WO1990003331A1

WO1990003331A1 - High surface area alpha-alumina and supercritical fluid synthesis thereof

Info

Publication number: WO1990003331A1
Application number: PCT/US1989/003834
Authority: WO
Inventors: George P. Kinstle; James H. Heasley
Original assignee: Ferro Corporation
Priority date: 1988-09-30
Filing date: 1989-09-26
Publication date: 1990-04-05
Also published as: AU4316589A

Abstract

High surface area α-alumina free of transitional aluminas and having a BET surface area greater in the range of about 30 to at least about 100 m2/g, and a method of producing same comprising the step of heating an α-alumina precursor, such as an aluminum hydroxide or a hydrated alumina, with a solvent, such as methanol, under conditions of temperature and pressure that are supercritical for the solvent. The resultant surface area may be controlled by the addition of water to the starting materials.

Description

HIGH SURFACE AREA ALPHA-ALUMINA AND SUPERCRITICAL FLUID

SYNTHESIS THEREOF

Background of the Invention

1. Field of the Invention

This invention relates to alpha (α-) alumina and methods of synthesizing same and, more particularly, to high surface area α-alumina essentially free of

transitional aluminas and supercritical fluid synthesis thereof.

2. Description of the related art

High surface area aluminas are useful and desirable in the industry as catalysts or catalyst carriers. The efficiency of the catalytic action of the alumina or the amount of catalyst an alumina can carry increase with increasing surface area.

Alpha alumina, or corundum, has typically been prepared from various hydrated aluminas or aluminum hydroxides;

U. S. Patent No. 2,642,337 teaches conversion of alumina trihydrate or alumina monohydrate to alpha alumina by heating in the presence of steam at pressures below about 2500 p.s.i. and temperatures of about

400ºC.

Inoue et al. describe a process for producing wide pore aluminas by heating gibbsite in an alcohol to obtain boehmite or mixtures of boehmite, gibbsite and X*-alumina having BET surface areas from 11 to 62 m²/g, depending upon the carbon number of the alcohol.

Subsequent calcination at 800ºC is reported to yield vide pore aluminas having BET surface areas ranging from 41 to 140 m²/g.

Kingsley et al., teach a method of synthesizing fine particle α-alumina by the combustion of an hydrated aluminum nitrate-urea mixture at about 500ºC. The resultant product was characterized as having a BET surface area of 8.30 m²/g.

α-alumina has also been obtained by the calcination of Bayer process calcined aluminas. Slightly calcined products containing fine particle α-alumina exhibit surface areas reported as high as 50 m²/g; these slightly calcined products contain a considerable portion of transition aluminas. Cleaner or purer α- alumina (80-100%) products have been reported to exhibit surface areas of 0.5 - 20 m²/g.

α-alumina catalyst carriers produced from gibbsite have been described as having surface areas from 0.01 - about 50 m²/g; other α-alumina carriers synthesized from pseudoboehmite have been reported as having "greater" but unspecified surface areas.

None of the art discloses a clean preparation consisting essentially of α-alumina free of transitional aluminas and having a surface area of at least about 30 m²/g.

Summary of the Invention

The invention provides a method for producing high surface area α-alumina essentially free of transitional aluminas and having a BET surface area from about 30 m²/g to at least about 100 m²/g. The method comprises the step of heating an α-alumina precursor with a solvent under conditions of temperature and pressure that are supercritical for the solvent. Supercritical conditions may be achieved in a closed system or in a pipe reactor for continuous processing. The precursor may be an aluminum hydroxide or a hydrated alumina; the solvent may be an alcohol. The resultant surface area may be controlled by the use of a solvent/ water mixture.

As used in the specification and claims, α-alumina refers to the stable, anhydrous form of AI₂O₃.

Supercritical or supercritical conditions refer to pressures and temperatures above those commonly

reported as being critical for a given solvent.

Transitional aluminas refer to crystalline forms of alumina other than a , as are known in the art.

BET surface area refers to the surface area determined according to the Brunauer-Emmett-Teller method. High surface area refers to a surface area in the range of 30 to at least 100 m²/g.

Accordingly, the invention provides a method of preparing high surface area α-alumina essentially free of transitional aluminas comprising the step of heating an α-alumina precursor in contact with a solvent under solvent supercritical conditions of temperature and pressure. The solvent is an alcohol, preferably an alcohol having 1 - 5 carbon atoms, and most preferably, methanol. The precursor is an aluminum hydroxide or a hydrated alumina, preferably a trihydrated alumina (Al₂O_O ·3H₂O), and most preferably one having a surface area of about 5 m²/g to about 40 m²/g.

Ideally, the invention provides a method of preparing high surface area α-alumina essentially free of transitional aluminas comprising the step of heating a low surface area trihydrated alumina having a surface area of at least about 5 m²/g with methanol at a supercritical temperature and pressure for a period of time effective to convert the hydrated alumina to α- alumina with a surface area of at least about 50 m²/g; the process may be carried out in a closed system, or in a pipe reactor to provide for a continuous process.

