under mild supercritical conditions while being immersed in the solvent used for But It was found that to be.

Among them, the organic solvent to be substituted is C
_n H _{(2n + 1)} OH (In the formula, n = 2 to 4, provided that tert-C
It has been found that either one of ₄ H ₉ OH) and acetone is most desirable.

[0011]

According to the method for producing a porous alumina body of the present invention, the progress of gelation is controlled by controlling the retention time after gelling the alumina precursor sol obtained by hydrolysis of aluminum alkoxide. As a result, a gel having an arbitrary pore size and pore volume is obtained, and the solvent in the pores is replaced with an organic solvent having a high solubility with the dispersion medium used for supercritical drying, and under supercritical conditions. By drying, it is possible to dry while maintaining the pore structure after substitution with the organic solvent, and it is possible to easily control the pore diameter and the pore volume depending on the progress state of the gelation.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The method for producing an alumina porous body of the present invention will be described in detail below.

In the present invention, an aluminum alkoxide solution diluted with an organic solvent or the like is hydrolyzed by adding water diluted with an organic solvent to prepare an alumina precursor sol, which is then allowed to stand at room temperature for a certain period of time to form a gel. Turn into

Thereafter, the obtained alumina precursor gel is maintained at a temperature not higher than the boiling point of the solvent of the aluminum alkoxide solution, and the amount of the solvent reduced in the alumina precursor gel is controlled by limiting the wet gel state, that is, A gel having an arbitrary pore diameter and pore volume suitable for the intended use is prepared by controlling the progress of gelation.

Next, water, alcohol, and other additives produced by hydrolysis remaining in the pores and voids of the gel have a high solubility in a dispersion medium used under supercritical conditions and have a higher critical temperature. After replacing with a low organic solvent, the organic solvent is removed under supercritical temperature and pressure conditions and dried to obtain a dry gel having a target pore diameter and pore volume structure.

The dried gel thus obtained is calcined in air at a predetermined temperature to obtain an alumina porous body having a desired pore size and pore volume.

In the present invention, as a method for producing the alumina precursor sol, it is preferable to use a sol-gel method by hydrolysis of aluminum alkoxide, and as the aluminum alkoxide, iso-propoxide, n-butoxide, sec- Butoxide, tert
-Butoxide and the like can be used.

Further, the alkoxide is replaced with 2-propanol, 2-butanol, 2-methoxyethanol, 2-
After diluting with an organic solvent such as ethoxyethanol, preferably adding a compound that reduces the reactivity of the alkoxide to the alkoxide solution, 2-propanol,
Hydrolysis is performed by adding dropwise a mixed solution of water and an organic solvent having high compatibility with water, such as 2-butanol, 2-methoxyethanol, and 2-ethoxyethanol, at room temperature with sufficient stirring to perform hydrolysis to give an alumina precursor. Body sols can also be made.

The aluminum alkoxide needs to have high solubility in the solvent used for dilution, and it is desirable that the solvent has high solubility in water. For example, in the case of iso-propoxide, 2-methoxyethanol and 2-ethoxyethanol are sec
With respect to butoxide, it is desirable to use 2-propanol, 2-butanol, 2-methoxyethanol, 2-ethoxyethanol as the organic solvent.

At this time, the optimum amount of water to be added varies depending on the amount of the stabilizer, but if the amount is too small, gelation does not substantially occur, and if it exceeds 5 mols, the gelation rate is increased. Since it is too fast and a non-uniform gel is formed, the amount of water added is 0.1 mol or more and 5 mol or less with respect to 1 mol of aluminum alkoxide, and a uniform gel is formed after several hours to several days. In terms of being able to
The optimum amount is 3 mol or more and 3 mol or less.

The compound that reduces the reactivity of the alkoxide is R ₁ R ₂ NCH ₂ CH ₂ OH (wherein
R ₁ or R ₂ is H, CH ₃ , C ₂ H ₅ , C
Examples thereof include ethanolamine represented by any one of H ₃ CH ₂ OH), β-diketone, acetic anhydride, acetoacetic acid ester, and dicarboxylic acid ester.

On the other hand, the amount of the stabilizer added is 0.1
If the amount is less than 2 mols, the effect as a stabilizer is not substantially obtained. If the amount exceeds 2 mols, gelation does not occur even if hydrolyzed, or the gelation requires an extremely long time. 0 to mol.
It is preferably 1 mol or more and 2 mol or less.

The method for producing the alumina precursor sol of the present invention is not limited to the above production method.

Next, as a method of drying under supercritical conditions, a method of heating and raising the temperature and pressure up to the critical point of the organic solvent used for substitution and drying the organic solvent in that state,
Alternatively, there is a method in which a dispersion medium such as methanol, ethanol, dichlorodifluoromethane, carbon dioxide, etc. is used, and this dispersion medium reaches a supercritical state, and is replaced with the organic solvent in the pores and voids to remove the solvent.

