WO1993017968A1

WO1993017968A1 - Preparation of stabilized alumina having enhanced resistance to loss of surface area at high temperatures

Info

Publication number: WO1993017968A1
Application number: PCT/US1993/002104
Authority: WO
Inventors: Lewis Bernard Decker, Jr.; Kevin J. Mclaughlin
Original assignee: Vista Chemical Company
Priority date: 1992-03-12
Filing date: 1993-03-10
Publication date: 1993-09-16
Also published as: EP0630356A4; CA2131795C; EP0630356A1; CA2131795A1

Abstract

A process for preparing stabilized alumina having increased surface area retention at high temperature in which a gel of a boehmite alumina which has been obtained by hydrothermally treating an aqueous mixture of a precursor beohmite alumina having a pH of from about 5 to about 9 for a period of time sufficient to convert the greater portion of the precursor boehmite alumina to a colloidal sol is subjected to working as, for example, by using a sufficient shearing force for a sufficient period of time to produce a worked boehmite alumina which has an increase in pore volume of at least about 30 percent and an increase of median pore radius of at least about 20 percent, a stabilizer being added to the boehmite alumina, the stabilizer being an oxide of a metal such as barium or a metal included in the lanthanide series of metals or a compound of such metals which converts to an oxide at elevated temperatures.

Description

PREPARATION OF STABILIZED ALUMINA HAVING ENHANCED RESISTANCE TO LOSS OF SURFACE AREA AT HIGH TEMPERATURES

BACKGROUND OF THE INVENTION

1. Field of the Invention

5 The present invention relates to a process for producing alumina which can be converted to catalyst supports exhibiting enhanced resistance to loss of surface area when subjected to high temperatures.

2. Description of the Background

One of the key requirements of a catalyst support or substrate such as alumina

10 (Al₂O₃) is high surface area. Increased surface area allows for deposition of the catalytically active species, enhances reactivity between the catalytically active species and the reactants and, in general, makes for a more efficient catalyst support. In the case of catalyst supports of alumina used in catalytic converters for automobiles, i.e. autocatalyst supports, high surface area is particularly desirable because of short

15 residence times between reactants and catalytic species, the desire to minimize the size of the catalytic converter and hence the need for a high efficiency catalyst.

A particular problem with autocatalyst supports involves the high temperatures to which the supports are subjected. High temperatures deleteriously effect the structural integrity of the catalyst support resulting in a loss of surface area. In

20 effect, the elevated temperatures cause the catalyst to collapse on itself.

It is known that stabilizers such as oxides of barium and the lanthanide series of elements can stabilize autocatalysts in the sense that the loss of structural integrity of the support is retarded. In particular, oxides of barium, lanthanum or other lanthanide elements have been used in alumina based autocatalyst supports as heat

25 stabilizers.

^ SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a process for producing stabilized alumina which can be used in catalyst supports and other structural substrates requiring high surface area. Still another object of the present invention is to provide a catalyst support exhibiting enhanced resistance to structural degradation at high temperatures.

The above and other objects of the present invention will become apparent from the description given herein and the appended claims.

According to the process of the present invention, a stabilized alumina of enhanced resistance to high temperature surface area loss is prepared by forming a gel of a boehmite alumina, the boehmite alumina being obtained by hydrothermally treating an aqueous mixture of a precursor boehmite alumina having a pH of from about 5 to about 9 for a period of time sufficient to convert the greater portion of the precursor boehmite alumina to a colloidal sol. The gel is subjected to working, i.e. by using a sufficient shearing force for a sufficient period of time to produce a worked boehmite alumina and increase the pore volume by at least 30 percent and the median pore radius by at least 20 percent. A stabilizer is added to the boehmite alumina, the stabilizer being an oxide of a metal such as barium or a metal included in the lanthanide series of metals or a compound of such metals which converts to an oxide at elevated temperatures. Mixtures of such stabilizers can be employed if desired, the amount of the stabilizer used being sufficient to decrease loss of porosity of a calcined alumina produced from the worked alumina.

