JPH0818824B2 - Method for producing alumina-based porous gel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for producing an alumina-based porous gel which is suitable for a high temperature catalyst carrier and which can maintain a high specific surface area even at a high temperature.

[Conventional technology]

Conventionally, catalysts used in chemical processes are usually
As long as it is resistant to temperatures of 600 ° C or less, it does not cause any problems in ordinary chemical processes, so even if ceramic catalysts are generally used, their heat resistance did not become a problem. Devices with processes, especially catalytic combustors, are expected to be applied to gas turbines, boilers, jet engines, etc., and are required to withstand high temperature environments of 1000 ° C or higher due to demands such as reduction of NOx generation and improvement of combustion efficiency. Ceramics are being used.

Such ceramics are produced, for example, by appropriately mixing and mixing powders of various metal oxides, molding, and firing to obtain a desired composition and shape. However, in this method, since the starting material powder itself has a low specific surface area, the specific surface area is further reduced by high temperature calcination, and it cannot be said that the catalyst has high efficiency.

Therefore, in order to produce ceramics having a higher specific surface area, a sol-gel method of firing a gel formed by mixing and hydrolyzing a metal alkoxide, for example, an aluminum alkoxide or a barium alkoxide in place of powder as a starting material is INTERNATIONAL. SYMPOSIUM ON FINE CERAMICS A
It is described in Hirota Arai's document entitled "Development of Heat-Resistant Catalyst Material in High Temperature Catalytic Combustion" in RITA'87.

[Problems to be Solved by the Invention]

However, in the above-mentioned sol-gel method, since it is generated as powder, it needs to be reformed in order to be integrated, and it is the most commonly used temperature as a combustion catalyst.
The specific surface area around 1200 ° C was as small as 50 to 60 g / m ² at most, and it could not be said that the specific surface area was sufficiently large.

The present invention solves the above problems, and has a high specific surface area at high temperature by a sol-gel method and is an integral bulky alumina alone or an alumina composed of a composition of alumina and another metal oxide. It is an object of the present invention to provide a method for producing a precursor gel of a porous porous ceramic, that is, an alumina porous gel.

[Means for solving the problem]

The present invention is a method for producing an alumina-based porous gel by hydrolyzing an alumina-based precursor comprising an aluminum alkoxide alone or a mixture of a compound of aluminum alkoxide and a metal other than aluminum, in the alumina-based precursor. For aluminum alkoxide, alkanolamine, β-keto acid ester, β
A step of mixing one or more diketone compounds in an amount of 0.1 to 5 times by mole to form an alumina-based precursor mixture, and optionally a hydrolyzable reaction in the mixture in the presence of a base catalyst. The step of hydrolyzing and gelling the precursor mixture with 0.5 to 2 moles of water relative to the number of groups, and an organic solvent that occupies most of the liquid in the gel or a mixture system containing at least the organic solvent A method for producing an alumina-based porous gel, which comprises the step of drying the gel after passing through a critical state, by which the above problems can be solved.

In the present invention, aluminum alkoxide and alkanolamine, which are components of an alumina-based precursor composed of aluminum alkoxide alone or a mixture of aluminum alkoxide and a compound of a metal other than aluminum, β
Mixing with keto acid ester and β-diketone compound (hereinafter sometimes referred to as modifier) involves reaction, and both can be mixed at room temperature or under heating, so that miscibility and reaction uniformity From the aspect, it is preferable to carry out in the presence of an organic solvent. However, mixing is possible without the presence of an organic solvent.
The organic solvent used here is preferably one that dissolves an aluminum alkoxide, specifically methanol,
Ethanol, n-propanol, iso-propanol, s
Alcohols represented by ec-butanol, etc., aromatic hydrocarbons represented by toluene, benzene, xylene, etc., tetrahydrofuran, dimethylformamide, carbon tetrachloride, etc. are exemplified, but alcohols are preferable from the viewpoint of solubility. .

