JPH046180A - Production of porous ceramics of alumina system - Google Patents

Abstract

PURPOSE:To obtain the porous ceramics of an alumina system having a high specific surface area at a high temp. by hydrolyzing a precursor mixture, which is obtd. by mixing a specific reforming agent and silicon alkoxide into alumina alkoxide, and gelatinizing the mixture, then calcining this gel after supercritical drying. CONSTITUTION:The precursor mixture of the alumina system is formed by mixing 0.1 to 5 times mol one or >= 2 kinds of alkanol amine, beta ketoate and betadiketone compd. (reforming agent) and 0.1 to 3mol% silicon alkoxide into the aluminum alkoxide. The precursor mixture is the hydrolyzed and gelatinized with water of 0.5 to 2 times the mol of the number of the reaction groups which exist in the above-mentioned mixture and are obtd. by hydrolyzing in the presence of a basic catalyst at need. The gel is dried by way of the supercritical state of the org. solvent occupying the greater part of the liquid component in the gel or the mixed system contg. this org. solvent. The dried gel is subjected to a heating treatment in a nonoxidative atmosphere.

Description

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

[Conventional technology]

Catalysts traditionally used in chemical processes are typically 6
Ceramic catalysts did not pose a problem 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. equipment involving chemical processes;
In particular, catalytic combustors are expected to be applied to gas turbines, boilers, shared engines, etc., and ceramics that can withstand high-temperature environments of 1000°C or higher are used to reduce NO emissions and improve combustion efficiency. This is becoming more and more common.

Such ceramics are manufactured, for example, by suitably mixing powders of various metal oxides, molding, and firing 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 metal alkoxide or barium alkoxide instead of powder as the starting material is fired, is an INTERNATIONAL method.
SYMPO5IUM ON FINE CER
It is described in a document by Hiromichi Arai entitled "Development of heat-resistant catalyst materials for high-temperature catalytic combustion" in AMIC3ARTT^87.

[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
At around 00°C, alumina transitions to α phase and grain growth occurs, resulting in a specific surface area of 50 to 60 g at most.
/m2, and the specific surface area was not sufficient.

The present invention solves the above problems and produces 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, an alumina-based porous gel. The problem to be solved is to provide a manufacturing method for.

[Means for Solving the Problems] The present invention provides one of alkanolamine, β-keto acid ester, and β-diketone compound for aluminum alkoxide.
O, 1 to 5 times the mole of the species or two or more species, and 0.1 mol% to 30 mol''A to the aluminum alkoxide
silicon alkoxide? A step of kneading to form an alumina-based precursor mixture, optionally in the presence of a base catalyst, with water in an amount of 0.5 to 2 times the number of hydrolyzable reactive groups present in the mixture. hydrolyzing the precursor mixture;
a step of gelling, a step of drying the gel through a super-e-boundary state of an organic solvent that accounts for most of the liquid content in the gel, or a mixed system containing at least the organic solvent, and a step of drying the gel in a non-oxidizing atmosphere. The present invention is a method for producing alumina-based porous ceramics, which is characterized by comprising a step of heat-treating the ceramics, and thereby the above-mentioned problems can be solved.

In the present invention, the formation of an alumina-based precursor mixture by mixing an aluminum alkoxide, an alkanolamine, a β-keto acid ester, a β-diketone compound (hereinafter sometimes referred to as a modifier) and a predetermined amount of silicon alkoxide involves a reaction. Both can be mixed at room temperature or under heating, and 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 and the retardant, and specifically methanol, ethanol, n-propanol, 1so-propanol, 5
Examples include alcohols typified by ec-buknol, aromatic hydrocarbons typified by toluene, hensen, xylene, etc., tetrahydrofuran, dimethylformamide, carbon tetrachloride, etc., but alcohols are preferred from the viewpoint of solubility. .

In the case of forming this alumina-based precursor mixture, the order of mixing the aluminum alkoxide, the modifier, and the silicon alkoxide is not particularly limited.

