JP2000007309A - Production of porous oxide powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain porous oxide powder having large specific surface area using a simple process. SOLUTION: An oxide particle having insufficiently proceeded oxidation state is produced by properly selecting the spraying and burning conditions of an emulsion in a process to form a W/O-type emulsion by mixing an aqueous solution containing a dissolved metal salt with an organic solvent and a dispersing agent and a process to synthesize the oxide particle by spraying and burning the W/O-type emulsion. The specific surface area of the oxide particle is remarkably increased by contacting a water-containing solution with the oxide particle synthesized by the above process because the bond between the hydroxy group and the nondecomposed metal salt remaining on the crystallite surface of the synthesized particle is broken to generate a porous structure on the surface of the particle. The specific surface area can be controlled by selecting the combustion condition, the kind of the aqueous solution, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a porous oxide powder such as alumina or silica.

[0002]

2. Description of the Related Art Porous alumina is useful as various carriers, adsorbents, paints and additives. In addition, the raw material powder of ceramics such as silica and alumina sometimes contains aggregates that are not easily pulverized by ball mill mixing. Therefore, many methods for producing a porous oxide powder such as alumina or silica have been proposed.

For example, in the invention of a method for producing low bulk density alumina disclosed in JP-A-1-32196 (hereinafter referred to as a first prior art), an alumina hydrogel formed at a pH of 6 to 11 and at a temperature of 50 ° C. or higher is used. Was added to the mixture under conditions of a temperature of 50 ° C. or higher and a pH of 6 to 11 and in the coexistence of a sulfate group to obtain a pseudo-boehmite gel which crystal-grows and forms a sparse aggregate. After washing with water, spray drying and firing are performed to produce alumina with low bulk density.

In the method for producing alumina disclosed in Japanese Patent Publication No. 1-17733 (hereinafter referred to as a second prior art), a slurry containing aluminum hydroxide seed is heated to 50 ° C.
The temperature is maintained at the above, and the pH of the aqueous solution is adjusted to 5 or less or 11 or less by adding an acid or alkali substantially containing no precipitated ions, and then adjusted to pH 6 to 11 by adding a neutralizing agent containing aluminum. The main point is to repeat this step a plurality of times.

[0005] Furthermore, in Japanese Patent Application Laid-Open No. 6-285358, the sustained-release metal oxide hollow fine particles and a method for producing the same (hereinafter referred to as a third conventional technique) are disclosed in Japanese Patent Application Laid-Open No. H06-285358. The droplets are 0.1 to 500 μm droplets, and the droplets are sent to a high-temperature reactor in a gas-liquid mixed phase using a carrier gas, and the metal salts contained in the droplets are thermally decomposed inside the reactor. After producing metal oxide hollow fine particles having a porous and hollow structure by contacting the fine particles with one or more kinds of liquid substances under a condition in which the inside of the fine particles is at a lower pressure than the outside, It is characterized in that a liquid substance is contained inside the porous part and / or the hollow part of the fine particles.

[0006] In the invention of Japanese Patent Application Laid-Open No. 9-294929, the porous hollow particles and the method for producing the same (hereinafter referred to as a fourth prior art), the first continuous phase containing a solid component is contained in the first continuous phase. A primary emulsion in which a dispersed phase is dispersed, in which a gas is dissolved under pressure in the first continuous phase, as a second dispersed phase, which is further dispersed in a second continuous phase. A secondary emulsion is produced by the method, and the secondary emulsion is formed under normal pressure to form hollow particles.The solidified component is solidified, and then the first dispersed phase portion of the primary emulsion is removed. This is a method for producing particles.

[0007]

However, in the first prior art, in order to form an alumina hydrogel, it is necessary to regulate the pH to 6 to 11, and a pseudo-boehmite gel which forms a coarse aggregate is required. PH to obtain
It is necessary to grow crystals in the presence of sulfate groups under the conditions of 6 to 11, which involves pH control during the process and the process itself is extremely complicated.

[0008] In the second prior art, an acid or alkali is added to an aqueous slurry containing seed aluminum hydroxide to adjust the pH to 5 or less or 11 or more, and then a neutralizing agent containing aluminum is added. PH 6 ~
In this case, the pH is repeatedly adjusted to 11, and as in the first conventional technique, there is a problem that complicated pH control is involved. Furthermore, any of the prior arts is generally difficult to apply to the synthesis of a composite oxide powder because the precipitation pH varies depending on the element type.

In the third prior art using the spray pyrolysis method, if the size of the spray droplets is small (ｍ1 μm) in order to obtain a submicron powder, the efficiency of forming hollow metal oxide fine particles becomes poor. Conversely, efficient spraying conditions (sprayed droplets to 1
0 μm), it is necessary to reduce the metal salt concentration in the solution to obtain a submicron fine powder, which is similarly inefficient.

