JPH07315832A - Silica-modified alumina sol and its production - Google Patents

Abstract

PURPOSE:To obtain a silica-modified alumina sol having a sharp particle size distribution, excellent in transparency and stability with time and free from coagulation tendency in a wide pH range of 1-13 by modifying the surface of crystalline alumina particles with silica. CONSTITUTION:This silica-modified alumina sol consists of crystalline alumina fine particles having silica-modified surfaces. The fine particles preferably contain silica in the range of 0.1-50 pts.wt. based on 100 pts.wt. of alumina on the basis of the oxide. This method for producing silica-modified alumina sol is to add a polymerizable silicon compound (e.g. an alkali metal silicate, an alkoxide of silicon) to an alkaline alumina sol containing fine particles of crystalline alumina dispersed in water and age the resulting sol.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alumina sol which is stable in a wide pH range and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, various alumina sols and methods for producing the same have been known. For example, JP-A-59-78925 discloses an acidic alumina sol composed of pseudo-boehmite crystals, and JP-A-59-195527 discloses an alkaline alumina sol composed of alumina hydrate. .

However, these conventional sols are aggregated when the pH is changed such that the acidic alumina sol is made alkaline or the alkaline alumina sol is made acidic, so that there is a limitation in use in each application. It was At present, no stable alumina sol is known at any pH within a wide range of pH 1 to pH 13.

Further, the present inventors have found the existence of inventions relating to various complex oxide sols in the process of earnestly researching to obtain a stable alumina sol. For example, Japanese Examined Patent Publication No. 59-17047 discloses a method for producing an amorphous aluminosilicate sol in which colloidal particles of 3 to 90 nm are dispersed by controlling the pH of a heel sol solution to grow the particles, and also in Japanese Examined Patent Publication No. 47. No. 26,959 discloses a method for producing a sol of positively charged colloidal silica particles coated with alumina. However, a stable alumina sol obtained by modifying the surface of alumina fine particles contemplated by the present invention with an inorganic oxide other than alumina is not known.

[0005]

The object of the present invention is to obtain a silica-modified alumina sol which does not cause aggregation at any pH within a wide range of pH 1 to pH 13, has a sharp particle size distribution and is excellent in transparency and stability over time. It is to provide a manufacturing method.

[0006]

The silica-modified alumina sol according to the present invention is characterized by comprising crystalline alumina fine particles whose surface is modified with silica.

From the viewpoint of dispersion stability of the fine particles, it is preferable that the fine particles contain silica in an amount of 0.1 to 50 parts by weight based on 100 parts by weight of alumina. The silica-modified alumina sol has a solid content concentration of 0.5% by weight and an absorbance of 1.0 at a wavelength of 560 nm.
The following is preferable. The silica-modified alumina sol of the present invention can stably maintain the sol state at any pH within the range of pH 1 to pH 13.

The method for producing an alumina sol according to the present invention is characterized by adding a polymerizable silicon compound to an alkaline alumina sol in which fine crystalline alumina particles are dispersed in water and aging.

The alkaline alumina sol is preferably obtained by adding an alkaline substance to an acidic alumina sol in which crystalline alumina fine particles are dispersed in water, and bringing it into contact with an anion exchanger.

[0010]

DETAILED DESCRIPTION OF THE INVENTION In the present invention, the crystalline alumina fine particles (hereinafter sometimes referred to as colloid particles) constituting the silica-modified alumina sol include aluminum oxide and aluminum hydroxide. Examples of aluminum oxide include γ, η, δ, ρ, χ, θ, κ, and α crystal forms, and examples of aluminum hydroxide include boehmite, pseudoboehmite, gibbsite, diaspore, and bayerite. And one or more of these can be used as the dispersoid. Since the crystal form of the alumina fine particles can be detected by X-ray diffraction, and the crystal form of the alumina fine particles modified with silica described below can be similarly detected, it should be distinguished from amorphous silica / alumina. You can

In the silica-modified alumina sol of the present invention, silica is chemically bonded to and / or physically adsorbed on the surface of the alumina fine particles, so that the silica is entirely or partially adsorbed on the surfaces of the alumina fine particles. It refers to a state in which the coated dispersoid is uniformly dispersed in the dispersion medium.