In the present invention, the silicon alkoxide reacts with the aluminum alkoxide through water by the hydrolysis reaction of the alumina-based precursor mixture, and a gel in which the silicon atoms are bonded is formed, and a gel derived from the aluminum alkoxide, silicon alkoxide, etc. is formed. Since organic solvents such as alcohol are generated, the gel after gel formation (obviously this also includes areas other than the gel) contains the generated organic solvent, the above-mentioned added organic solvent, the modifier, etc. Therefore, there will be an organic solvent.

The organic solvent is removed with the drying of Kel through a supercritical state (hereinafter referred to as supercritical drying), but if desired, a mixed system of the organic solvent and other chemical substances, for example, Drying of the gel may be carried out using a mixed system of a solvent and carbon dioxide. In this case, the supercritical state refers to the critical temperature (
It refers to a state in which the organic solvent or the mixture exceeds the critical pressure (Tc) and the critical pressure (P, ), and refers to the state of a supercritical fluid 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.

As mentioned above, by removing the gel solvent under its supercritical state, a bulky gel (aerogel) without shrinkage is produced. This airgel is a superporous material containing a large amount of fine pores and having a high specific surface area.

It is possible to maintain a high specific surface area by heat-treating this airgel in a non-oxidizing atmosphere, but if a high-density gel (xerogel) with a small amount of pores is produced, stress will occur during drying, resulting in several Although it often breaks into pieces or pieces, this does not occur because stress is not generated due to the nature of supercritical drying (the airgel is produced with excellent integrity. Therefore, the airgel integrated by the present invention It is possible to directly synthesize high specific surface area porous ceramics with a desired shape.

On the other hand, in the case of normal pressure drying, in which the gel formed with or without the silicon alkoxide is dried by gradual evaporation of the solvent under normal atmospheric pressure rather than supercritical drying, shrinkage occurs during drying. As a result, the above-mentioned high-density gel (xerogel) with a small amount of pores is produced, which easily becomes dense with heating and cannot maintain a high specific surface area.

In addition, in the present invention, the supercritically dried gel is calcined at high temperature by using silicon alkoxide in an amount of 0.1 mol % to 30 mol Iχ based on the aluminum alkoxide as the starting material, so that the α phase transition temperature of alumina is lowered. Compared to the case where this is not used, it is possible to obtain an alumina-based porous gel having a crystalline structure that can maintain a high specific surface area even in scientific processes that are applied to higher temperatures because the temperature can be made slower than in the case where this is not used.

In this case, the amount of silicon alkoxide added is 0.1 m
ol! If it is less than 30 mo1%, the effect of the additive will not be exhibited, which is not preferable, and if it is more than 30 mo1%, a new crystal phase such as mullite will be formed, and the melting point will be lowered, making it easy to sinter, which is not preferable.

The silicon alkoxide is not particularly limited as long as it has the above functions and has an alkoxy group, but examples include Si (OR) a (R represents an alkyl group having 1 to 5 carbon atoms, etc. ), R'5i(OR
”)s(R' represents an aromatic group such as an aryl group such as a phenyl group, a non-aromatic group such as an alkyl group having 1 to 5 carbon atoms, etc., and R2 represents an alkyl group having 1 to 5 carbon atoms, etc. ), partial hydrolysates thereof, and condensates thereof (for example, those with a polymerization number of 2 or more), etc., and these may be used alone or in appropriate combinations. be able to.

In addition to aluminum alkoxide and silicon alkoxide, starting materials for forming the alumina-based precursor mixture of the present invention and the hydrolyzed gel thereof include aluminum alkoxide and silicon alkoxide.
Other metal alkoxides (e.g. PO(OR)3, Na
ORXMg(OR)z, KORlCa(OR)z,
Ti(OR)4.5r(OR)z, Y(OR):l,
Zr (OR) s, Ba (OR) 2, etc., where R is an alkyl group such as methyl, ethyl, etc.), inorganic compounds such as other metal salts (for example, halides such as NaCl, MgBrz, TiCIa, NaN0., KzSO4, Ca(NO
nitrates, sulfates, etc.), organometallic compounds other than alkoxides (e.g., CH3COONa, (CH3
Acetate salts such as COO)2Ca, (COONa)z, (
complexes with chelate compounds such as oxalates such as COOK), EDTA, and NT8), and in some cases highly reactive metals and oxide fine powders (e.g., CaO1Ti).
ZrOz, 5iOz, P2O6, ZrO2, etc.) can be used in combination.

