PROCESS FOR PRODUCING LITHIUM ALUMINATE POWDER HAVING LARGE SPECIFIC SURFACE AREA

Abstract: A lithium aluminate powder having a large specific surface area can be produced by contacting a raw material powder of lithium aluminate with water singly or in the presence of an organic solvent having one or more hydroxyl groups to form a hydrate, followed by dehydration with heating.

Description: BACKGROUND OF THE INVENTION This invention relates to a method for producing lithium aluminate (LiAlO₂) powder the specific surface area of which having been enlarged by increasing its porosity. Lithium aluminate (LiAlO₂) is resistant to the corrosive action of alkali carbonate, and it is used as a carrier of modified catalyst for internal modification type molten carbonate fuel cells. According to prior processes, lithium aluminate is produced by heat-treating a mixture of lithium salt and aluminum salt, a mixture of lithium salt and aluminum oxide or a mixture of lithium hydroxide and aluminum oxide. Since all these prior processes work with solid substances as starting material, the starting material must be pulverized and mixed with greatest care in order to prevent the product from contamination of unreacted material as an impurity. Further, since these processes require a relatively high reaction temperature, the resulting powdery product generally has a small specific surface area. On the other hand, according to a prior invention disclosed in Japanese Patent Application Kokai (Laid-Open) No. 53-140300, β-lithium aluminate is produced by precipitating Li₂O·Al₂O₃·nH₂O from a solution containing lithium ion and ionic or elementary aluminum and heat-treating the precipitate at a temperature of 200°C to 800°C. Although the above-mentioned problem can be avoided because the reaction of this process is carried out in a solution, this process requires regulation of the molar ratio of lithium to aluminum and thorough washing of the precipitate to prevent the contamination of ionic impurities. In that specification, specific surface area of the lithium aluminate obtained by that invention is not mentioned. The above-mentioned prior techniques are disadvantageous in the point of contamination of the product by impurities. In order to avoid this contamination, a number of additional steps are considered necessary. Further, in these production processes, no sufficient measure is taken for obtaining a product having a large specific surface area. SUMMARY OF THE INVENTION An object of this invention consists in overcoming the above-mentioned disadvantages of prior techniques by providing a process for producing a lithium aluminate powder having a large specific surface area and well grown pores. This invention provides a process for producing a lithium aluminate powder having a large specific surface area which comprises contacting a γ-lithium aluminate powder either singly or in the presence of an organic solvent having at least one hydroxyl group with water to prepare a hydrate, followed by heating and dehydrating the hydrate. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a transmission electron microscopic photograph illustrating the particle structure of lithium aluminate crystal particle which has been provided with a large specific surface area and made porous; Figure 2 and Figure 3 are scanning electron microscopic photographs illustrating the particle structures of starting γ-LiAlO₂ powder and its hydrated crystals, respectively; Figure 4 and Figure 5 are graphs illustrating specific surface area and pore volume distribution, respectively, of a hydrate heated at various temperatures; Figure 6 and Figure 7 are graphs illustrating specific surface area and pore volume distribution, respectively, of a hydrate heated at 650°C for various periods of time; and Figure 8 and Figure 9 are graphs illustrating the hydration velocity of starting powders. DESCRIPTION OF THE PREFERRED EMBODIMENTS According to this invention, a lithium aluminate powder having large specific surface area is produced in the following manner. (1) A process for producing a lithium aluminate powder having a large specific surface area, which comprises contacting a γ-lithium aluminate powder with water to convert it to a hydrate, followed by heating the hydrate to decompose the hydrate and remove the combined water. (2) A process for producing a lithium aluminate powder having a large specific surface area, which comprises contacting a γ-lithium aluminate powder with a mixture comprising water and an organic solvent having at least one hydroxyl group such as an alcohol to convert the lithium aluminate powder to its hydrate, followed by heating the hydrate to decompose the hydrate and remove the combined water. (3) A process for producing a lithium aluminate powder having a large specific surface area, which comprises contacting a γ-lithium aluminate powder with a mixture comprising water and an organic solvent having at least one hydroxyl group while applying a shearing force to the powder to convert the powder to its hydrate, followed by heating the hydrate to decompose the hydrate and remove the combined water. (4) A process for producing a lithium aluminate powder, which comprises contacting a γ-lithium aluminate powder with water or with a mixture comprising water and an organic solvent having at least one hydroxyl group to convert the powder to a hydrate substantially represented by the following formula: Li₂Al₂O₄·7H₂O followed by heating the hydrate to decompose the hydrate and remove the combined water to make the powder porous. In the above-mentioned processes, the heating temperature is preferably 300°C to 1,000°C. For example, when heated at 650° to 