Alumina sol, process for preparing the same, process for preparing alumina molding using the same, and alumina-based catalyst prepared thereby

An alumina sol, containing a small amount of water of crystallization inside the crystal lattice of boehmite, having high mechanical strength, properties suitable for use as an alumina adsorbent or carrier, and the ability to be formed without additional concentration. The alumina sol comprises a fibrous boehmite having the molecular formula Al.sub.2 O.sub.3.1.05-1.30 H.sub.2 O with the weight average diameter and the weight average length being respectively in the range of 3 to 50 nm and in the range of 30 to 3000 nm, and has an alumina concentration of 15 to 60% by weight.

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What is claimed is:

1. A process for producing an alumina sol comprising fibrous crystalline boehmite exhibiting .rho.-crystal structure, comprising mixing alumina produced by rapid heatingdehydration of aluminum hydroxide with alumina having higher solubility than boehmite in an amount of 0 to 95% by weight in terms of oxide, adding at least one inorganic monobasic acid, and then subjecting the resulting mixture to hydrothermal treatmentat a temperature of 70 to 350.degree. C. in the presence of water, said hydrothermal treatment being carried out with stirring accompanied by substantially slight shearing, without stirring or a combination thereof, and said mixture having a chemicalcomposition by mol represented by aAl.sub.2 O.sub.3.bHA.cH.sub.2 O wherein HA is a sum of effective acid and a, b and c are numbers satisfying the following relation:

wherein the k value is in the range of 0.0005 to 0.01, and containing alumina in a concentration of 5% to 60% by weight.

2. The process for producing an alumina sol as claimed in claim 1, wherein the inorganic monobasic acid contains at least one acid selected from hydrofluoric acid, hydrochloric acid, nitric acid and acids made by dissociation of their aluminumsalts.

3. The process for producing an alumina sol as claimed in claim 1, wherein the alumina having higher solubility than boehmite, that is one starting alumina to be mixed, at least partially comprises one or more substances selected from gibbsite,bayerite, nordstrandite, amorphous hydrated alumina, amorphous alumina, .chi.-alumina, .gamma.-alumina and .eta.-alumina.

4. The process for producing an alumina sol as claimed in claim 1, wherein the hydrothermal treatment is carried out with stirring accompanied by substantially slight shearing, without stirring, or a combination thereof throughout the treatmentexcept during the period of heating up and the initial period of treatment, and a heating medium set apart from the reactants by a reactor wall comprises a temperature-controlled heating medium.

5. A process for producing an alumina sol comprising fibrous crystalline boehmite exhibiting .rho.-crystalline structure, comprising mixing alumina produced by rapid heating dehydration of aluminum hydroxide with alumina having higher solubilitythan boehmite in an amount of 0 to 95% by weight in terms of oxide, adding at least one lower aliphatic monocarboxylic acid, and then subjecting the resulting mixture to hydrothermal treatment at a temperature of 70 to 350.degree. C. in the presence ofwater, said hydrothermal treatment being carried out with stirring accompanied by substantially slight shearing, without stirring or a combination thereof, and said mixture having a chemical composition by mol represented by aAl.sub.2O.sub.3.bHA.cH.sub.2 O wherein HA is a sum of effective acid and a, b and c are numbers satisfying the following relation:

wherein the k value is in the range of 0.002 to 0.05, and containing alumina in concentration of 5 to 60% by weight.

6. The process for producing an alumina sol as claimed in claim 5, wherein the lower aliphatic monocarboxylic acid contains at least one acid selected from formic acid, acetic acid, propionic acid and acids made by dissociation of their aluminumsalts.

7. The process for producing an alumina sol as claimed in claim 5, wherein the alumina having higher solubility than boehmite, that is one starting alumina to be mixed, at least partially comprises one or more substances selected from gibbsite,bayerite, nordstrandite, amorphous hydrated alumina, amorphous alumina, .chi.-alumina, .gamma.-alumina and .eta.-alumina.

8. The process for producing an alumina sol as claimed in claim 5, wherein the hydrothermal treatment is carried out with stirring accompanied by substantially slight shearing, without stirring, or a combination thereof throughout the treatmentexcept during the period of heating up and the initial period of treatment, and a heating medium set apart from the reactants by a reactor wall comprises a temperature-controlled heating medium.