WO2005014482A2 - PROCESS FOR PREPARING BOEHMITIC ALUMINAS HAVING A HIGH α-CONVERSION TEMPERATURE - Google Patents

PROCESS FOR PREPARING BOEHMITIC ALUMINAS HAVING A HIGH α-CONVERSION TEMPERATURE Download PDF

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WO2005014482A2
WO2005014482A2 PCT/EP2004/007988 EP2004007988W WO2005014482A2 WO 2005014482 A2 WO2005014482 A2 WO 2005014482A2 EP 2004007988 W EP2004007988 W EP 2004007988W WO 2005014482 A2 WO2005014482 A2 WO 2005014482A2
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aging
hydrolysis
aluminas
acids
calcination
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PCT/EP2004/007988
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French (fr)
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WO2005014482A3 (en
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Kai DÖLLING
Andrea Brasch
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Sasol Germany Gmbh
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Priority to JP2006519893A priority Critical patent/JP5132149B2/en
Priority to EP04741108A priority patent/EP1646583B1/en
Priority to DK04741108.7T priority patent/DK1646583T3/en
Priority to US10/564,244 priority patent/US8147795B2/en
Priority to AT04741108T priority patent/ATE554052T1/en
Publication of WO2005014482A2 publication Critical patent/WO2005014482A2/en
Publication of WO2005014482A3 publication Critical patent/WO2005014482A3/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
    • B01J21/02—Boron or aluminium; Oxides or hydroxides thereof
    • B01J21/04—Alumina
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/03—Precipitation; Co-precipitation
    • B01J37/031—Precipitation
    • B01J37/033—Using Hydrolysis
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00—Compounds of aluminium
    • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00—Compounds of aluminium
    • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
    • C01F7/34—Preparation of aluminium hydroxide by precipitation from solutions containing aluminium salts
    • C01F7/36—Preparation of aluminium hydroxide by precipitation from solutions containing aluminium salts from organic aluminium salts
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00—Compounds of aluminium
    • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
    • C01F7/44—Dehydration of aluminium oxide or hydroxide, i.e. all conversions of one form into another involving a loss of water
    • C01F7/447—Dehydration of aluminium oxide or hydroxide, i.e. all conversions of one form into another involving a loss of water by wet processes
    • C01F7/448—Dehydration of aluminium oxide or hydroxide, i.e. all conversions of one form into another involving a loss of water by wet processes using superatmospheric pressure, e.g. hydrothermal conversion of gibbsite into boehmite
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/10—Particle morphology extending in one dimension, e.g. needle-like
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/20—Particle morphology extending in two dimensions, e.g. plate-like
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/60—Particles characterised by their size
    • C01P2004/64—Nanometer sized, i.e. from 1-100 nanometer
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/12—Surface area
    • C01P2006/13—Surface area thermal stability thereof at high temperatures
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/14—Pore volume
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/16—Pore diameter
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/32—Thermal properties
    • C01P2006/33—Phase transition temperatures
    • C01P2006/36—Solid to solid transition temperatures