The aluminum alkoxide used in the present invention has the general formula (RO)+! (R: alkyl group), specifically R is methyl, ethyl, n-propyl, 1so-propyl, n-butyl, 1so-butyl, 5
Examples include ec-butyl and tert-butyl.

The alkanolamines to be mixed and reacted with aluminum alkoxide etc. used in the present invention include monoethanolamine, mono-n-propazuramine, mono-1so-
propatoolamine, jetanolamine, di-1so-
Propertoolamine, triethanolamine, Tojan S
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 above alkanolamines, β-keto acid esters, β-cite I
- The sum of one or more of the compounds is 0.1 to 5 times the mole of aluminum alkoxide, and Q, 1 mol% to 30 times the mole of aluminum alkoxide.
A mol % of silicon alkoxide is mixed and reacted with the aluminum alkoxide to form an alumina-based precursor mixture. 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 salt 4 catalyst, such as ammonia, pyridine, piperidine, piperazine, to reduce the number of hydrolyzable reactive groups present in the mixture (for example, the ( RO), unreacted RO of Aff
group, and the number of groups formed by the reaction of the alkanolamine, β-keto acid ester, β-diketone compound with the RO group, that is, the number of RO groups in the starting material aluminum alkoxide or the silicon alkoxy 1ξ, and the aluminum alkoxide. Examples include the number of alkoxide groups in metal alkoxides other than those added or metal alkoxides produced from simple metals. ) 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 mole, gelation will be difficult, and if it is more than 2 times the amount of water, particles will tend to form, 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 A]20i
-3iOz series, Alz03-3i02-MgO series (cordierite), A1203-3iO□-BaO1A
lzO: +-5iOz-BaO-SrO, 8I□Q3-
5iO□-CaO1A]20:+-5i02-FeO1
A1zO+-5i02-LiO2, 81203-ZrO
z, A1203-5iO□-Na02, and the like.

1 Effect] The feature of the present invention is that in synthesizing an alumina-based porous gel by a sol-gel method, aluminum alkoxide is combined with modifiers such as alkanolamine, β-keto acid ester, and β-diketone compound and the presence of silicon alkoxy 1-” Under,
By hydrolyzing with an appropriate amount of water, an integrated homogeneous alumina-based gel is formed instead of a powder, which is then integrated by supercritical drying to form a homogeneous alumina-based porous gel without shrinkage or cracking. Obtained gel (aerogel)
By performing high-temperature firing treatment under a non-oxidizing atmosphere (for example, N2, NH, Nato), it is possible to produce ceramics that maintain a high specific surface area that cannot be obtained by high-temperature firing treatment under a normal atmosphere. .

In the present invention, the wet gel produced by hydrolysis contains the above-mentioned organic solvent, and since this organic solvent, especially alcohol-based solvent, exchanges reacts with the hydroxyl groups of AI, Si, etc. of the gel, supercritical drying removes this organic solvent. When the solvent is removed, it has the effect of promoting the residual rate of the battering machine solvent compared to a normal drying process.

Further, the residue is carbonized during the firing process, and the amount of carbon is also considered to be increased compared to the normal drying process.

Then, the volatilization of the produced substance is promoted by the reduction reaction described below between SiO□ produced from silicon alkoxide and the carbon produced in a non-oxidizing atmosphere, or by the pores formed by supercritical drying, and as a result, fine pores are formed. In addition, in this high-temperature firing process, Si derived from silicon alkoxide has the function of stably maintaining the intermediate alumina state and delaying the transition to the α phase, so it is not exposed to high temperatures. Also, a bulky high-temperature catalyst carrier can be obtained while maintaining a high specific surface area.

SiO Since it is considered that the above-mentioned reaction hardly occurs because the oxide is combined with atmospheric oxygen, it is obvious that an increase in the number of pores according to the present invention cannot be expected.