In the present invention, the modification amount of silica with respect to alumina is 100 parts by weight of alumina.
It is preferably in the range of 1 to 50 parts by weight, particularly preferably in the range of 1 to 20 parts by weight. When the amount of modification of silica is less than 0.1 part by weight, the amount of silica covering the surface of the alumina particles is small, so that the surface characteristics of alumina remain strong and the dispersion stability in a wide pH range becomes low. The desired effect cannot be obtained. On the other hand, when the amount of silica modified exceeds 50 parts by weight, not only the characteristics of the alumina sol are lost, but also the dispersion stability of the particles is not further improved, resulting in poor economic efficiency.

It is preferable that the silica-modified alumina fine particles have an average particle diameter of 1 to 500 nm, particularly 2 to 100 nm. If the alumina fine particles are fine, the silica-modified alumina sol of the present invention has excellent transparency,
When a silica-modified alumina sol having a solid content concentration of 0.5% by weight is measured with a spectrophotometer, the absorbance at a wavelength of 560 nm is 1.0 or less, and particularly 0.5 or less.

In the silica-modified alumina sol of the present invention, since the surface of the alumina fine particles which are dispersoids is modified with silica, the pH is from 1 to p as in the case of silica colloid particles.
It is stable in a wide range of H13. Solid content concentration 1
A sol in which phosphoric acid or sodium hydroxide is added to a silica sol modified with silica by weight to adjust the sol to pH 2 and pH 8 does not precipitate alumina fine particles even when treated with a centrifuge at 1000 rpm for 30 minutes. On the other hand, a conventionally known alkaline or acidic alumina sol is adjusted to pH 2 and pH 8 by adding phosphoric acid or sodium hydroxide to a sol having a solid content concentration of 1% by weight and treated with a centrifugal separator at 1000 rpm for 30 minutes. When it does, it separates into two layers and lacks stability. The silica-modified alumina sol of the present invention is very stable in a wide range of pH 1 to pH 13, and has the characteristics of alumina sol, and is useful for various applications described later.

The dispersion medium of the silica-modified alumina sol may be water, or may be an organosol by substituting it with an organic solvent by a known method such as distillation under reduced pressure or ultrafiltration. As the organic solvent, alcohols, glycols, esters, ketones, aromatic solvents and the like can be used, and specifically, methanol, ethanol, propanol, ethylene glycol, propylene glycol, glycerin, acetone, methyl ethyl ketone, methyl. Cellosolve, ethyl cellosolve, dimethylformamide, N-
Examples thereof include organic solvents such as methyl-2-pyrrolidone. Further, by subjecting the surface of the colloidal particles to a known method such as a silane coupling agent, a titanium coupling agent, a surfactant, a higher fatty acid ester, or a salt thereof, low polarities such as xylene, toluene, dimethylethane, etc. A sol having an organic solvent as a dispersion medium can also be used.

Since the silica-modified alumina sol of the present invention has a low viscosity and is stable, the solid content concentration can be increased. The solid content concentration is usually adjusted in the range of 1 to 40% by weight.

Next, a method for producing a silica-modified alumina sol will be described.

As described above, in the production method according to the present invention, a crystalline silicon compound is added to an alkaline alumina sol in which crystalline alumina fine particles are dispersed in water and aged, and the alkaline alumina sol is, for example, It can be prepared by the following method.

A conventionally known alumina hydrate sol obtained by peptizing crystalline aluminum hydroxide with an acid, or
After adding an alkaline substance to an acidic alumina sol having a pH of 7 or less, such as the alumina sol described in Japanese Patent Application No. 5-261601 filed by the applicant of the present application, and contacting it with an anion exchanger to form a sol, The ion exchanger is separated to obtain an alkaline alumina sol. The pH of the alkaline alumina sol is preferably 7 or more, particularly preferably 9 to 13.

Among the above-mentioned raw material alumina sols, in particular, the method described in Japanese Patent Application No. 5-261601, that is, the raw material alumina powder is selected from γ, η, δ, ρ, χ, θ, κ, and α forms. Used in combination with one or more kinds of alumina composed of a single crystal or a plurality of crystals, the alumina powder is suspended in water, and a predetermined amount of acid is added to the suspension, and then cations are added. Add exchange resin, stir and mix,
Next, an acidic alumina sol obtained by separating the cation exchange resin is suitable.

The concentration of the acidic alumina sol is preferably in the range of 0.5 to 20% by weight, and the stirring and mixing with the anion exchanger are carried out until the alumina fine particles return to colloidal form, but usually from room temperature to 100. 0.5 ~ in the temperature range of ℃
When it is treated for 50 hours, it is deflocculated and becomes a colloid. When the acidic alumina sol contains a slight amount of an alkali component such as sodium, it is not necessary to add the alkali again.