In an optional embodiment of the invention, the stabilizer can be added to a calcined product obtained by calcining the worked (sheared) boehmite alumina.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The aluminas which can be treated according to the process of the present invention are boehmite aluminas which have been hydrothermally treated under conditions to convert the greater portion of the boehmite alumina to a colloidal sol, the thus hydrothermally treated aluminas forming the starting material boehmite alumina for use in the process of the present invention. The beohmite alumina which is hydrothermally treated, hereinafter referred to as precursor boehmite alumina, is preferably, although not necessarily, obtained by the hydrolysis of an aluminum alkoxide in the well known fashion. The aluminum alkoxide (trialkoxide) can be produced, in the well known manner, by reacting a low molecular weight alcohol, a linear or branched chain, with an aluminum-bearing material. Such aluminum- bearing materials include pure aluminum and mixed alloy scrap. Typical methods for preparing such aluminum alkoxides are shown, for example, in U.S. Patent No. 4,242,271 , incorporated herein by reference for all purposes. The aluminum alkoxide can be hydrolyzed in the well known manner, such as by the process taught in U.S. Patent No. 4,202,870, incorporated herein by reference for all purposes. Especially preferred are aluminas obtained from the hydrolysis of aluminum alkoxides derived from Ziegler Chemistry in the well known manner. While the preferred feedstock used as the precursor alumina is an alumina slurry, particularly a slurry produced by the hydrolysis of aluminum alkoxides, it will be recognized that aluminas from other sources can be formed into slurries and hydrothermally treated to produce the precursor alumina.

The starting material boehmite alumina used in the process of the present invention can be obtained according to the process disclosed and claimed in U.S. Patent No. 4,676,928, incorporated herein by reference for all purposes. Basically, the process disclosed in U.S. Patent No. 4,676,928 involves taking a precursor boehmite alumina, forming the precursor alumina into an aqueous slurry or mixture, the pH being in the range of from about 5 to about 9, and then heating the aqueous slurry of the precursor alumina at elevated temperatures, generally about 70 °C or greater, for a sufficient period of time to convert the greater portion of the precursor boehmite alumina to a colloidal sol.

In using the process disclosed in U.S. Patent No. 4,676,928 to form the starting material boehmite alumina used in the present process, a colloidal sol can be employed. Alternately, a colloidal sol which has been dried to form a dried powder can be formed into an aqueous dispersion and used. In either event, the alumina content will range from about 15 to about 55 percent-by- weight calculated as Al₂O₃, depending on whether or not a gelling agent is employed. In cases where a gelling agent is employed, the gel will normally contain from about 15 to about 25 percent- by-weight Al₂O₃. In the absence of a gelling agent, the gel will generally contain from about 35 to about 55 percent-by-weight Al₂O₃.

Generally the process is conducted by forming an aqueous slurry or dispersion, either as the sol as described above, or by dispersing a dried sol in an aqueous medium. Once the slurry of the starting material boehmite alumina has been formed, it must be gelled or thickened to increase the viscosity prior to being worked. The term "gel" as used herein refers to a suspension, colloidal in nature, in which shearing stresses below a certain finite value fail to produce permanent deformation. Gelling of the alumina slurry can be carried out simply by concentrating the slurry by the removal of water to form a viscous gel of increased alumina content. Additionally, or alternatively, the gelling of the dispersion can be carried out by the addition of gelling agents. Such gelling agents are generally water-soluble compounds which are well known by those skilled in the art to be compounds which will de- stabilize aqueous colloidal systems. Non-limiting examples of such gelling agents include mineral acids such as nitric acid, hydrochloric acid, etc., organic acids such as formic acid, acetic acid, etc., polyvalent metal salts, etc. For example, water- soluble salts of certain polyvalent metals such as the nitrates, chlorides, acetates, sulfates, etc., of metals such as aluminum, iron, magnesium, manganese, etc. can be used. When employed, such gelling agents will be added in an amount sufficient to increase the viscosity to the desired degree, i.e. until a gel is formed, amounts of from about 0.1 to about 50 percent-by-weight based on the weight of alumina in the gel being generally used.