800°C, it takes about 1 hour to decompose the bonded water compared to about 100 hours when standing at 10° to 50°C. The specific surface area of the lithium aluminate can be measured by, for example, a conventionally used nitrogen adsorption method. Preferably the specific surface area is 30 m²/g or more, more preferably 40 m²/g or more, for example, 40 to 100 m²/g. The organic solvent having at least one hydroxyl group includes, for example, lower alcohols such as methanol, ethanol, propanol, etc. The shearing force (or shear strength) can be applied by using conventionally used stirrers, mills, etc., under conventionally used conditions. In the above reactions, when contacting with water, one mole of LiAlO₂ incorporates about 3.5 moles of crystal water into its structure to form a hydrate represented by Li₂Al₂O₄·7H₂O as follows: 2LiAlO₂ + 7H₂O → Li₂Al₂O₄·7H₂O (4) Since this hydrate itself does not have a large specific surface area and forms to crystals somewhat larger than the original LiAlO₂ particles, the specific surface area tends to decrease in this reaction. However, if the hydrate crystal is heated, the crystal water in the crystal structure is eliminated, and the crystal is modified to a LiAlO₂ crystal having a greater specific surface area, preferably 2 times or more, than that of the original LiAlO₂ crystal. This is for the reason that the elimination of crystal water leaves minute pores in the structure and the pores thus formed are retained in the product. Next, this invention will be explained by way of the following examples. Example 1 One hundred grams of a starting γ-LiAlO₂ having a mean specific surface area of 22 m²/g and a mean particle diameter of about 0.1 µm was thrown into 350 g of distilled water and stirred at room temperature for 750 hours to prepare a hydrate. The hydrate thus obtained was separated from excessive water by filtration with suction and washed with ethanol. Figure 2 is a scanning microscopic photograph (SEM) of the starting γ-LiAlO₂ used in this example, and Figure 3 is a photograph of the hydrate crystal obtained by the above-mentioned process. While the starting powder was nearly spherical, the hydrate crystal had a hexagonal flaky shape. By X-ray diffraction, the hydrate was identified as Li₂Al₂O₄·yH₂O, and a thermogravimetric analysis revealed that the number of crystal water (y) was approximately equal to 7. Its crystal system was a hexagonal system, which corresponded to the crystal form observable in Figure 3. Further, the hydrate Li₂Al₂O₄·yH₂O was heat-treated at 650°C, 800°C and 900°C for one hour. In Figure 4, Curve 2 illustrates the specific surface areas of the heated products determined by N₂ adsorption method. The starting γ-LiAlO₂ had a specific surface area of 22 m²/g. However all the heated products had a larger specific surface area than above-mentioned, regardless of the heating temperature. Thus, when the heating temperature was 650°C, 800°C and 900°C, the specific surface area was increased to about 4 times, about 2.5 times and about 2 times, respectively, of the value of the starting material. Curve 3 is a comparative curve illustrating the specific surface area of the same starting γ-LiAlO₂ which had been heated as it was under the same conditions as above. Figure 5 illustrates the pore diameter distribution of heated product, wherein Curve 5 expresses that of a product heated at 650°C, Curve 6 is that of a product heated at 800°C, and Curve 7 is that of a product heated at 900°C. Curve 4 expresses the pore diameter distribution of starting γ-LiAlO₂. Compared with starting LiAlO₂, the products prepared by heat-treating the hydrate had evidently greater pore volumes. Figure 1 illustrates a transmitting electron microscopic photograph of a sample prepared by heating a hydrate at 650°C. Fine particles of 100-200 Å are observable in the particle, and pores of about 50 Å are formed between the particles. Example 2 In this example, the hydrate obtained in Example 1 was heated at 650°C for a period of 1, 3, 6 and 10 hours. In Figure 6, Curve 8 expresses the change in specific surface area as a function of heating time. The specific surface area decreases somewhat till the heating time reaches about 3 hours. After this time, the specific surface area does not change and keeps constant at about 60 m²/g. Curve 9 expresses the change in specific surface area observed when the starting γ-LiAlO₂ is heated under the same conditions as above. Figure 7 illustrates pore diameter distribution of the heated product, wherein Curve 5 denotes a heating at 650°C for one hour, Curve 10 denotes a heating at 650°C for 3 hours and Curve 11 denotes a heating at 650°C for 10 hours. Curve 4 expresses the pore diameter distribution of starting γ-LiAlO₂. In all these cases, pore volume is larger in heated product than in starting powder. Example 3 If the hydration is carried out with a large quantity of water, the quantity of by-product formed by hydrolysis contaminating the hydrate becomes not negligible. The present example is effectively applicable to such a case. For instance, 100 g of the same starting powder of γ-LiAlO₂ as in Example 1 (specific surface area 22 m²/g) was thrown into 300 cc of ethanol. While stirring the mixture with a stirrer, 96 g of distilled water was dropped into it over a period of 140 hours. This quantity of distilled water (96 g) corresponded to the quantity of crystal water in the hydrate represented by formula (4) formed from 100 g of γ-LiAlO₂. In Figure 8, Curve 13 expresses the hydration velocity under this condition, while Curve 12 expresses the hydration velocity under the condition of Example 1, wherein hydration rate (ordinate) is the ratio of