In addition, the amount of residual carbon can be adjusted in order to control the above reaction and physical properties such as the specific surface area of the target ceramic by selecting the conditions of the reaction system during the formation of the alumina-based precursor mixture. .

In this case, other carbon sources such as a simple substance such as carbon blank or a polymer such as phenol resin may be added to the reaction system during the formation of the alumina precursor mixture, if desired.

Thus, the high temperature catalyst support using the present invention is produced by calcining the supercritical dry gel preferably in the range of 800°C to 1200°C under a non-oxidizing atmosphere as described above. In addition, high-temperature catalysts are coated with a catalyst metal by immersing a high-temperature catalyst carrier in a solution containing the catalyst 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-2-butoxide C(2-BuO)aA
l) 9g and 4.75g of ethyl acetoacetate,
10ml of phthanol was added. A mixture of 0.033 g of water and 0.2 g of 2-butanol was added to a mixture of 0.36 g of silicon tetraethoxide and 0.2 g of 2-butanol, and the mixture was hydrolyzed. This was mixed and stirred into the aluminum tri-2-butoxide stabilized with ethyl acetoacetate. Next, a mixed solution of 2.07 g of water and 20 ml of 2-butanol was gradually added to this for hydrolysis.
The gel was maintained at 0°C for 72 hours. This wet gel was solvent-substituted with ethanol and placed in an autoclave for 230 min.
kg/cIIN, the solvent was removed in a supercritical state of ethanol at 270°C to obtain a dry gel. This dried gel was baked at 1200°C for 15 hours in a nitrogen atmosphere to give a specific surface area of 16
Alumina-based porous ceramics of 0 m2/g were obtained.

(Example 2) Aluminum tri-2-butoxide ((2-BuO): +
Mix 9 g of AI) and 4.75 g of ethyl acetoacetate,
10ml of 2-phthanol was added. Mix 0.98 g of silicon phenyltriethoxide and 0.2 g of 2-butanol, and add 0.07 g of water and 0.2 g of 2-butanol to this.
A mixture of these was added. This was mixed and stirred into the aluminum tri-2-butoxide stabilized with ethyl acetoacetate. Next, a mixed solution of 2.12 g of water and 20 ml of 2-butanol was gradually added to this for hydrolysis.
It was kept at 0°C for 74 hours to form a gel.

The solvent of this wet gel was replaced with ethanol, and the solvent was removed in an autoclave at 230 kg/c+jt and 270°C in the supercritical state of ethanol to obtain a dry gel.

This dried gel was fired at 1200° C. for 15 hours in a nitrogen atmosphere to obtain alumina-based porous ceramics with a specific surface area of 170 m/g.

(Comparative example) The supercritical dry gels prepared in Examples 1 and 2 were dried in air for 1
When fired at 200°C for 5 hours, the specific surface area is 90
It decreased to ~100m27g.

〔Effect of the invention〕

The present invention gradually removes the organic solvent present in the ceramic precursor gel in a supercritical state, and further increases the pores in the firing process of the supercritically dried gel, and furthermore, it Ceramic catalyst carriers or catalysts with excellent moldability and integrity that can maintain a high specific surface area can be produced, so they can contribute to a wide range of areas such as increasing the efficiency of high-temperature chemical processes and improving the efficiency of high-temperature equipment.

Claims (1)

[Claims] (1) 0.1 to 5 times the mole of one or more of alkanolamines, β-keto acid esters, and β-diketone compounds to the aluminum alkoxide, and 0.1 mol% to 30 mol% of silicon to the aluminum alkoxide. mixing alkoxides to form an alumina-based precursor mixture, optionally in the presence of a base catalyst;
A step of hydrolyzing and gelling the precursor mixture with ~2 times the molar amount of water, and drying the gel through a supercritical state of an organic solvent that accounts for most of the liquid content in the gel, or a mixture system containing at least the organic solvent. 1. A method for producing alumina-based porous ceramics, comprising the steps of: and heat-treating the dried gel in a non-oxidizing atmosphere.