It is generally necessary, when viscosifying the alumina dispersion, whether such be accomplished by concentrating the dispersion and/or the addition of gelling agents, to add sufficient acid to maintain the gelled alumina in a flowable condition. Generally speaking, monobasic acids such as nitric acid, hydrochloric acid, formic acid, acetic acid, and so forth can be employed. The amount of acid added should be kept to a minimum, consistent with achieving desired gelling, as increased acid decreases porosity.

Working or shearing of the gel to the desired extent can be accomplished in a variety of equipment and under widely varying conditions. In general, any apparatus which is capable of imparting high shear to viscous systems can be employed. Non-limiting examples of apparatus which can be used to carry out the working or shearing step include plastic melt viscometers, mullers commonly used for mixing paste-like materials, driers for preparing high viscosity pastes and gels and the like. Parameters such as shear rate, shear time, temperature, etc. will vary depending upon the concentration of alumina in the gel, the type of gelling agent employed, the type of precursor boehmite employed and the type of hydrothermal treatment applied to the precursor alumina to obtain the staring material boehmite used in the process of the present invention. In general, conditions of high shearing, high concentration of alumina in the gel and minimum acid concentration are preferred. Temperature can vary widely as from ambient to about 100°C. In general, the gel will be subjected to a sufficient shearing force, for a sufficient period of time to increase the pore volume by at least 30% and the median pore radius by at least 20% over that of the alumina in the unworked gel. Such increase in porosity parameters can be determined by techniques well known to those skilled in the art.

It can be shown by transmission electron microscopy (TEM) that ordinary boehmite which has not been treated according to the process of U.S. Patent No.

4,676,928, exists in the form of extensive aggregates of individual crystallites of relatively small size, i.e. less than about 5θA in thickness (020 plane). Such aluminas exhibit extensive aggregation of the crystallites, i.e. microgels. Aluminas which have been preparing according to the process of U.S. Patent No. 4,676,928, as seen by TEM, also exist as aggregates but unlike ordinary boehmite the microgels are made up of stacks of plate-like crystallites which are generally highly oriented. When the latter type of alumina starting material is treated according to the process of the present invention, and again as can be observed by TEM microscopy, the oriented, stacks of crystallites become much more randomly oriented or de-agglomerated resulting in a more open structure of the aggregates, i.e. increased porosity. Thus, to achieve the unexpected increase in porosity using the process of the present invention, it is necessary to employ a starting material alumina which has been prepared in accordance with the process of U.S. Patent No. 4, 676,928 or an equivalent wherein the alumina exists essentially as microgels comprising stacks of plate-like crystallites. Such staring material aluminas can be characterized as being comprised of microgels which are comprised of numerous, associated stacked crystallites on the order of from about 50 to about 150 nm in diameter, the individual crystallite size being on the order of from about 50 to about 15θA in thickness (020 plane). The process of the present invention includes the addition of a stabilizer to a boehmite alumina which has been worked, i.e. sheared, as described above. The term "stabilizer" or "stabilization", as used herein and with reference to the alumina obtained by the process described above, refers to a compound or process which acts to decrease or retard loss of surface area when the alumina, calcined to Al₂O₃, is subjected to elevated temperatures, i.e. 1000°C or greater, generally 1200°C or greater. The stabilizer can be an oxide of barium, an oxide of a lanthanide metal such as lanthanum, cerium, etc., a compound of barium which is converted to an oxide upon heating at an elevated temperature or a compound of a lanthanide metal which is converted to an oxide at an elevated temperature. Especially preferred stabilizers are oxides or barium or lanthanum, or a compound of barium or lanthanum which is converted to an oxide upon heating at an elevated temperature. In the more preferred method, a compound of barium or a lanthanide metal which can be converted to the oxide is used rather than the oxide thereof. This permits the stabilizer to be incorporated in the form of an aqueous solution or dispersion ensuring more uniform distribution of the stabilizer throughout the alumina.