[0010] The fourth prior art also has the disadvantage that the number of steps is large and the process is complicated. Furthermore, there are many methods for synthesizing porous oxide powders in addition to the first to fourth prior arts, but there are problems that the processes are complicated and costly.

The present invention has been made to solve the above-mentioned problems of the prior art in a method for producing a porous oxide powder, and is intended to be simple and inexpensive.
An object of the present invention is to provide a method for producing a porous oxide powder having a high specific surface area.

In order to solve the above-mentioned problems, the present inventors have focused on an emulsion combustion method as a method for easily synthesizing submicron oxide powder at low cost. However, this method is a very excellent method for synthesizing an oxide powder, but a porous oxide having a high specific surface area has not been synthesized so far. For example, the hollow alumina powder synthesized by emulsion combustion also has a specific surface area of about 50 m ² / g,
It was smaller than alumina synthesized by the solution method.

The inventors have further studied and found that, when producing oxide powder by the emulsion combustion method, if the progress of the oxide reaction is made insufficient by controlling the combustion conditions, etc. The surface of the crystal grains does not completely oxidize and bind, leaving hydroxyl groups, undecomposed metal salts, etc. locally on the surface of the crystal grains, and a water treatment process for bringing such powders into contact with a solution containing water. The present inventors have newly found that the bonding of the portion weakly bonded by the residual hydroxyl group or the like is broken when a porous structure is formed on the powder surface, thereby completing the present invention.

[0014]

According to the present invention, there is provided a method for producing a porous oxide powder, comprising the steps of: mixing an aqueous solution in which a metal salt is dissolved with an organic solvent and a dispersant to form a w / o emulsion; A step of spraying and burning the / o type emulsion to synthesize oxide powder; and a step of contacting the oxide powder synthesized in the step with a solution containing water.

The oxide powder synthesized by spraying and firing the above emulsion is preferably hollow. The structural change of the particles due to the process of contacting the powder synthesized by emulsion combustion with a solution containing water (hereinafter referred to as water treatment) occurs on the particle surface. Therefore, the effect of the water treatment is small for solid particles in which the ratio of the surface to the particle volume is small. On the other hand, the hollow particles synthesized by emulsion combustion have a very small thickness of the surface shell, so that the ratio of the surface to the particle volume is very large, and the effect of the water treatment is increased.

The composition of the oxide powder to be synthesized is not particularly limited. For example, a simple oxide such as alumina, titania, ceria, and zirconia may be used, or a composite oxide such as spinel in which one or more elements are added to these elements may be used.

The type of metal salt used as a raw material is not limited. Any water-soluble metal salt such as metal nitrate and metal acetate may be used. The metal salt concentration of the aqueous metal salt solution is not limited. An appropriate concentration may be set according to the solubility of the metal salt. However, if the concentration is extremely low, the efficiency of powder synthesis becomes poor, and the advantage of the present process of low cost is impaired, which is not desirable.

The type of the organic solvent used is not limited.
Like hexane, octane, kerosene, gasoline, etc.
Any organic solvent that can produce an aqueous solution and a w / o emulsion may be used. The type and amount of the dispersant used are not limited. Any of a cationic surfactant, an anionic surfactant, and a nonionic surfactant may be used, and an aqueous solution,
What is necessary is just to change the kind and addition amount of a dispersing agent according to the kind of organic solvent and the required water droplet diameter.

The mixing ratio of the aqueous solution component and the organic solvent component to be mixed when preparing the emulsion is not particularly limited. However, when the ratio of the water to be mixed exceeds 70%, the disperse phase of the emulsion and the dispersing medium may be inverted. w /
In order to obtain an o-type emulsion stably, the water ratio should be 7
0% or less is desirable.

The diameter of the water droplets in the emulsion can be arbitrarily controlled by the type and amount of the dispersant, and is not particularly limited. However, considering that hollow particles are more desirable, the water droplet diameter is desirably 100 nm or more. on the other hand,
If the water droplet diameter is larger than 10 μm, the reaction field is too large, so that a temperature distribution is easily formed during the combustion of the emulsion, and the combustion becomes unstable. Therefore, the water droplet diameter is desirably 10 μm or less.