In the present invention, the hydrolysis reaction of the alumina-based precursor mixture produces an organic solvent such as an alcohol derived from an aluminum alkoxide, etc., so that it is in the gel after gel formation (it is clear that it is also included in the gel). In this case, the organic solvent formed, the above-mentioned added organic solvent, and an organic solvent composed of the modifier and the like are present. The organic solvent is removed along with the drying of the gel that is made through a supercritical state (hereinafter, abbreviated as supercritical drying), but if desired, a mixed system of the organic solvent and other chemical substances, for example, the organic The gel may be dried in a mixed system of a solvent and carbon dioxide. The supercritical state in this case refers to a state in which the critical temperature (Tc) and the critical pressure (Pc) inherent to one or a mixture of the organic solvent or a mixture of the organic solvent and other chemical substances are exceeded. , For example, in a pressure-volume diagram of carbon dioxide, it refers to a state in which the discontinuity points in the isotherm are in the region of the isotherm above the critical temperature at which the discontinuity points coincide and become one point. In a state where the organic solvent or the mixture in the present invention cannot be distinguished from a liquid and a gas, that is, in a broad sense, a state in which the organic solvent or the mixture is not liquefied even when compressed to a high density, that is, a criticality peculiar to the substance. It means a state of "supercritical fluid" which is a general term for gases heated above a temperature.

Also, since the critical point differs depending on the substance, it is preferable to match it so that the state of the supercritical fluid can be surely controlled. Therefore, when the solvent in the wet gel is different from the desired solvent used for supercritical drying, Replace the solvent in the wet gel with the desired solvent used for supercritical drying. Specifically, the wet gel is immersed in a large amount of the solvent used for supercritical drying and left for several hours, and the solvent is exchanged by utilizing the diffusion of the solvent.

Therefore, the organic solvent that occupies most of the liquid content in the gel in supercritical drying means that the organic solvent used for the substitution is present in the gel-forming system when the solvent substitution is not performed. Means the organic solvent formed, the added organic solvent, the modifier, and the like.

On the other hand, in the case of normal pressure drying in which the solvent is gradually evaporated and dried under normal atmospheric pressure, a gel (xerogel) with a high density and a small amount of pores is formed due to shrinkage during drying, which is easy with heating. It is very dense and cannot maintain a high specific surface area. On the other hand, the gel solvent is removed under the supercritical state, and the gel solvent is removed from the gel without the volume change such as the case where the organic solvent, etc., which is the gel solvent changes from liquid to gas. A high gel (aerogel) is produced. Since this airgel is an ultraporous body containing a large amount of fine pores, it can maintain a high specific surface area even after being heated at a high temperature. Further, in the case of xerogel, when it is heated and dried under atmospheric pressure, there is volume expansion accompanied by the fact that the organic solvent of the liquid in the pores becomes vapor, which causes a large stress, causing a few stresses or pieces. In many cases, airgel has excellent integrity because it does not generate stress due to the nature of supercritical drying. Therefore, an integrated alumina-based porous gel having a high specific surface area can be produced.

The alumina-based precursor mixture of the present invention, as a starting material for forming the hydrolysis product gel thereof, other than aluminum alkoxide alone, other metal alkoxides other than aluminum alkoxide and other aluminum (e.g., NaOR, Mg ( OR) ₂ , Si (OR) ₄ , PO (OR) ₃ , KOR, Ca
(OR) ₂ , Ti (OR) ₄ , Sr (OR) ₂ , Y (OR) ₃ , Zr (OR) ₄ , Ba (OR) ₂
R is an alkyl group such as methyl and ethyl), an inorganic compound such as a metal salt thereof (for example, a halide such as NaCl, MgBr ₂ , TiCl ₄ , NaNO ₃ , K ₂ SO ₄ and Ca (NO ₃ ) ₂ ) Nitrates, sulfates, sulfates, etc.), organometallic compounds other than alkoxides (eg, acetates such as CH ₃ COONa, (CH ₃ COO) ₂ Ca, (COON
a) ₂ , oxalates such as (COOK) ₂ , complexes with chelate compounds such as EDTA and NTA), and in some cases finely reactive metals and oxide fine powders (eg, CaO, TiO ₂ , SiO ₂₎ , P
₂ O ₅ , ZrO _2, etc.) can be used together.