The stabilizer may be added at various points in the process. For example, the stabilizer can be added to the boehmite alumina prior to gelling, during the gelling or after the boehmite alumina is sheared. Thus, the stabilizer can be added to the boehmite alumina prior to the boehmite alumina being worked or after the boehmite alumina is worked. For example, the worked boehmite alumina can be dried and the stabilizer added to the dried, worked beohmite alumina. In an alternative embodiment of the present invention, the worked boehmite alumina can be dried and calcined to produce a calcined product, i.e. Al₂O₃, and the stabilizer added to the calcined product. The stabilizer will be added in an amount sufficient to decrease loss of porosity of a calcined alumina which is subjected to elevated temperatures. In general, the amount of the stabilizer added will be such as to provide a stabilizer content of from about 0.5 to about 20 weight percent based on Al₂O₃ whether in the boehmite alumina or in the calcined product.

It is believed that the unexpected stability of alumina prepared according to the process of the present invention results from the fact that the starting material boehmite is comprised of aggregations of individual pseudoboehmite crystallites, the crystallites being of a generally larger size, i.e. from about 50 to about 150 A in thickness (020 plan), than the conventional boehmite aluminas wherein the individual crystallites are generally about 50 A and smaller in thickness (020 plan). Further, in the staring material boehmite used in the process of the present invention the individual crystallites are plate-like structures which are generally arranged in an ordered, stacked configuration as can be seen by transmission electron microscopy (TEM). When such an alumina is subjected to working as by shearing, the individual crystallites become more randomly distributed, i.e. the stacks of crystallites are disoriented leaving voids or pores, i.e. greater porosity and higher surface area. This porosity provides for a reactive, accessible surface yielding higher catalytic activity. The incorporation of a stabilizer enhances the structural integrity of the alumina in the sense that when subjected to high temperature, the surface area remains, i.e. the alumina does not collapse upon itself. Thus, to achieve the unexpected, stabilized surface area retention using the process of the present invention, it is necessary to employ, as a starting material alumina, a boehmite alumina which has been prepared in accordance with the process of U.S. Patent No. 4,676,928 or an equivalent wherein the alumina exists essentially as microgels comprising stacks of plate-like crystallites. Such starting material aluminas can be characterized as being comprised of microgels which are comprised of numerous, associated stacked crystallites on the order of from about 50 to about 150 nm in diameter, the individual crystallite size being, as noted, on the order of from about 50 to about 15θA in thickness (020 plan).

The process of the present invention can be used to make catalyst supports which retain a high surface area, i.e. about 50 m²/g or greater upon calcination at 1200 °C for three hours.

To more fully illustrate the present invention, the following non-limiting examples are presented. The DISPAL^® aluminas used in the following examples are boehmite aluminas marketed by Vista Chemical Company and made in accordance with the teachings of U.S. Patent No. 4,676,928. In all cases surface area was obtained by the multi-point BET method.

Example 1

A series of samples were prepared by adding a predetermined amount of a 62.8 percent-by-weight lanthanum nitrate hexahydrate solution to a predetermined amount of DISPAL^® 120 alumina sol or DISPAL^® 180 alumina powder. The addition of the lanthanum solution resulted in gelation of the alumina sol. The alumina/lanthanum mixture was then worked on a Haake Torque Rheometer. The material was then removed from the rheometer/mixer, dried over night at 70°C, and then fired at 1200°C for three hours. The firing temperature and time were selected, to mimic the conditions that cause loss of surface area and porosity collapse, i.e. conditions a catalyst would experience during use at elevated temperatures such as in a catalytic converter. High surface areas, i.e. about 50 m²/g or greater, following such treatment at 1200 °C are indicative of a highly stable catalyst which would retain high surface area and provide improved catalytic activity for longer lifetimes under high temperature extremes. The results are shown in Table 1 below. Sample 1 is a control sample which was not worked but contained stabilizer.

As can be seen from the data in Table 1, samples prepared in accordance with the process of the present invention wherein the alumina is worked, i.e. sheared, and contains a stabilizer, exhibit high surface area retention, i.e. generally greater than about 50 πr/g even after being subjected to a temperature of 1200°C for three hours. This is to be contrasted with Sample 1 in which an unworked alumina containing stabilizer showed a surface area markedly less than 50 m²/g after being heated to 1200°C for three hours.

Examples 2-4 which follow demonstrate that retention of high surface area of calcined products is not achieved with conventional boehmite aluminas. In the examples, the CATAPAL^® aluminas used are conventional aluminas marketed by

Vista Chemical Company which have not been prepared in accordance with the process of U.S. Patent No. 4,676,928.