The combustion temperature can be controlled by the type and mixing ratio of the organic solvent in the emulsion, the flow rate of the emulsion, the amount of supplied oxygen, and the like. The combustion temperature is not particularly limited.
00-1000 degreeC is desirable. If the temperature is lower than 600 ° C., the combustion temperature is too low, and the organic solvent may not completely burn. On the other hand, when the temperature exceeds 1000 ° C., the oxidation reaction of the metal salt may proceed completely, and the effect of the water treatment may be lost. The combustion atmosphere is not particularly limited, but if oxygen is not sufficient, carbon components in the organic solvent may remain due to incomplete combustion. Therefore, it is desirable to supply oxygen (air) to such an extent that the organic solvent in the emulsion can be completely burned.

The oxidation reaction of the metal salt can be controlled by changing the temperature at the time of emulsion combustion synthesis. On the other hand, even when the reaction temperature cannot be changed due to the required powder composition, powder shape, and the like, the progress of the oxidation reaction can be controlled by heat-treating the synthetic powder in an electric furnace. The heat treatment temperature, heat treatment time, heat treatment atmosphere and the like in this case are not particularly limited.

The type of solution used for water treatment is not particularly limited, except that it contains water. Neutral water may be used, or an acidic solution such as nitric acid or hydrochloric acid, or an alkaline solution such as aqueous ammonia or aqueous sodium hydroxide may be used. Further, a solution obtained by mixing these solutions with a water-soluble organic solvent such as ethanol at an arbitrary ratio may be used. Further, the water treatment temperature and the water treatment time are not particularly limited. The state of the surface structure can be controlled by changing the type of the solution, the temperature of the water treatment, and the time of the water treatment. The method of contacting with a solution containing water is not particularly limited. For example, the synthesized oxide powder may be mixed with a solution containing water, or may be contacted with a sprayed solution.

[0024]

The emulsion combustion method heats water droplets containing metal salts by burning an organic solvent in a w / o emulsion.
This is a short-time process in which water evaporation and metal oxidation occur instantaneously. Oxide particles are thought to be synthesized by the following mechanisms: 1) water evaporation on the surface of water droplets, 2) shrinkage of water droplets and nucleation of crystallites on the surface of water droplets, 3) growth and sintering of crystallites that have formed nuclei. . At this time, there are cases where the synthetic particles become solid particles and cases where the synthetic particles become hollow particles depending on the rate of water droplet shrinkage and crystallite sintering.

Depending on the composition of the synthetic powder, the type of the organic solvent, and the combustion conditions, under conditions where the oxidation reaction proceeds sufficiently, oxide crystallites of several tens of nm or less that have nucleated and grown on the surface of the water droplets are completely removed. And one oxide particle is synthesized.
On the other hand, when the oxidation reaction does not proceed sufficiently, individual crystallite surfaces are not completely oxidized and bonded, and hydroxyl groups, undecomposed metal salts, etc. may remain locally on the crystallite surface. it is conceivable that. In the latter case, it is considered that when the water treatment is performed, the weakly bonded portion is broken by the residual hydroxyl group and the like, and a porous structure is formed on the particle surface.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (Example 1) 0.1 to 2 mo prepared by dissolving commercially available aluminum nitrate in ion-exchanged water.
The aqueous phase was 1 / L aluminum nitrate aqueous solution. Commercial kerosene was used as the organic solvent. As the dispersant, sun soft No. manufactured by Taiyo Kagaku Co., Ltd. 818H was used. The addition amount was 5 to 10 wt% with respect to kerosene. The kerosene containing the dispersant was used as an oil phase. The aqueous phase and the oil phase were mixed so that the aqueous phase / oil phase = 50-70 / 50-30 (vol%). Using a homogenizer, the mixed solution was subjected to 1000-2.
The mixture was stirred for 5 to 30 minutes at a rotation speed of 0000 rpm, and w / o
A type emulsion was obtained. From the result of observation with an optical microscope, the diameter of the water droplet in the emulsion was about 1 to 2 μm.

The prepared mixed emulsion was sprayed and burned using an emulsion combustion reactor to synthesize an oxide powder. In the synthesis, the sprayed emulsion burns completely,
In addition, the process was performed while controlling the flow rate of the emulsion, the amount of air (the amount of oxygen), and the like so that the flame temperature became a constant temperature of 600 to 800 ° C. The obtained powder was collected by a bag filter installed at the rear of the reaction tube. As a result of TEM observation, the alumina powder synthesized under these conditions was hollow particles having a surface shell thickness of 10 to 20 nm.

1 to 10 g of the synthetic powder was added to 1 part of ion-exchanged water.
0-1000 cc. The obtained suspension was stirred at room temperature for 1 to 240 minutes using a magnetic stirrer. The suspension after stirring was filtered and further washed several times with ion-exchanged water. Dry the powder remaining on the filter paper,
Disintegrate, shape observation (SEM) and specific surface measurement (BE)
T) was performed. The SEM photograph in FIG. 1 is a photograph showing the particle structure obtained after the water treatment in Example 1.