Water may be included in the method of the invention in a quantity effective to control the resultant surface area of the α-alumina.

The invention further provides an α-alumina catalyst or catalyst carrier consisting essentially of α-alumina and having a surface area from about 30 m²/g to at least about 100 m²/g. Additionally, the invention provides an alumina preparation consisting essentially of α-alumina free of transitional aluminas and having a surface area from about 30 to at least about 100 m²/g.

The process of the invention involves stirring an alcoholic slurry of Al₂O₃·3H₂O at temperatures at or above 350ºC in a closed system for a period of time effective for conversion of the Al₂O₃·3H₂O to high surface area alumina essentially free of transitional aluminas. A closed system is provided by, e.g., an autoclave having a volume proportional to that of the alcoholic slurry so that at synthesizing temperature, the pressure is all due to the volatized alcohol, and both the temperature and the pressure are supercritical for that alcohol. In the case of a methanolic slurry in a, e.g., one gallon autoclave, a temperature of 350ºC will result in a pressure of about 3400 p.s.i. For methanol, P_c = 1174.5 p.s.i., and T_c = 239.4ºC. After a suitable period of time, the solvent can be vented off at the operating temperature or the system can be cooled to ambient temperature and filtered. In either case, the product is a high surface area (30 - >100 m²/g) α- alumina essentially free of transitional aluminas.

The addition of water to the alcoholic slurry starting material has been discovered to affect the final surface area of the product of the inventive method.

Various starting materials, end products and synthesis conditions are listed in Table I. The experimental conditions under which the data shown in

Table I were generated generally as described, wherein a slurry of about 50 grams of starting material in about a liter of solvent was heated with stirring at the noted temperature in a one gallon autoclave. After about two hours, the autoclave was either vented to cause the volatiles to escape or allowed to cool to ambient temperature, followed by filtration of the product. The method of solvent removal did not appear to affect the surface area of the final product, only the amount of trace impurities.

*The starting material was sold as a trihydrated Alumina; X-ray analysis showed it to be Aluminum hydroxide.

**Synthesis was aborted due to pressure build up in the system.

*** X-ray data indicated a mixture of AI₂O₃ and Al2θ3.xH₂O phases; the loss of weight upon heating to 1000'C indicated Al₂O₃·1.5H₂O

Reaction a in Table I shows that it is possible to convert trihydrated alumina having a surface .area of 10-11 m²/g to α-alumina having a surface area of 50 m²/g. Comparison of reaction a with reactions b and c appears to indicate that the surface area of the final product is related to the surface area of the starting material. While not wishing to be bound by any theory, or by the possibility that reaction parameters, such as, e.g., the size of the reaction vessel, may be

responsible for such data, a minimum surface area of starting material may be a requisite of the method. It is contemplated, however, that starting materials having surface areas below about 5 m²/g are within the scope of the invention as long as their treatment with a solvent under supercritical conditions results in a high surface area α-alumina. Likewise comparison of reactions a, g, and h would indicate that a temperature in excess of 330ºC is required to convert a methanolic slurry of trihydrated Al₂O₃ to α-alumina. Again, the appearance of a need for a specific minimum temperature may be due simply to such a feature as the size of the starting vessel. It is therefore contemplated that temperatures below 330ºC may be employed in the process as long as the combination of temperature and pressure is such as to create supercritical conditions for a given solvent. Notwithstanding the indication that arises from a comparison of reactions a, d, and e, limiting the process of the invention to methanol is not intended, as these particular data may be due to reaction vessel capacity. Therefore, alcohols having a carbon number of 1 to 5 are contemplated as equivalents, as are other volatile solvents capable of being subjected to

supercritical conditions within a closed systems.

Still other features and advantages and a full understanding of the invention will become apparent to those skilled in the art from the drawing and the following description of the preferred embodiments.

Brief description of the drawing

Figure 1 is an X-Ray Spectrum of α-alumina prepared by the method of the invention.

Description of the preferred embodiments The preferred embodiment and best modes of the invention are illustrated and described by the following specific examples.

Example 1

Fifty (50) grams of Alcoa Hydral-705 brand

Al₂O₃.3H₂O (available from Alcoa Chemicals, Bauxite, Arkansas) having a surface area of about 10 m²/g was slurried with 1200 ml laboratory grade methanol and loaded into an Autoclave Engineers 1 gallon autoclave. The slurry was heated with stirring to 350'C and 3400 p.s.i. It was kept at this temperature and pressure for 2 hours, after which time, the volatiles were vented. During venting, the temperature fell to about 275ºC. When venting was complete, nitrogen gas was flowed through the autoclave as it cooled to ambient

temperature. The product was a white powder having a noticeably large volume than the Al₂O₃.3H₂O precursor. X-ray analysis of the product showed it to be a clean α- alumina essentially free of transitional aluminas. BET surface analysis of the product gave a value of 50 m²/g compared to the low surface area of the precursor.

