JPH03199120A - Production of alumina-based porous gel - Google Patents

Abstract

PURPOSE:To produce the gel capable of maintaining a large specific surface even at a high temp. appropriate for a high-temp. catalyst carrier by mixing an aluminum alkoxide and a specified compd. to obtain an alumina-based precursor, hydrolyzing and gelling the precursor and supercritically drying the product. CONSTITUTION:An aluminum alkoxide expressed by (RO)3Al (R is alkyls) and 0.1-5mols, based on the alkoxide, of >=1 kind among alkanolamines (e.g. diethanolamine, etc.) and the esters of beta-ketonic acids (e.g. ethyl acetoacetate) and beta-diketonic compd. (e.g. acetylacetone, etc.) to form an alumina-based precursor. The precursor is hydrolyzed and gelled with 0.5-2mols of water, based on the reactive group in the precursor capable of being hydrolyzed, in the presence of a basic catalyst (e.g. pyridine, etc.), as required, and the obtained gel is dried via the supercritical state of the org. solvent constituting the greater part of the liq. in the gel or the mixed system contg. the solvent. Consequently, the gel capable of maintaining the large specific surface even at a high temp. appropriate for a high-temp. catalyst carrier is obtained.

Description

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

[Conventional technology]

Catalysts traditionally used in chemical processes are typically 6
Ceramic catalysts did not pose any problems in ordinary chemical processes if they could withstand temperatures below 00°C, so even when ceramic catalysts were commonly used, their heat resistance was not a problem.However, in recent years, high-temperature thermochemical Devices that involve processes, especially catalytic combustors, are expected to be applied to gas turbines, boilers, jet engines, etc., and are used to reduce NOx generation,
Due to the demand for improved combustion efficiency, ceramics that are resistant to high-temperature environments of 1000° C. or higher are being used.

Such ceramics are manufactured, for example, by suitably mixing powders of various metal oxides, forming a bottom shape, and firing the powders in order to obtain a desired composition and shape. However, in this method, since the starting raw material powder itself has a low specific surface area, the specific surface area is further reduced by high-temperature calcination, and the catalyst cannot be said to be highly efficient.

Therefore, in order to produce ceramics with a higher specific surface area, the sol-gel method, in which a gel formed by mixing and hydrolyzing a metal alkoxide, such as aluminum alkoxide or barium alkoxide, instead of a powder as a starting material, is fired is used.
ON FINBCERAMIC3ARITA '87
This is described in Hiromichi Arai's article titled ``Development of heat-resistant catalyst materials for high-temperature catalytic combustion.''

[Problem to be solved by the invention]

However, in the above-mentioned sol-gel method, since it is produced as a powder, reformation is required to integrate it.
In addition, 12
The specific surface area at around 00℃ is at most 50 to 60 g/m
2, and could not be said to have a sufficiently large specific surface area.

The present invention solves the above problems and provides a precursor gel for alumina-based porous ceramics that has a high specific surface area at high temperatures and is integrated using a sol-gel method, that is,
The object of the present invention is to provide a method for producing an alumina porous gel.

[Means to solve the problem]

The present invention provides aluminum alkoxide with alkanolamine, β-keto acid ester, and β-diketone compound.
A process of mixing 0.1 to 5 times the mole of the species or two or more species to form an alumina-based precursor mixture, optionally in the presence of a base catalyst, to remove the hydrolyzable reactive groups present in the mixture. a step of hydrolyzing and gelling the precursor mixture with water in an amount of 0.5 to 2 times the mole of This is a method for producing an alumina porous gel characterized by comprising a step of drying the gel through a critical state, and thereby the above-mentioned problems can be solved.

In the present invention, the mixing of aluminum alkoxide with alkanolamine, β-keto acid ester, and β-diketone compound (hereinafter sometimes referred to as a modifier) involves a reaction, and the two are mixed at room temperature or under heating. It is possible,
From the viewpoint of miscibility and uniformity of reaction, it is preferable to carry out the reaction in the presence of an organic solvent.

However, mixing is possible even without the presence of an organic solvent. The organic solvent used here is preferably one that dissolves the aluminum alkoxide, specifically alcohols such as methanol, ethanolopen n-propanol, 1so-propanol, 5ec-butanol, etc.
Examples include aromatic hydrocarbons typified by toluene, benzene, xylene, etc., tetrahydrofuran, dimethylformamide, carbon tetrachloride, etc., but alcohols are preferred from the viewpoint of solubility.

In the present invention, organic solvents such as alcohols derived from aluminum alkoxide etc. are generated by the hydrolysis reaction of the alumina-based precursor mixture, so it is clear that the organic solvents such as alcohols derived from aluminum alkoxide etc. are included in the gel after gel formation (it is clear that it also includes areas other than the gel). An organic solvent consisting of the produced organic solvent, the above-mentioned added organic solvent, the modifier, etc. is present. The organic solvent is removed as the gel dries through a supercritical state (
(hereinafter abbreviated as supercritical drying), if desired, the gel may be dried in a mixed system of the organic solvent and other chemical substances, for example, a mixed system of the organic solvent and carbon dioxide. . In this case, the supercritical state refers to a state in which the critical temperature (Tc) and critical pressure (Pc) specific to the organic solvent or mixture or the mixture of the organic solvent and other chemical substances are exceeded. , refers to a supercritical fluid state in which the organic solvent or the mixture exhibits properties intermediate between a liquid and a gas.