Comparative Example 1 A powder synthesized by emulsion combustion was prepared in the same manner as in Example 1 except that the water treatment was not performed. The same evaluation as in Example 1 was performed on the powder of Comparative Example 1. The SEM photograph in FIG. 2 is a photograph showing the particle structure of the emulsion combustion synthetic powder before the water treatment.

(Test Results) As is clear from the photograph of FIG. 2, it is understood that the particle surface is very smooth. The specific surface area of the powder was 46 m ² / g. This value is based on the assumption that the surface shell is smooth and nitrogen adsorption occurs on both sides (inside and outside) of the surface shell when measuring the specific surface area, the amount of aluminum ion in one water droplet, the particle size of the synthetic powder, the surface shell thickness, It almost corresponded to the specific surface area calculated from the density. Therefore, the particle surface was considered to be smooth from the viewpoint of the specific surface area. In contrast, in the water-treated powder of Example 1, it can be seen that the smooth surface structure collapsed as shown in FIG. 1 and an uneven structure on the order of several tens of nm appeared.

(Examples 2 to 4) As Example 2, the same procedure as in Example 1 was carried out except that the powder obtained by the emulsion combustion synthesis in Example 1 was changed from a deionized water treatment solution to a 1 mol / L nitric acid aqueous solution. Water treatment and specific surface area measurement were performed in the process. In Example 3, the powder prepared by the emulsion combustion synthesis in Example 1 was prepared by changing the water treatment solution from ion-exchanged water to 1%.
Water treatment and specific surface area measurement were performed in the same process as in Example 1, except that the aqueous ammonia solution was changed to a mol / L aqueous ammonia solution. Example 4 Example 4 was the same as Example 1 except that the powder obtained by emulsion combustion synthesis in Example 1 was changed from a water treatment solution from ion-exchanged water to a mixed solution of ion-exchanged water and ethanol (1: 1 by volume). Water treatment and specific surface area measurement were performed in the same process.

(Examples 5 to 7, Comparative Example 2) The aqueous phase in the emulsion prepared in Example 1 was changed to 0.1 to 2 mol / L.
0.1 to 2 mol / L aluminum nitrate and sodium nitrate mixed aqueous solution (mixing ratio;
The molar ratio was changed to Na / Al = 1/99), and Na-containing alumina was synthesized under the same conditions as in Example 1. The synthetic powder was hollow particles having a very thin surface shell as in Example 1. Of the obtained synthetic powders, a synthetic powder not subjected to the subsequent water treatment was used as Comparative Example 2 to measure the specific surface area, and the synthetic powders obtained as Examples 5 to 7 were subjected to the following water treatments, respectively. The surface area was measured.

In Example 5, this synthetic powder was used in Example 1.
Water treatment was carried out using ion-exchanged water, and the specific surface area was measured. In Example 6, this synthetic powder was used in Example 2.
Water treatment was performed using a 1 mol / L nitric acid aqueous solution in the same manner as described above, and the specific surface area was measured. As Example 7, this synthetic powder was treated with a 1 mol / L aqueous ammonia solution as in Example 3, and the specific surface area was measured.

(Examples 8 to 10, Comparative Example 3) The aqueous phase in the emulsion prepared in Example 1 was adjusted to 0.1 to 2 mol /
0.1 to 2 mol / L from aluminum nitrate aqueous solution
Was changed to a mixed aqueous solution of aluminum nitrate and magnesium nitrate (mixing ratio: molar ratio: Mg / Al = 1/99).
The contained alumina was synthesized under the same conditions as in Example 1. In addition,
As in Example 1, the synthetic powder was hollow particles having a very thin surface shell. A part of the obtained synthetic powder was used as Comparative Example 3 to measure the specific surface area. The remaining synthetic powder was subjected to the following water treatment, and the specific surface area was measured as Examples 8 to 10.

That is, as Example 8, the obtained synthetic powder was treated with ion-exchanged water as in Example 1, and the specific surface area was measured. As Example 9, the obtained synthetic powder was treated with a 1 mol / L aqueous nitric acid solution as in Example 2, and the specific surface area was measured. As Example 10, the obtained synthetic powder was prepared in the same manner as in Example 3.
Water treatment was performed using a 1 mol / L aqueous ammonia solution, and the specific surface area was measured. Table 1 collectively shows the specific surface areas measured in the above Examples 1 to 10 and Comparative Examples 1 to 3.