Example 2 100 g of CATAPAL A^® alumina and 452 g deionized water were placed in a Baker-Perkins Muller and sheared for 20 minutes. The resulting material was dried at 66°C and calcined three hours as 1200°C. The surface area on the calcined product was determined to be 5.8 m²/g.

ff.vampte 3

25 g of CATAPAL A^® alumina, 32 g deionized water and 2.42 g lanthanum nitrate solution (61.1 wt.% lanthanum nitrate) were mixed for 10 minutes and dried at 66°C. The resulting powder was calcined three hours at 1200°C. The resulting calcined product was found to have a surface area of 26.1 m²/g.

Example 4

700 g CATAPAL A^® alumina, 452 g deionized water and 69.79 lanthanum nitrate solution (61.1 wt. % lanthanum nitrate) were placed in a Baker-Perkins Muller and sheared for 20 minutes. The resulting material was dried at 66 °C and calcined three hours at 1200°C. The calcined product was found to have a surface area of

42.5 m²/g.

As can be seen from a comparison of the surface area of the calcined products obtained in Examples 2-4, although both working and stabilizing result in a calcined product which retains surface area as contrasted with a CATAPAL A^® alumina which has not been worked and/or stabilized, the surface area remains below about 50 m²/g. In this regard, a CATAPAL A^® alumina which has not been worked (sheared) or stabilized has a surface area of 4.7 m /g after calcining for three hours at 1200°C.

The following examples (5-8) demonstrate that when an alumina such as that prepared according to U.S. Patent No. 4,676,928 is employed, the combination of working and stabilizing results in an end product which retains a surface area of greater than about 50 m²/g even when calcined at 1200°C for three hours.

Example 5

A sample of DISPAL^® 18N4-80 alumina powder was calcined three hours at 1200°C and found to have a surface area of 4.7 m²/g.

700 g DISPAL^® 18N4-80 alumina and 452 g deionized water were placed in a Baker-Perkins Muller and sheared for 20 minutes. The resulting material was dried at 66°C and calcined three hours at 1200°C. The calcined material was found to have a surface area of 9.1 m²/g.

Example 7

100 g DISPAL^® 18N4-80 alumina, 100 g deionized water and 2.69 g lanthanum nitrate solution (61.1 wt. % lanthanum nitrate) were mixed for 10 minutes and dried at 66°C. The resulting powder was calcined three hours as 1200°C. The calcined material was found to have a surface area of 35.4 m²/g.

Example 8 700 g DISPAL^® 18N4-80 alumina, 452 g deionized water and 75.24 lanthanum nitrate solution (61.1 wt. % lanthanum nitrate) were placed in a Baker- Perkin Muller and sheared for 20 minutes. The resulting material was dried at 66°C and calcined three hours at 1200 °C. The resulting calcined material was found to have a surface area of 52.9 n /g.

As can be seen from a comparison of Examples 5-8, the combination of working and stabilizing (Example 8) DISPAL^® alumina, i.e. aluminas prepared in accordance with the teaching of U.S. Patent No. 4,676,928, results in a dramatic increase in retained surface area of the final, calcined product, i.e. a surface area of greater than 50 m²/g is obtained even after the material has been subjected to a temperature of 1200°C for three hours.

Example 9

100 g of DISPAL^® 18N4-20 alumina and 4.54 g barium acetate powder were mixed for 10 minutes and dried at 66°C. The resulting powder was calcined three hours at 1200°C. The calcined material was found to have a surface area of 63 m²/g.

Example 10

700 g of DISPAL^® 18N4-80 alumina, 602 g deionized water, and 103.64 g barium acetate powder were placed in a Baker-Perkins Muller and sheared for 20 minutes. The resulting gel was dried at 66°C and calcined three hours at 1200°C. The calcined material was found to have a surface area of 72.1 m²/g.