The aluminum alkoxide used in the present invention is generally represented by (RO) ₃ Al (R: alkyl group), and specifically, R is methyl, ethyl, n-propyl, is
There are o-propyl, n-butyl, iso-butyl, sec-butyl, tert-butyl and the like.

Examples of alkanolamines to be mixed and reacted with the aluminum alkoxide used in the present invention include monoethanolamine, mono-n-purpanolamine, and mono iso.
-Propanolamine, diethanolamine, diiso-
Propanolamine, triethanolamine, tri-iso
-Propanolamine and the like, and likewise the β-keto acid ester include ethyl acetoacetate, methyl acetoacetate, ethyl malonate, diethyl malonate and the like, and the β-diketone compound includes acetylacetone and the like.

The alkanolamine, β-keto acid ester, and β-diketone compound are mixed and reacted with the aluminum alkoxide so that the sum of one kind or two or more kinds thereof is 0.1 times to 5 times the mol of the aluminum alkoxide, and the alumina is mixed. A system precursor mixture is formed. in this case,
When the amount is 0.1 times or less, the effect of controlling the hydrolysis rate in the next step is small and a homogeneous gel cannot be obtained. On the other hand, if the amount is 5 times or more, it is not preferable because it is too stable and gelation becomes difficult.

The alumina-based precursor mixture is, if necessary, in the presence of a base catalyst, for example, ammonia, pyridine, piperidine, piperidine, the number of hydrolyzable reactive groups present in the mixture (for example, the (RO) ₃ Al unreacted RO groups, and the number of reaction products groups of the alkanolamine, β-keto acid ester, β-diketone compound and the RO group, that is, the number of RO groups of the starting aluminum alkoxide, further aluminum The number of alkoxide groups of the added metal alkoxide other than the alkoxide or the metal alkoxide generated from the metal simple substance is included.), And the hydrolysis is performed with 0.5 to 2 moles of water. In this case, if the amount of water is 0.5 times or less the gelation is difficult, and if it is 2 times or more the particles tend to be formed, and the desired homogeneous gel cannot be produced. The alumina-based porous gel obtained after supercritical drying of the present invention withstands high-temperature conditions, and when it is fired, it is possible to obtain ceramics. As an example of the ceramics, Al ₂ O ₃ , Al ₂ O ₃ -BaO system (BaO ・ 6 Al
_{2 O 3), Al 2 O} 3 -SiO 2 system, Al ₂ O ₃ -TiO ₂ system, Al ₂ O ₃ -ZrO
₂ system, Al ₂ O ₃ -SiO ₂ -MgO system (cordierite), and the like.

[Action]

A feature of the present invention is that when synthesizing an alumina-based porous gel by the sol-gel method, an alkanolamine, a β-keto acid ester, and a β-diketone compound are used as a modifier for an aluminum alkoxide and hydrolyzed with an appropriate amount of water. This is to form an integrated homogeneous alumina-based gel instead of powder, and to form an integrated homogeneous alumina-based gel without shrinkage and cracks by supercritical drying. The gel (aerogel) thus obtained can maintain a high specific surface area even when exposed to a high temperature, and is suitable as a precursor of a high temperature catalyst carrier.

The high temperature catalyst carrier can be produced by firing the precursor in the range of 800 ° C to 1200 ° C. The high-temperature catalyst coats the catalyst metal by immersing the high-temperature catalyst carrier in a solution containing the catalyst metal. Then, this is heat-treated to obtain a high temperature catalyst.

〔Example〕

Hereinafter, specific examples of the present invention will be described, but the present invention is not limited thereto.