Also, since the critical point differs depending on the substance, it is preferable to match the critical point because it ensures control of the state of the supercritical fluid. Therefore, if the solvent in the wet gel is different from the desired solvent used for supercritical drying, , exchange the solvent in the wet gel with the desired solvent used for supercritical drying. Specifically, a wet gel is immersed in a large amount of the solvent used for supercritical drying, left for several hours, and the solvent is exchanged using diffusion of the solvent.

Therefore, the organic solvent that occupies most of the liquid content in the gel in supercritical drying refers to the organic solvent used for replacement when solvent replacement is performed, and the organic solvent that is present in the gel production system when solvent replacement is not performed. It means an organic solvent consisting of the organic solvent produced, the organic solvent added above, the modifier, etc.

On the other hand, in the case of normal pressure drying, in which the resulting gel is dried by gradual evaporation of the solvent under normal atmospheric pressure, shrinkage occurs during drying, resulting in the formation of a gel (xerogel) with high density and low porosity, and it is easy to dry with heating. It is not possible to maintain a high specific surface area when it is densely deposited. On the other hand, by removing the gel solvent under its supercritical state, a bulky gel (aerogel) without shrinkage is produced.

Since this airgel is a wintering pore body containing a large amount of fine pores, it can maintain a high specific surface area even after high temperature heating. Furthermore, in the case of xerogel, stress occurs during drying,
Although it often breaks into several pieces or pieces, airgel has excellent integrity because no stress is generated due to the nature of supercritical drying. Therefore, it is possible to directly synthesize porous ceramics having a desired shape from the integrated airgel.

As starting materials for forming the alumina-based precursor mixture of the present invention, and by extension its hydrolyzed gel, aluminum alkoxide alone, as well as aluminum alkoxide and other metal alkoxides other than aluminum (e.g., NaOR, Mg( OR)2, Si(OR)4, P
O(OR)3, KDR, Ca(OR)2, Ti(OR)
,,5r(OR)2,Y(OR)3,Zr(ORL,
Ba(OR), etc. R is an alkyl group such as methyl, ethyl, etc., and inorganic compounds such as metal salts thereof (e.g., NaCl, M
halides such as gBr2, TiCl4, NaNO3,
K2So, , nitrates and sulfates such as Ca(NO3)2,
sulfates, etc.), organometallic compounds other than alkoxides (e.g., CHsCOONa).

(CH3COO) 2 Acetate such as Ca, (COONa
),, (COOK) 2nd grade oxalate, BDTA,
complexes with chelate compounds such as NTA>, and in some cases highly reactive metals and oxide fine powders (e.g. C
an, Tie, 5102, P2O5, ZrO2, etc.)
etc. can be used together.

The aluminum alkoxide used in the present invention is represented by the general formula (RO) s to A (R: alkyl group), and specifically R is methyl, ethyl, n-propyl, 1so-propyl, n- Butyl, 1so-butyl,
Examples include 5ec-butyl and tert-butyl.

The alkanolamines mixed and reacted with aluminum alkoxide etc. used in the present invention include monoethanolamine, mono-n-purpanolamine, mono-1SO
-propertoolamine, jetanolamine, di-1so
-Propertoolamine, triethanolamine, tri-1
Examples of the β-keto acid ester include ethyl acetoacetate, methyl acetoacetate, ethyl malonate, diethyl malonate, and the like. Examples of the β diketone compound include acetylacetone.

The alkanolamine, β-keto acid ester, and β-diketone compound are mixed and reacted with the aluminum alkoxide such that one or more of the 411s is 0.1 to 5 times the mole of the aluminum alkoxide. , an alumina-based precursor mixture is formed. In this case, if the amount is 0.1 times the mole or less, the effect of controlling the hydrolysis rate in the next step is small and a homogeneous gel cannot be obtained. Moreover, if it is 5 times the mole or more, it is not preferable because it is too stabilized and gelation becomes difficult.

The alumina-based precursor mixture is optionally prepared in the presence of a basic catalyst such as ammonia, pyridine, piperidine, piperesine, etc. to reduce the number of hydrolyzable reactive groups present in the mixture (for example, the (RO) , unreacted R of 3Af
O group, and the number of groups formed by the reaction between the alkanolamine, β-keto acid ester, β-diketone compound and the RO group,
That is, the number of RO groups in the starting material aluminum alkoxide, and the number of alkoxide groups in the metal alkoxide produced from an added metal alkoxide other than aluminum alkoxide or a metal simple substance, etc. ) for 0
．． It is hydrolyzed with 5 to 2 times the mole of water.