As can be seen from Examples 9 and 10, the combination of stabilization with a barium containing material and working provides a marked increase in retained surface area (compare the surface area of the calcined material from Examples 9 and

10 with the surface area of the calcined materials in Examples 5-7). Although, as can be seen from Example 9, the presence of barium stabilization alone gives a surface area of greater than 50 Mm²/g, barium presents certain toxicity problems not presented by the use of lanthanum. However, it can be seen that the use of both barium stabilization and working gives sharply increased retained surface area (note Example 10).

Example 11

100 g CATAPAL A^® alumina, 500 g deionized water, and 12.59 g barium acetate powder was mixed for 15 minutes and dried at 66°C. The resulting powder was calcined three hours as 1200°C. The calcined material had a surface area of 43.4 m7g.

Example 12

700 g CATAPAL^® A alumina, 452 g deionized water, and 88.13 g barium acetate powder were placed in a Baker-Perkins Muller and sheared for 20 minutes. The resulting gel was dried at 66°C and calcined three hours at 1200°C. The calcined material was found to have a surface area of 45.8 m²/g.

As can be seen from the data in Examples 11 and 12, while the addition of stabilizer and working on a conventional boehmite alumina, i.e. an alumina not made in accordance with the teaching of U.S. Patent No. 4,676,928, results in increased, retained surface area, the retained surface area is substantially less than 50 m²/g.

E flm l? 13 600 g DISPAL^® 18N4-25 alumina sol and 24.90 g aluminum nitrate solution

(50 wt. % aluminum nitrate, 50 wt. % deionized water) were mixed to form an alumina gel. The gel was sheared on a Haake Torque Rheometer for 10 minutes at 60°C, 110 rpm. The Al₂O₃ content of the sheared gel was 26.8 percent. 53.0 g of the sheared gel, 2.6 g barium acetate powder, and 80.0 g deionized water were mixed for 10 minutes and dried at 66°C. The resulting powder was calcined three hours at 1200°C. The calcined material was found to have a surface area of 67.8 m²/g. Example 14

55.7 g of the sheared gel of Example 13 were dried at 66°C. The resulting dried gel (18 g), 2.77 g barium acetate powder and 80.0 g deionized water were mixed for 10 minutes and dried at 66°C. The resulting powder was calcined three hours at 1200°C. The calcined material was found to have a surface area of 68.7 m²/g.

Example 15

52.24 g of the sheared gel of Example 13 were dried at 66°C. The dried gel was calcined two hours at 250°C followed by 24 hours at 600°C. The resulting material was mixed for 10 minutes with 2.59 g barium acetate powder and 20.0 g deionized water. The slurry was dried at 66°C and the resulting powder calcined three hours at 1200°C. The calcined material was found to have a surface area of 70.6 m²/g.

Example 16

53.0 g of the sheared gel of Example 13, 1.84 g lanthanum nitrate solution (61.1 wt. % lanthanum nitrate), and 80.0 g deionized water were mixed for 10 minutes and dried at 66°C. The resulting powder was calcined three hours at 1200°C. The calcined material was found to have a surface area of 50.2 m²/g.

Example 17

55.75 g of the sheared gel of Example 13 were dried at 66°C. The resulting dried gel (18 g), 1.94 g lanthanum nitrate solution, and 80.0 g deionized water were mixed for 10 minutes and dried at 66°C. The resulting powder was calcined three hours at 1200°C. The calcined material was found to have a surface area of 47.7 m²/g.

Example 18 52.24 g of the sheared gel of Example 13 were dried at 66°C. The resulting dried gel was calcined two hours at 250°C, followed by 24 hours at 600°C. The resulting material was mixed for 10 minutes with 1.82 g lanthanum nitrate solution (61.1 wt.% lanthanum nitrate) and 20.0 g deionized water. The slurry was dried at 66°C and the resulting powder calcined three hours at 1200°C. The calcined material was found to have a surface area of 52.2 nr/g.

As can be seen from a comparison of Examples 13-18, the combination of working (shearing) and the use of a stabilizer results in an alumina which, after calcining at 1200°C for three hours, in general, retains a surface area of greater than about 50 m²/g. As can be seen from these examples, best results are obtained in terms of retained surface area when the worked or sheared gel is first dried and calcined and the stabilizer then added to the calcined material. Compare, for example, the retained surface area obtained by the procedure of Examples 15 and 18. In general, however, the data in Examples 13-18, as well as the other examples, demonstrate that the stabilizer can be added after the gel has been worked, after the gel has been worked and dried, after the gel has been worked, dried and calcined, and the retained surface area still remains above about 50 m /g.