Example 1 Aluminum trisec-butoxide [(sec-BuO) ₃ Al] 1
0 g and 7 ml 2-butanol are mixed and to this is added 5.28 g ethyl acetoacetate. A mixture of 2.16 g of H ₂ O and 21.17 ml of 2-butanol was gradually added to this, and then at 60 ° C. for 72 hours.
And gelled. After replacing the solvent in the obtained wet gel with ethanol, the initial wetness was set to 8 using a CO ₂ supercritical extraction device.
CO under conditions of 0 ℃, 160kg / cm ² and final 120 ℃, 200kg / cm ^2.
₂ was flowed and ethanol was extracted and removed to obtain an integrated dry gel body. The specific surface area of this dried gel was 550 m ² / g.

When baked at 1000 ℃ for 5 hours, the specific surface area is 130m ² / g
Porous alumina ceramics were produced.

(Example 2) 9.88 g of (sec-BuO) ₃ Al and 0.450 g of metal barium were added to 10 ml of 2
-Pour in butanol and reflux. After the metal barium was completely dissolved, 8.96 g of diethanolamine was added as a modifier, and a mixed solution of 2.24 g of water and 20 ml of 2-butanol was gradually added to this, and the solution was refluxed for another 2 hours, then 1N-NH 2 ₃ a
2 ml of q was added to obtain a homogeneous alumina-barium wet gel. The wet gel was aged at 60 ° C. for 7 days, then put in an ethanol solvent and left for day and night for solvent replacement. This gel is CO ₂ supercritical extraction equipment at 80 ℃, 160kg / cm ² for about 6 hours,
Next, CO ₂ was flowed at 120 ° C. and 200 kg / cm ² for about 12 hours to extract ethanol to obtain an integrated alumina-barium porous body. The specific surface area of this dried gel was 580 m ² / g.

When baked at 1000 ℃ for 5 hours, the specific surface area is 110m ² / g
Porous alumina ceramics were produced.

(Example 3) 9.88 g of (sec-BuO) ₃ Al and 0.450 g of metal barium were added to 10 ml of 2
-Pour in butanol and reflux. After the barium metal is completely dissolved, 5.27 g of ethyl acetoacetate is added as a modifier. Then 2.16 g of H ₂ O is mixed with 20 ml of 2-butanol and slowly added to the (sec-BuO) ₃ Al solution. Then, it was kept at 60 ° C. for 7 days for gelation and aging. The solvent of this gel was replaced with ethanol, and ethanol was removed with an autoclave at 230 kg / cm ² and 270 ° C. in a supercritical state of ethanol to obtain a dry gel. The specific surface area of this dried gel was 420 m ² / g.

When this was fired at 1200 ° C. for 5 hours, porous alumina ceramics having a specific surface area of 95 m ² / g was produced.

Comparative Example 3 The dry gel obtained by gradually evaporating the solvent of the wet gel prepared in Example 1 at atmospheric pressure was 550 m ² / g.
The specific surface area was reduced to 3 m ² / g by firing at 0 ° C.

〔The invention's effect〕

The present invention, by gradually removing the organic solvent present in the gel produced by hydrolysis in a supercritical state, shrinkage,
An integrated alumina-based porous gel without cracks is obtained. This gel (aerogel) can maintain a high specific surface area even when exposed to high temperatures, and is therefore suitable for use at high temperatures. Also, this alumina-based porous gel is 800
Alumina-based ceramics can be obtained by firing in the range of up to 1200 ° C. This product has an extremely wide range of contributions to high efficiency of high temperature chemical process, improvement of efficiency of high temperature apparatus and the like.

Claims (1)

[Claims] 1. A method for producing an alumina-based porous gel by hydrolyzing an alumina-based precursor comprising an aluminum alkoxide alone or a mixture of an aluminum alkoxide and a compound of a metal other than aluminum. Alkanolamine, β-keto acid ester, β
A step of mixing one or more diketone compounds in an amount of 0.1 to 5 times by mole to form an alumina-based precursor mixture, and optionally a hydrolyzable reaction in the mixture in the presence of a base catalyst. The step of hydrolyzing and gelling the precursor mixture with 0.5 to 2 moles of water relative to the number of groups, and an organic solvent that occupies most of the liquid in the gel or a mixture system containing at least the organic solvent A method for producing an alumina-based porous gel, comprising the step of drying the gel after passing through a critical state.