The alkali used in the above method is a hydroxide of an alkali metal element such as sodium hydroxide, potassium hydroxide or lithium hydroxide, ammonia, tetramethylammonium hydroxide or N-methyl-2-.
Examples thereof include nitrogen compounds such as pyrrolidone and trimethylamine, and these alkalis are used alone or in combination. In producing the alkaline alumina sol, the amount of alkali added is 1 × 10 ^{−3 to} 0.1 mol, preferably 5 × 10 ⁻³ , per 1 mol of alumina (Al ₂ O ₃ ).
It is desirable to be in the range of 0.05 mol.

As the anion exchanger, any one having anion exchange ability can be used. In addition to a strongly basic or weakly basic anion exchange resin, a chelate resin, an ion exchange membrane, an ion exchange filter, etc. Is exemplified. These anion exchangers may be used alone or in combination. In the case of an ion exchange resin, it is usually alumina (Al ₂ O ₃ ) 1k.
It is used in a ratio of 1 to 20 liters with respect to g.

In the above production method, when the acid which is the stabilizer for the acidic alumina sol is removed by the anion exchanger and passes through the isoelectric point (pH 7 to 9) of alumina, the alumina fine particles are accompanied by aggregation, On the side, alumina reacts with alkali to produce an aluminum salt, and the aluminate ion of this aluminum salt is removed by the anion exchanger to regenerate the alkali, so that the reaction with alumina is repeated, and the alumina fine particles are It is deflocculated into colloidal particles (fine particles). In this method, even if the amount of alkali with respect to the alumina is small, the alkali is regenerated and repeatedly acts on the alumina, and therefore the alumina fine particles can be sufficiently deflocculated. Further, since the alumina sol contained in the obtained alumina sol is very small, that is, the content of the salt in the sol is small, the colloidal particles sufficiently form an electric double layer and the stability of the sol is high.

A polymerizable silicon compound is intermittently or continuously added to the above-mentioned alkaline alumina sol and aged to prepare a silica-modified alumina sol. As the polymerizable silicon compound used in the present invention, an alkali silicate, or a silicic acid solution obtained by dealkalizing a dilute aqueous solution of an alkali silicate with a cation exchange resin, sodium metasilicate,
There are sodium orthosilicate, alkoxide of silicon or a derivative thereof, for example, tetramethoxysilane, tetraethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane, phenyltrimethoxysilane, triphenylmethoxysilane, vinyltrimethoxysilane,
Examples thereof include alkoxysilanes such as trivinylmethoxysilane, and these silicon compounds are used alone or in a mixture.

The addition of the polymerizable silicon compound to the alkaline alumina sol is preferably carried out at a pH of the sol higher than 7, preferably in the range of 9 to 13, and more preferably in the range of 10 to 12, and the temperature is It is desirable to carry out at room temperature to 200 ° C. The aging is performed by stirring for a time sufficient for the silicon compound to polymerize and modify the surface of the alumina fine particles within the above temperature range, for example, for 0.5 to 50 hours. Even if a polymerizable silicon compound is added to the acidic alumina sol, the polymerization of silica does not occur in the acidic region, so the surface of the alumina particles cannot be modified with silica.

[0027]

EXAMPLES The present invention will be described in detail below with reference to examples.

Reference Example 1 Commercially available aluminum oxide powder (specific surface area: 110 m ² /
g, average particle diameter of powder: 0.6 μm, main crystal form: δ form,
100 g of hydrochloric acid content (value converted as HCl: 0.4% by weight) and 400 g of pure water were mixed, and while stirring at room temperature, a strong acidic cation exchange resin (manufactured by Mitsubishi Kasei,
400 cc of DAIAION, SK-1B) was gradually added, and after stirring for 20 hours, the ion exchange resin was removed to obtain an acidic alumina colloid aqueous solution. The average particle size of the alumina fine particles dispersed in this aqueous alumina colloid solution was 50 nm. The obtained acidic alumina colloidal aqueous solution was concentrated to give a 20 wt% colloidal solution as an oxide.