In the present invention, the latter method is preferred,
As the dispersion medium in the supercritical state, it is necessary to satisfy the conditions that the critical temperature is around room temperature, the critical pressure is low, the solubility is high, and it is inexpensive and easily available, and that it is not toxic. is there.

Carbon dioxide is most suitable as such a dispersion medium, and the temperature and pressure of 31.0 ° C. or more and 7.38 MPa or more, which are the critical points of carbon dioxide, are desirable as supercritical conditions, and further 50 It is most preferable to dry under the temperature and pressure conditions of not less than 0 ° C and not less than 15 MPa.

In addition, the organic solvent is excellent in solubility in carbon dioxide used as a dispersion medium, and therefore C _n H
_{(2n + 1)} OH (in the formula, n = 2 to 4, but tert-C ₄ H
Optimum is one of ₉ ) (excluding ₉ OH) and acetone.

[0028]

EXAMPLES Hereinafter, the method for producing the porous alumina body of the present invention will be described in detail based on one example.

First, in a alkoxide solution containing aluminum secondary butoxide, 2 methoxyethanol and NN dimethylethanolamine in a molar ratio of 1 to 6: 1, water and 2 methoxyethanol are in a molar ratio of 1: 3. After the solution consisting of (1) was added dropwise at room temperature to prepare an alumina precursor sol, the alumina precursor sol was allowed to stand in the atmosphere at room temperature for 4 days for gelation.

The obtained gel was allowed to stand under the holding conditions of each temperature and each period shown in Table 1, and the water in the gel, 2-methoxyethanol, other additives, etc. in the gel were limited to the limit. After the solvent was volatilized, the solvent remaining in the pores and voids of the gel was replaced with ethanol.

Next, carbon dioxide was used as a dispersion medium for supercritical drying and supercritical drying was carried out under the temperature and pressure conditions shown in Table 1, and the obtained dried gel was dried in the air at 500 to 900.
Firing was performed at a temperature of ° C to prepare an alumina porous body for evaluation.

Using the thus obtained alumina porous body for evaluation, the pore diameter and the pore volume were measured by a nitrogen adsorption measuring device.

[0033]

[Table 1]

As is clear from the table, in the sample numbers 16, 17, and 18 which are comparative examples, the pore diameters are all extremely small on the order of 3 nm, and the pore volume is also small as a whole. It can be seen that the pore diameter is 8.27 nm or more, and the pore volume is 0.83 cc / g or more.

Based on the results shown in Table 1, the alumina precursor was obtained by holding the alumina precursor at 60 ° C. for 1 to 15 days and then supercritically drying the dried gel to be calcined at 500 ° C. in the atmosphere. FIG. 1 shows the relationship between the retention period of A, and the pore diameter and pore volume of the alumina porous body.

As is clear from the figure, the pore diameter and the pore volume of the alumina porous body can be arbitrarily controlled by controlling the retention period, that is, the decrease amount of the solvent in the alumina precursor gel within the state of the wet gel. You can see that you can control.

[0037]

As described above, according to the method for producing an alumina porous body of the present invention, an alumina precursor gel produced from an aluminum alkoxide has an arbitrary pore size depending on the selection of holding conditions, and Since it is possible to obtain a gel with a large pore volume, by supercritical drying the gel, it is possible to dry while maintaining the pore structure,
An alumina porous body having a controlled pore diameter and a high pore volume can be manufactured by a simple operation procedure.

Therefore, in the method for producing an alumina porous body of the present invention, various filters having a desired pore size and sufficient pore volume, a separating material, a heat insulating material, a soundproofing material, or a functional material such as a catalyst or an enzyme. It is possible to efficiently obtain a highly accurate alumina porous body having excellent properties for various uses such as the carrier.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a retention period of an alumina precursor gel and a pore diameter and a pore volume of an alumina porous body according to the present invention.

Claims (2)

[Claims] 1. An alumina precursor sol obtained by hydrolyzing an aluminum alkoxide is gelled, and the obtained alumina precursor gel is kept at a temperature not higher than the boiling point of the solvent in the aluminum alkoxide solution to obtain a wet gel. In that state, the solvent in the pores and voids of the alumina precursor gel is replaced with an organic solvent that is a dispersion medium, and then the organic solvent is removed by supercritical drying, and then the dried alumina precursor gel is baked. A method for producing an alumina porous body, comprising: 2. The organic solvent is C _n H _{(2n + 1)} OH (wherein
n = 2 to 4, except tert-C ₄ except H ₉ OH) and a manufacturing method of an alumina porous body according to claim 1, characterized in that any kind of acetone.