actual quantity of crystal water to the theoretical quantity of crystal water. It is apparent from Figure 8 that the hydration velocity in this invention (Curve 13) is comparable to that of Curve 12. Since in this example a necessary minimum quantity of water was used and the alcohol exhibited an inhibitory effect on hydrolysis, no impurity (i.e. hydrolyzate) was found in the hydrate obtained by thermal aging for 100 hours followed by separation by filtration. Example 4 Herein is shown one example of hydration process successfully applicable to a case in which starting powder has a small specific surface area. In Figure 9, Curve 14 expresses the hydration velocity observed when a γ-LiAlO₂ having a specific surface area of 3 m²/g is hydrated under the same conditions as in Example 1. The velocity herein shown is much lower than that expressed by Curve 12 of Figure 8 (specific surface area 22 m²/g). The increase in specific surface area after heating and dehydration greatly varies depending on the hydration rate. If heating and dehydration are carried out in a state of low hydration rate, the increase in specific surface area is not high. In this example, 100 g of a starting γ-LiAlO₂ powder having a specific surface area of 3 m²/g and 350 g of distilled water were thrown into a porcelainous ball mill, and hydration was carried out while pulverizing the powder by rotating the mill. At this time, the hydration velocity increased as expressed by Curve 15 of Figure 9. A similar effect can be achieved by simultaneously carrying out hydration and pulverization not only with ball mill but also with mortar grinder, vibrating mill and the like. As the hydration progresses and a layer of hydrate is formed on the surface of starting powder, the hydration velocity decreases. The ultimate hydration rate is lower when the initial specific surface area is smaller. This example is effective for breaking the hydrate layer formed on the surface thereby promoting the contact between unhydrated material and water. Example 5 If the amount of water added to starting material is decreased and the starting material particles are left in a state of intimate mutual contact, the crystal of the hydrate grows while filling the interstices between starting material particles and combining the individual particles, whereby a rigid product is formed. Thus, in this example, 20 g of γ-LiAlO₂ was mixed with 19 g of distilled water and a powder of a substance making a large contact angle with molten salt, such as metal nitride and the like. The mixture was homogenized by means of a kneader, then the mixture was formed in a desired shape. After hydration and hardening had took place, it was heated and dehydrated. The formed product of lithium aluminate thus prepared contained a molten salt-repelling material in a state of immobilized dispersion, and therefore it is useful as a carrier for internally modified catalyst used in the fuel circuit of fuel cell in order to modify the directly fed hydrocarbon such as CH₄ or the like. According to this invention, starting material powder of lithium aluminate is contacted with water and heat-treated. By this simple procedure, specific surface area of lithium aluminate can be increased to 1.5 times or more, preferably 2 times or more, e.g. 2 to 4 times, and pores of about 50 Å can be formed. Accordingly, when the product of this invention is used as a carrier for internally modified catalyst placed in the fuel circuit of molten carbonate type fuel cell, it can exhibit a high modifying activity and a prolonged lifetime. Further, when it is used as a starting material for producing electrolyte plates of molten carbonate type fuel cells, the maintenance of performance of electrolytic solution can be improved owing to its capillary action.

Claims: A process for producing a lithium aluminate powder having a large specific surface area, which comprises contacting a γ-lithium aluminate powder with water to form a hydrate, followed by heating the hydrate to decompose the hydrate and remove the combined water. A process for producing a lithium aluminate powder having a large specific surface area, which comprises contacting a γ-lithium aluminate powder with water and a mixture comprising water and an organic solvent having at least one hydroxyl group to prepare a hydrate, followed by heating the hydrate to decompose the hydrate and remove the combined water. A process for producing a lithium aluminate powder having a large specific surface area, which comprises contacting a γ-lithium aluminate powder with a mixture comprising water and an organic solvent having at least one hydroxyl group, hydrating said powder while applying a shearing force to the powder, and heating the resulting hydrate to decompose the hydrate and remove the combined water. A process for producing a lithium aluminate powder having a large specific surface area, which comprises contacting a γ-lithium aluminate powder with water or with a mixture comprising water and an organic solvent having at least one hydroxyl group to convert the powder to a hydrate substantially represented by the following formula: Li₂Al₂O₄·7H₂O followed by heating the hydrate to decompose the hydrate and remove the combined water. A process according to any of the Claims 1 to 4, wherein the decomposition of the hydrate and removal of combined water from the hydrate of γ-lithium aluminate are carried out at a temperature of 300°C to 1,000°C.

Inventors: HIGASHIYAMA, KAZUTOSHI, ITO, SHINGO C/O GIJUTSU KENKYUSHO, MURAKAMI, TADAYOSHI, YOSHIOKA, SUSUMU
Applicants: HITACHI, LTD., THE TOKYO ELECTRIC POWER CO., INC.
Application year: 1989
Main classification: C01F7
IPC codes: H01M8, C01F7