Example 1 While stirring 200 g of the acidic alumina colloidal solution (solid content concentration 20% by weight) obtained in Reference Example 1 and 180 g of pure water at room temperature, 20 g of 1% by weight sodium hydroxide was added. After that, strong basic anion exchange resin (manufactured by Mitsubishi Kasei, D
200 cc of AIAION, SA-20A) was gradually added, and after stirring for 20 hours, the ion exchange resin was removed to obtain a stable aqueous alkaline alumina colloid solution.
The average particle size of the colloidal particles was 40 nm, and the crystal form was δ type.

An aqueous sodium silicate solution (silica concentration: 5% by weight, sodium hydroxide concentration:
(3.3% by weight), and then heated to 150 ° C., 400 g of 1% by weight silicic acid solution was added over 5 hours, and after stirring for 1 hour, the solution was cooled to room temperature. Subsequently, it was washed with an ultrafiltration membrane (SIP-1013, manufactured by Asahi Kasei) while adding pure water, and then concentrated to give 20% by weight as an oxide.
A colloidal aqueous solution of The average particle size of the colloidal particles dispersed in this silica-modified alumina colloidal aqueous solution is 6
It was 0 nm. The crystal form of the colloidal particles was δ type, which was the same as the crystal form before the treatment. The crystal form was measured by an X-ray diffractometer, and the average particle size was measured by a light scattering particle size distribution analyzer.

The preparation conditions of the colloidal solution thus prepared are shown in Table 1 together with Examples 2 to 6 and Comparative Example 1 below.
Shown in. The properties of this colloidal solution and colloidal particles were measured and observed by the following methods, and the results are shown in Tables 2 and 3, respectively.

(1) Average Particle Size of Colloidal Particles Acidic alumina sol is diluted with distilled water, and is then measured by a light scattering particle size distribution analyzer (manufactured by Hiac-Leuco, NICOMP-37).
0) was used.

(2) Particle size distribution of colloidal particles It was measured by the same measuring device as above. The symbols in Table 3 have the following meanings. ○: One particle size distribution is sharp Δ: One particle size distribution is broad ×: Multiple particle size distribution

(3) Crystal form of colloidal particles After drying the colloidal aqueous solution with a freeze dryer, 110
A sample dried at 20 ° C. for 20 hours was measured using a high-power X-ray diffractometer (RINT-1400, manufactured by Rigaku Denki) under the conditions of Cu tube, 60 KV, and 200 mA, and JCPD
The crystal form was qualified using S software.

(4) Absorbance of colloidal solution The colloidal solution is diluted with distilled water to a solid content concentration of 0.5.
The absorbance at 560 nm was measured with a spectrophotometer (manufactured by Hitachi, Ltd., U-2000) with respect to the sample in which the weight% was set.

(5) Dispersion stability of colloidal solution A sample prepared by diluting an aqueous colloidal solution with distilled water to a solid concentration of 1% by weight and adding phosphoric acid or sodium hydroxide to adjust the pH to 2 and 8 3 at 1000 rpm with a small centrifuge (domestic centrifuge, H-18)
Centrifugation was performed for 0 minutes. The symbols in Table 2 have the following meanings. ○: Colloidal solution does not settle and separate ×: Colloidal solution does not settle and separate

(6) Change with time of colloidal solution A sample in which an aqueous colloidal solution was allowed to stand for 1 month was diluted with distilled water to a solid concentration of 1% by weight, and 5% by weight of phosphoric acid was added to adjust the pH to 2. For a sample adjusted to pH8, 1000 with a small centrifuge (H-18 manufactured by domestic centrifuge)
Centrifugation was performed at rpm for 30 minutes. The symbols in Table 2 have the following meanings. ○: Colloidal solution does not settle and separate ×: Colloidal solution does not settle and separate

(7) Viscosity of colloidal solution A sample obtained by diluting an aqueous colloidal solution with distilled water to a solid content concentration of 10% by weight was measured at 25 ° C. by an Ostwald viscometer.

(8) Zeta potential of silica-modified alumina colloidal particles An ultrasonic zeta potential measuring device (ESA-manufactured by Matec) was used to dilute the colloidal aqueous solution with distilled water to obtain a solid content concentration of 2% by weight and a pH of 8. 800).