[0036]

[Table 1]

As shown in Table 1, the specific surface areas of the powders after water treatment of Examples 1 to 10 which are examples of the present invention are the specific surface areas of the powders before water treatment of Comparative Examples 1 to 3 which are comparative examples. It can be seen that it is dramatically improved as compared with. Examples 2 to 10
When the powder was observed by SEM, the surface structure was broken as in Example 1, although the degree varied depending on the powder composition and water treatment conditions. Further, as in Example 4, in the mixed solution of water and alcohol, since the chance of contact between the powder and water is small, the collapse of the surface structure is small, and the specific surface area is increased as compared with the case of treating only with water. Is small.
This means that the surface structure is controlled by changing the mixing ratio of water and alcohol, and an arbitrary specific surface area can be obtained between the untreated product and the water-treated product. Further, although the cause is not clear, the specific surface area was larger in the order of the aqueous ammonia treated product> the ion-exchanged water treated product> the nitric acid aqueous solution treated product in the range of the examples. This means that the specific surface area can be controlled by the pH of the solution used for the water treatment.

(Examples 11 to 14) The alumina powder synthesized in Example 1 was subjected to a heat treatment in an atmosphere at a temperature of 700 to 1000 ° C. for 4 hours using an electric furnace. The powder after the heat treatment was subjected to water treatment using ion-exchanged water and the specific surface area was measured as in the example. Table 2 shows the measured specific surface areas. Note that the heat treatment temperature in each example is the same as that in Example 1.
1, 700 ° C., Example 12 at 800 ° C., Example 13
In Example 14, the temperature was 900 ° C., and in Example 14, the temperature was 1000 ° C.

[0039]

[Table 2]

As shown in Table 2, it is found that the specific surface area of any of the powders after the heat treatment is improved by the water treatment. In addition, the specific surface area of the powder after water treatment decreases as the heat treatment temperature increases. When the powders of Examples 11 to 14 were observed by SEM, collapse of the surface structure was observed as in Example 1, although the degree varied depending on the heat treatment conditions. However, as the heat treatment temperature increased, the collapse of the surface structure became smaller and tended to approach the state of Comparative Example 1 (FIG. 2). It is considered that this is because the oxidation reaction partially progressed by the heat treatment, the hydroxyl groups, undecomposed metal salts, and the like remaining on the crystallite surface were reduced, and the surface structure was less likely to collapse. This result means that the surface structure can be controlled also by heat treatment after synthesis, and any arbitrary specific surface area can be obtained between the untreated product and the water-treated product.

[0041]

According to the method for producing a porous oxide powder of the present invention, a step of mixing an aqueous solution in which a metal salt is dissolved with an organic solvent and a dispersant to form a w / o emulsion,
In the step of spraying and burning the / o type emulsion to synthesize the oxide powder, by appropriately selecting the spraying and burning conditions of the emulsion, oxide particles in which the progress of the oxidation reaction is insufficient can be obtained. The step of contacting the oxide particles synthesized in this step with a solution containing water breaks the bonds of hydroxyl groups and undecomposed metal salts remaining on the crystallite surface of the synthesized particles, and a porous structure appears on the powder surface As a result, the specific surface area is dramatically improved. Further, the specific surface area can be controlled by the combustion conditions, the type of the aqueous solution, and the like.

[Brief description of the drawings]

FIG. 1 is an SEM photograph showing the particle structure of a porous oxide particle obtained by the method of the present invention.

FIG. 2 is an SEM photograph showing a particle structure of an oxide particle which is not subjected to a water treatment according to the method of the present invention.

────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] August 18, 1998 (1998.8.1)
8)

[Procedure amendment 1]

[Document name to be amended] Drawing

[Correction target item name] Fig. 1

[Correction method] Change

[Correction contents]

FIG.

[Procedure amendment 2]

[Document name to be amended] Drawing

[Correction target item name] Figure 2

[Correction method] Change

[Correction contents]

FIG. 2

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Nobuo Kamiya Inventor F-term (reference) in Toyota Central Research Institute, Inc. 41 No. 41, Chukumi Yokomichi, Nagakute-cho, Aichi-gun, Aichi (reference) 4G042 DA01 DB09 DC03 DE14 4G065 AA01 AA06 AA07 AB03X AB32X BA07 BB03 BB06 CA04 CA21 DA04 FA01 4G076 AA02 AB07 BA06 CA12 DA01 DA23 DA25

Claims (1)

[Claims] 1. A step of mixing an aqueous solution in which a metal salt is dissolved with an organic solvent and a dispersant to form a w / o emulsion, and a step of spraying and burning the w / o emulsion to synthesize an oxide powder. Contacting the oxide powder synthesized in the above step with a solution containing water.