Heating a portion of the product to about 1000ºC resulted in a 1-2% loss in weight.

The analytical data for the product is shown in

Example 2

The synthesis of α-alumina as described in Example 1 was carried out with Alcan SF-11 brand Al₂O₃.3H₂O (available from Alcan Chemicals, Cleveland, Ohio).

Alcan SF-11 has a surface area of about 11 m²/g.

Otherwise, all the conditions were the same as described in Example 1. Likewise, the product was comparable in surface area with a BET value of 50 m²/g.

The X-ray spectrum of the α-alumina is shown in Figure 1.

The anal tical data for the product was as follows

Example 3

The synthesis as described in Example 1 was repeated, except that rather then venting the volatiles , the system was cooled to ambient temperature, the methanol filtered off and the product washed and filtered with additional methanol, followed by water. The product was then dried at 120'C. The analytical data as shown in Example 1 was unchanged except that the level of some of the impurities was lowered, most notably that of Na₂O from 0.50 to 0.25%.

Example 4

A synthesis was carried out as described in Example 1 except that the starting material was Alcan UF35E brand ^A1(OH)³ (available from BA Chemicals Ltd.,

Buckinghamshire, England) (the starting material was labeled as a trihydrated alumina; X-ray analysis showed it to be aluminum hydroxide). UF35E has a surface area of 35 m²/g. X-ray analysis of the product showed it to be α-alumina essentially free of transitional aluminas. The peaks in the X-ray were very broad, as was not unexpected for this product which gave a BET surface area of 96 m²/g.

Example 5

A synthesis was carried out as described in Example 1 except that the starting slurry was prepared with 1000 ml methanol and 200 ml water. After the system was vented and cooled, the product did not appear as

voluminous as when only methanol was used. X-ray data showed the product to the clean α-alumina essentially free of transitional aluminas. The X-ray peaks were much sharper than those for the product of Example 1, indicating a larger crystal size. BET surface area analysis yielded a value of 1 m²/g.

Many modifications and variations of the invention will be apparent to those skilled in the art in light of the foregoing detailed disclosure. Therefore, it is to be understood that, within the scope of the appended claims, the invention can be practiced otherwise than as specifically described.

Claims

We claim: CLAIMS

1. A method of preparing high surface area α- alumina comprising the step of heating an α-alumina precursor in contact with a solvent under supercritical conditions of temperature and pressure for a period of time effective to convert the precursor to α-alumina essentially free of transitional aluminas and having a surface area from about 30 m²/g to at least about 100 m²/g.

2. The method of claim 1 wherein the step of heating is carried out in a closed system.

3. The method of claim 1 wherein the step of heating is carried out in a continuous pipe reactor.

4. The method of claim 1 wherein the solvent is selected from the group of alcohols having 1 - 5 carbon atoms .

5. The method of claim 1 wherein the solvent is methanol.

6. The method of claim 1 wherein the temperature is greater than about 350ºC.

7. The method of claim 1 wherein the precursor is an aluminum hydroxide or a hydrated alumina.

8. The method of claim 7 wherein the hydrated alumina is Al₂O₃·3H₂O having a surface area ranging upward from about 5 m²/g.

9. The method of claim 7 wherein the aluminum hydroxide has a surface area ranging upward from about 5 m²/g.

10. A method of preparing high surface area α- alumina comprising the step of heating a low surface area hydrated alumina or aluminum hydroxide having a surface area of about 5 m²/g to about 40 m²/g with methanol at a supercritical temperature and pressure for a period of time effective to convert the hydrated alumina or aluminum hydroxide to α-alumina with a surface area from about 30 m²/g to at least about 100 m²/g.

11. The method of claim 10 comprising the step of including water in the methanol in a quantity effective to control the surface area of the α-alumina.

12. The method of claim 10 wherein the hydrated alumina is Al₂O₃·3H₂O.

13. An α-alumina catalyst or catalyst carrier consisting essentially of α-alumina and having a surface area in the range from about 30 m²/g to at least about 100 m²/g.

14. The α-alumina catalyst or catalyst carrier of claim 14 having a surface area in the range from about 40 m²/g to about 100 m²/g.

15. An α-alumina preparation consisting

essentially of α-alumina free of transitional aluminas and having a surface area in the range from about 30 m²/g to at least about 100 m²/g.

16. The α-alumina of claim 15 having a surface area in the range from about 40 m²/g to about 100 m²/g.

17. An α-alumina prepared according to the method of claim 1.

18. An α-alumina prepared according to the method of claim 9.

19. An α-alumina prepared according to the method of claim 10.

20. An α-alumina prepared according to the method of claim 12.