Example 2 While stirring 500 g of the silica-modified alumina colloidal aqueous solution (solid content concentration 20% by weight) obtained in Example 1 and 500 g of pure water at room temperature, a strong acidic cation exchange resin (Mitsubishi Kasei Co., Ltd.) was added. Made, DAIAION, SK-1B) 200c
After adding c, stirring for 2 hours and performing deionization,
The ion exchange resin was removed. Next, while adding ethanol to the colloid solution, an ultrafiltration membrane (made by Asahi Kasei, SI
P-1013) is used to replace the solvent to give an oxide of 20
A silica-modified alumina sol having a weight% of ethanol as a dispersion medium was obtained. 1% by weight of this colloidal solution as an oxide
After diluting with ethanol so that it would be, centrifugation was performed at 1000 rpm for 30 minutes in a small centrifuge, but no precipitation of colloidal particles was observed.

Example 3 Acidic alumina colloidal solution prepared in the same manner as in Reference Example 1 (solid content concentration: 20% by weight, average particle diameter: 60 n)
m, main crystal form: γ form) 1000 g and pure water 2950 g and 1
A strong basic anion exchange resin (manufactured by Mitsubishi Kasei, D
(AIAION, SA-10A) 1000 cc was gradually added, and the mixture was stirred for 20 hours. Then, after cooling to room temperature, the ion exchange resin was removed to obtain an alkaline alumina sol. Sodium silicate solution (silica concentration: 5% by weight, sodium hydroxide concentration:
3.3% by weight), 20 g of this aqueous colloidal solution was heated to 98 ° C., 120 g of a 5% by weight silicic acid solution was added over 10 hours, and the same treatment as in Example 1 was carried out to obtain an oxide. As a result, a 20 wt% colloidal aqueous solution was obtained.

Example 4 An organosol (solids concentration: 20% by weight) using methanol as a dispersion medium was prepared in the same manner as in Example 2 by using the silica-modified alumina colloidal aqueous solution of Example 3.
After adding 50 g of 1-propanol to 00 g, 15 g of triethoxymonovinylsilane (KBE-1003, manufactured by Shin-Etsu Chemical Co., Ltd.) and 160 g of toluene were added thereto with stirring, and the mixture was stirred and mixed for 1 hour. Subsequently, the solvent was replaced with a rotary evaporator while adding toluene to this to obtain a silica-modified alumina sol having 10% by weight of toluene as an oxide as a dispersion medium. This colloidal solution was diluted with toluene so as to be 1% by weight as an oxide, and then centrifuged at 1000 rpm for 30 minutes in a small centrifuge, but no precipitation of colloidal particles was observed.

Example 5 While stirring 2000 g of commercially available acidic alumina sol (manufactured by Catalysts & Chemicals Industries, solid content concentration: 10% by weight, average particle size: 20 nm, crystal form: pseudo-boehmite type) and 5990 g of pure water at room temperature, After adding 10 g of 1% by weight sodium hydroxide thereto, a strongly basic anion exchange resin (manufactured by Mitsubishi Kasei, D
2000 cc of AIAION, SA-30A) was gradually added, and after stirring for 20 hours, the ion exchange resin was removed to obtain an alkaline alumina colloid aqueous solution. Sodium silicate solution (silica concentration:
5% by weight, sodium hydroxide concentration: 3.3% by weight) 3
After adding 0.3 g, the colloidal aqueous solution was heated to 98 ° C. and 1600 g of a 5 wt% silicic acid solution was added over 10 hours, and then treated in the same manner as in Example 1 to obtain 10
A weight% colloidal aqueous solution was obtained.

Example 6 While adding 90 g of ethylene glycol to 100 g of the silica-modified alumina colloidal aqueous solution (solid content: 10% by weight) obtained in Example 5, heating solvent substitution was carried out with a rotary evaporator to obtain 10% as an oxide. % Silica glycol-modified alumina sol was obtained. After diluting this colloidal solution with distilled water so as to be 1% by weight as an oxide, 100 with a small centrifuge.
After centrifugation at 0 rpm for 30 minutes, no precipitation of colloidal particles was observed.

Comparative Example 1 The acidic alumina colloidal solution (solid content concentration 20% by weight) obtained in Reference Example 1 was diluted with distilled water to 1% by weight as an oxide, and then 5% by weight phosphoric acid was added. The pH was adjusted to 2. When this solution was allowed to stand for a while, precipitation of agglomerated alumina colloidal particles was observed, and a stable aqueous colloidal solution could not be obtained.