In this case, if the amount of water is less than 0.5 times the molar amount, gelation will be difficult, and if the amount of water is more than 2 times the molar amount, particles will tend to be generated, making it impossible to produce the desired homogeneous gel. Ceramics obtained by firing the alumina-based porous gel obtained after supercritical drying of the present invention include Al2O3, A120
3-Ban type (BaO-6A1203), A1203-
3i02 series, A1203-TiO□ series, A1203-2
Examples include r02 series, Al2O3S+02-MgO series (cordierite), and the like.

[Effect]

The feature of the present invention is that when synthesizing an alumina-based porous gel by the sol-gel method, realkanolamine, β-keto acid ester, and β-diketone compound are used as modifiers for aluminum alkoxide, and an appropriate amount of water is used to synthesize the alumina-based porous gel. By hydrolysis, an integrated homogeneous alumina-based gel is formed instead of a powder, and by supercritical drying, an integrated homogeneous alumina-based porous gel without shrinkage or cracking is formed.

The gel (aerogel) obtained thereby can maintain a high specific surface area even when exposed to high temperatures, and is suitable as a precursor for a high-temperature catalyst carrier.

The high-temperature catalyst carrier is manufactured by firing the precursor at a temperature in the range of 800°C to 1200°C. In addition, high-temperature catalysts
A high-temperature catalyst carrier is immersed in a solution containing the catalytic metal to coat it with the catalytic metal. This is then heat-treated to form 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 tri-5ec-butoxide [(sec-Bu
n) Mix 5All 1Og and 7ml of 2-butanol and add 5.28g of ethyl acetoacetate. To this, 8.02, 16g and 2-butanol 21.17
ml mixture was gradually added and then heated to 60°C for 72 h.
It was kept and allowed to gel. After replacing the solvent of the obtained wet gel with ethanol, using a CD or supercritical extraction device, the initial stage was 80°C and 160 kg/cm', and the final stage was 120°C and 200 kg/cm'.
CO2 was flowed under the condition of kg/cm', and ethanol was extracted and removed to obtain an integrated dry gel body. The specific surface area of this dry gel was 550 m'/g. This is 100
After firing at 0° C. for 5 hours, porous alumina ceramics with a specific surface area of 130 m 2 /g was obtained.

(Example 2) (sec-Bu[l) 19.688 g of 3A and 0.450 g of metallic barium are placed in 10 ml of 2-butanol and refluxed. After the metallic barium is completely dissolved, 8.96g of jetanolamine is added as a modifier, and 2.0g of water is added to this.
A mixture of 24 g and 20 ml of 2-butanol was gradually added, and the solution was refluxed for an additional 2 hours.
2 ml of was added to obtain a homogeneous alumina-barium wet gel.

This wet gel was aged at 60° C. for 7 days, then placed in an ethanol solvent, and the solvent was replaced day and night.

This gel was extracted using a CD□ supercritical extraction device at 80℃ and 160 kg.
/cm2 for about 6 hours, then 120℃, 200kg/cm
An integrated alumina-barium porous material was obtained by flowing CO□ for about 12 hours at No. 2 and extracting ethanol. The specific surface area of this dry gel was 580 m2/g.

This was baked at 1000℃ for 5 hours, and the specific surface area was 110m.
Porous ceramics having a composition of 2/g BaO.6Al2O3 was obtained.

(Example 3) (Sec-Bun) 19.688 g of 3A and 0.450 g of metallic barium are placed in 1Qml of 2-butanol and refluxed. After the metallic barium is completely dissolved, 5.28 g of ethyl acetoacetate is added as a modifier. Next 8202.
Mix 16 g with 20 ml of 2-butanol and gradually (
sec-Bun) Add to 3Al solution. After that, 60
The mixture was kept at ℃ for 7 days to gel and ripen. This gel was replaced with ethanol using a cyautocrape apparatus (230 kg/230 kg).
cm2. Ethanol was removed in a supercritical state of ethanol at 270°C to obtain a dry gel. The specific surface area of this dry gel was 420 m2/g. This was fired at 1200° C. for 5 hours to obtain porous ceramics with a specific surface area of 95 m 2 /g.

(Comparative Example 3) The dry gel obtained by gradually evaporating the solvent from the wet gel prepared in Example 1 at normal pressure had a dry gel of 55 +2/g, but when this was calcined at 1200°C, the specific surface area was It decreased to 3m2/g.

〔Effect of the invention〕

Claims (1)

[Claims] (1) A step of mixing 0.1 to 5 times the mole of one or more of alkanolamine, β-keto acid ester, and β-diketone compound to aluminum alkoxide to form an alumina-based precursor mixture, as necessary. a step of hydrolyzing and gelling the precursor mixture with 0.5 to 2 times the mole of water based on the number of hydrolyzable reactive groups present in the mixture in the presence of a base catalyst according to the method; A method for producing an alumina porous gel, comprising the step of drying the gel through a supercritical state of an organic solvent that accounts for most of the liquid content, or a mixed system containing at least the organic solvent.