The foregoing disclosure and description of the invention is illustrative and explanatory thereof, and various changes in the method steps may be made within the scope of the appended claims without departing from the spirit of the invention.

Claims

What Is Claimed Ts:

1. A process for preparing stabilized alumina comprising: forming a gel of boehmite alumina, said boehmite alumina being obtained by hydrothermally treating an aqueous mixture of precursor beohmite alumina having a pH of from about 5 to about 9 for a period of time sufficient to convert the greater portion of said precursor boehmite alumina to a colloidal sol; subjecting said gel to a sufficient shearing force for a sufficient period of time to produce a worked boehmite alumina and increase the pore volume by at least 30% and the median pore radius by at least 20%; and adding a stabilizer to said boehmite alumina, said stabilizer being selected from the class consisting of an oxide of barium, an oxide of a lanthanide metal, a compound of barium that is converted to an oxide upon heating at an elevated temperature, metals converted to an oxide upon heating at an elevated temperature, and mixtures thereof, said stabilizer being a compound of a lanthanide metal that is added in an amount sufficient to decrease loss of surface area of a calcined alumina

< produced from said worked boehmite alumina.

2. The process of Claim 1 wherein a gelling agent is added to form said gel.

3. The process of Claim 1 wherein forming said gel is accomplished by concentrating an aqueous dispersion of said boehmite alumina.

4. The process of Claim 2 wherein said gelling agent comprises a water- soluble compound which will de-stabilize aqueous colloidal systems.

5. The process of Claim 4 wherein said gelling agent comprises aluminum nitrate.

6. The process of Claim 4 wherein said gelling agent comprises magnesium nitrate.

7. The process of Claim 1 wherein an aqueous dispersion of said boehmite alumina is formed, said gel being formed by treating said dispersion of said boehmite alumina.

8. The process of Claim 7 wherein said aqueous dispersion is formed into said gel by concentrating said dispersion.

9. The process of Claim 7 wherein said gel is accomplished by adding a gelling agent to said aqueous dispersion.

10. The process of Claim 8 wherein said gel contains from about 35 to about 55 percent-by-weight alumina.

11. The process of Claim 9 wherein said gel contains from about 15 to about 35 percent-by-weight alumina.

12. The process of Claim 1 wherein said stabilizer is added in an amount of from about 0.5 to about 20 percent-by-weight based on Al₂O₃ content of said boehmite alumina.

13. The process of Claim 1 wherein said stabilizer is a compound of lanthanum.

14. The process of Claim 1 wherein said stabilizer is a compound of barium.

15. The process of Claim 1 wherein said stabilizer is added to said boehmite alumina prior to said shearing.

16. The process of Claim 1 wherein said stabilizer is added to said boehmite alumina after said shearing.

17. A process for preparing a stabilized, calcined alumina comprising: forming a gel of boehmite alumina, said boehmite alumina being obtained by hydrothermally treating an aqueous mixture of precursor beohmite alumina having a pH of from about 5 to about 9 for a period of time sufficient to convert the greater portion of said precursor boehmite alumina to a colloidal sol; subjecting said gel to a sufficient shearing force for a sufficient period of time to produce a worked boehmite alumina and increase the pore volume by at least 30% and the median pore radius by at least 20%; calcining said worked boehmite alumina to produce a calcined product; and adding a stabilizer to said calcined product, said stabilizer being selected from the class consisting of oxides of barium and lanthanide metals, compounds of barium and lanthanide metals are converted to oxides upon heating at an elevated temperature, and mixtures thereof, said stabilizer being added in an amount sufficient to decrease loss of surface area of said calcined product.

18. The process of Claim 17 wherein said stabilizer is added in an amount of from about 0.5 to about 20 percent-by-weight based on Al₂O₃ content of said calcined product.

19. The process of Claim 17 wherein said stabilizer is a compound of lanthanum.

20. The process of Claim 17 wherein said stabilizer is a compound of barium.