[0046]

TABLE 1 U B A de soluble liquid tone made conditions mixing ratio of solids concentration pH silica alkali (* 1) Type added ratio (* 2) (* 3) (* 4) adding ratio (* 5) Example 1 5 NaOH 1.28 × 10 ^-2 10 10.9 0.1 Example 3 5 KOH 0.45 × 10 ^-2 5 11.0 0.3 Example 5 10 NaOH 0.13 × 10 ^-2 2.5 10.5 0.4 (* 1) Mixing ratio = ion exchange resin ( cc) / Al ₂ O ₃ (g) (* 2) Addition ratio = equivalent of alkali / equivalent of Al (* 3) Solid content concentration of acidic alumina sol (wt%) (* 4) pH of alkaline alumina sol (* 5) Addition ratio = weight of SiO ₂ / weight of Al ₂ O ₃

[0047]

TABLE 2 Co B A de soluble liquid sexual shaped solids absorbance dispersion stability temporal stability ζ potential pH Viscosity Concentration pH 2 pH 8 pH 2 pH 8 Example 1 20wt% 0.2 ○ ○ ○ ○ -15mV 9.0 2cp Example 3 20 0.4 ○ ○ ○ ○ -22 8.9 2 Example 5 10 0.1 ○ ○ ○ ○ -29 9.0 5 Comparative example 1 20 0.2 × × × × +20 4.6 4

[0048]

[Table 3] (* 1) Modification ratio = SiO ₂ weight / Al ₂ O ₃ weight

[0049]

The silica-modified alumina sol of the present invention has p
It is stable in a wide range of H1 to pH13 without causing aggregation, and is also excellent in stability over time.
Further, since the particle size distribution is sharp and the transparency is high, it is useful for various uses as described below, in addition to the use as a catalyst carrier.

(1) Since it has a large binder strength, it can be used as a binder for various refractories to obtain a high-strength molded article. For example, when it is used as a binder in the lost wax method for precision casting, it has excellent strength. You can get the mold. When used as a binder for inorganic fibers such as ceramic sheets, inorganic fibers having excellent tensile strength can be obtained. (2) Since it has little volume change or thermal expansion at high temperature, it can be used as a refractory material containing alumina and alumina.

(3) Since the hardness of colloidal particles is high, it is excellent when used as a hard coat film in combination with various abrasives, various cutting tools, or matrices such as resins and hydrolysates such as ethyl silicate. The effect is obtained. Further, when used as an abrasive, the polishing rate can be adjusted depending on the difference in particle size or crystal form. (4) Since it has a large Young's modulus and a small thermal expansion coefficient, when it is added to a coating composition such as an electromagnetic steel sheet, the tension of the coating can be significantly improved and a low iron loss steel sheet can be obtained.

(5) In particular, since the particle size distribution is narrow, mechanical strength and dimensional stability can be greatly improved when it is used as a filler for various plastics and rubbers. For example, when it is used as a filler for a polyester film, it is possible to obtain a secondary axis-oriented film having high strength, smoothness, and excellent abrasion resistance and slipperiness. (6) In addition, it is suitable for use as a surface treating agent for papermaking, a cosmetic compounding agent, a paint compounding agent, a fiber texture improving agent, a lubricant, a thickener, and an aluminum casting alloy.

The method for producing an alumina sol according to the present invention comprises:
The present invention provides an optimum method for producing the novel silica-modified alumina sol. In particular, when the alkaline alumina sol as a raw material is obtained by adding an alkaline substance to an acidic alumina sol in which crystalline alumina fine particles are dispersed in water and bringing it into contact with an anion exchanger, alkali is regenerated. Since it repeatedly acts on alumina, the alumina fine particles can be sufficiently deflocculated even if the amount of alkali added to alumina is small, which is economical as a manufacturing process. Further, since the content of the salt contained in the generated alkaline alumina sol is small, the stability of the sol is high and it can be said that this is an excellent production method.

Claims (4)

[Claims] 1. A silica-modified alumina sol comprising crystalline alumina fine particles whose surface is modified with silica. 2. The fine particles of alumina 10 based on oxide.
The silica-modified alumina sol according to claim 1, which contains 0.1 to 50 parts by weight of silica with respect to 0 parts by weight. 3. A method for producing a silica-modified alumina sol, which comprises adding a polymerizable silicon compound to an alkaline alumina sol in which crystalline alumina fine particles are dispersed in water and aging. 4. The alkaline alumina sol is obtained by adding an alkaline substance to an acidic alumina sol in which crystalline alumina fine particles are dispersed in water and bringing the alkaline alumina sol into contact with an anion exchanger. 4. The method for producing a silica-modified alumina sol according to 3.