Definitions

  • the present invention relates to a process for preparing boehmitic aluminas by hydrolysis of aluminium alcoholates in aqueous, alkaline solution. It further relates to boehmitic aluminas prepared by this process or aluminas obtained by calcination and to their uses.
  • the usefulness of an alumina-based catalyst carrier in general and for car exhaust gas catalysis in particular is characterised by physical properties, such as specific surface areas, pore volumes, and high surface stability.
  • the intensity of the ⁇ -Al 2 O 3 conversion temperature i.e. the temperature at which conversion into the alpha phase of the Al 2 O 3 takes place, is a measure of high surface stability.
  • this temperature can be as high as approx. 1150°C and rarely max. 1300°C.
  • the conversion temperature and thus the surface stability can be increased to a certain extent for example by doping with foreign metals, which, however, would result in contamination of the catalyst carrier and restrict its uses.
  • an object of this invention to provide a process for preparing boehmitic aluminas having ⁇ -Al 2 O 3 conversion temperatures above 1200°C and large pore volumes and surface areas, which does not result in contamination with foreign metals or foreign anions.
  • the problem is solved by a process for preparing boehmitic aluminas by hydrolysis of aluminium alcoholates in aqueous, alkaline solution, wherein the hydrolysis is carried out at pH values above 8.5, preferably from 9 to 11 and the hydrolysis and/or the aging of the mixture resulting from the hydrolysis, preferably at least the hydrolysis, is carried out in the presence of substituted carboxylic acids, the salts thereof or their derivatives which during hydrolysis and/or the hydrothermal aging are at least partially converted into the free carboxylic acid or the dissociated form thereof having at least one additional substituent (in addition to the carboxy group of the carboxylic acid) selected from the group comprised of carboxy-, hydroxy-, oxo-, and amino groups.
  • the aging is conducted for at least 30 min and more preferred as hydrothermal aging for at least 30 min and most preferred by employing stirring / mixing.
  • the dried products have ⁇ -Al 2 O 3 conversion temperatures above 1200°C.
  • the substituted carboxylic acid or its salt is added to the aqueous premix for hydrolysis in quantities of from 0.1 to 0.5 wt.%, preferably 0.2 to 0.4 wt.%, calculated as free acid and referring to the total mass, and preferably comprises independently hereof 2 to 12 carbon atoms, most preferably 2 to 8.
  • substituted carboxylic acids include carboxylic acids which furthermore have one or more carboxy-, hydroxy-, oxo-, or amino group(s) or a combination thereof, particularly di- or tricarboxylic acids, hydroxycarboxylic acids, hydroxydicarboxylic acids, hydroxytricarboxylic acids, dihydroxydicarboxylic acids, oxocarboxylic acids, and amino acids. Hydroxydicarboxylic acids, hydroxytricarboxylic acids, dihydroxydicarboxylic acids, oxocarboxylic acids, and amino acids are preferred.
  • ammonium salts including for example alkanol ammonium salts
  • derivatives of the carboxylic acid employed according to this invention which at least partially set free in the premix for hydrolysis the free acid or the dissociated form thereof.
  • Examples of useful substituted carboxylic acids within the meaning of the present invention include 2-hydroxypropionic acid, 2-oxopropanoic acid, hydroxybutanedi- carboxylic acid, dihydroxybutanedicarboxylic acid, 2-hydro xypropane- 1,2,3 - tricarboxylic acid (citric acid), L-aspartic acid, L-serine, glycine, L-leucine, L-tyro- sine, or L-tryptophane.
  • hydroxybutanedicarboxylic acid dihydroxybutanedicarboxylic acid, 2-hydr- oxypropane- 1,2, 3 -tricarboxylic acid (citric acid), L-aspartic acid, L-serine, glycine, or L-leucine.
  • the boehmitic aluminas prepared according to the invention can be subjected to additional hydro- thermal aging.
  • the aged products then have a conversion temperature above 1350°C, preferably above 1400°C.
  • the aging step is carried out at temperatures ranging from 80°C to 250°C, preferably from 120°C to 220°C, most preferably from 200°C to
  • Aging normally takes place from more than 1 hour or more than 2 hours to max. 20 hours for example, preferably 4 to 6 hours, and is preferably performed in a slurry having a solids content of preferably 2 to 17 wt.% prior to aging, most preferably 5 to 10 wt.%, referring to the total mass and calculated as Al 2 O 3 .
  • the term 'slurry' as used herein is defined as a heterogeneous suspension of solid alumina hydrate in water.
  • the present invention also relates to high-purity boehmitic aluminas prepared according to the process of the invention, which comprise for example less than 40 ppm of sodium and less than 50 ppm of sulfate.
  • Said boehmitic aluminas preferably have a lamellar or acicular crystal structure, depending on the type of carboxylic acid employed.
  • the lamellar crystal structure is a structure wherein the crystals form plates. According to the acicular crystal structure crystals in the form of needles are build.
  • the high-temperature boehmitic aluminas have large pore volumes and high specific surface areas.
  • aluminas including alumina hydrates prepared according to the process of the invention are suitable as catalyst carriers.
  • aluminium alcoholates comprising per Aluminium atom at least one alcoholate group are used for preparing high-purity boehmitic aluminas.
  • the aluminium alcoholates can be prepared for example by the Ziegler process, wherein preferably one purification step is filtration.
  • the aluminium alcoholates can be prepared for example from C ⁇ - to C 24 -alcohols or mixtures thereof.
  • the process of the invention yields high-purity boehmitic aluminas having particularly regular crystallite structures and significantly higher ⁇ -Al 2 O 3 conversion temperatures, especially after the slurry aging step.
  • ' ⁇ -Al 2 O 3 conversion temperature' refers to the temperature at which during the thermal degradation of aluminium hydroxides the ⁇ -Al 2 O 3 (also termed corundum) is formed.
  • the final step of the known exothermal crystal lattice remodelling resulting in the energetically most favourable structure takes place at this temperature.
  • lattice remodelling is concomitant with a drastic reduction of surface area and pore volume.
  • a high conversion temperature has the advantage of providing a larger surface area and pore volume even at high temperatures and prior to conversion into ⁇ -Al 2 O 3 .
  • the boehmitic aluminas prepared according to the process of the invention excel by unusually regular crystal structures and the special physical properties attributable thereto. Having access to the boehmitic aluminas of the invention and their special physical properties is of great importance with respect to the further development of alumina-based catalyst carriers.
  • this invention therefore also relates to the aging and calcination of follow-up products obtained by hydrolysis.
  • the dried products obtained from unaged slurry are outstanding in that their conversion into ⁇ -Al 2 O 3 takes place at above 1200°C.
  • the dried products obtained for example from slurry aged at 210°C for 5 hours excel by even higher ⁇ -Al 2 O conversion temperatures of about 1400°C and much higher.
  • Pore volume and surface area of hydrothermally aged products are considerably higher in comparison with a product which was hydrolysed without additives. Aging can be done in closed apparatuses under the pressure generated thereby.
  • the ⁇ -Al 2 O 3 conversion temperatures of the dried products obtained from slurries aged at 210°C increase to nearly 1400°C.
  • the ⁇ -Al 2 O 3 conversion temperatures of the dried products obtained from slurries aged at 210°C increase to above 1400°C.
  • pore volume and specific surface area increase overproportionately with narrow and monomodal pore radii distribution. Both ⁇ -Al 2 O 3 conversion temperature and pore volume can be further increased with defined pore radii, when using hydroxytricarboxylic acids.
  • the -Al 2 O conversion temperature can be influenced by the type of carboxylic acid as well as by the amount of acid added.
  • a larger quantity of substituted carboxylic acid in the premix for hydrolysis may lower the -Al 2 O 3 conversion temperature due to strong hindrance of the crystallite growth.
  • a smaller quantity of substituted carboxylic acid would support precipitation of a conventional boehmitic alumina (not incorporated in the present invention).
  • the optimum amount in the premix for hydrolysis would be 0.1 to 0.5 wt.% in order to reach the highest possible ⁇ -Al 2 O 3 conversion temperature.
  • the hydrolysis temperature preferably ranges from 50°C to 95°C, particularly from 70°C to 95°C. It is essential that the pH value of the premix for hydrolysis be in the alkaline range, preferably above 8.5, more preferably above 9, most preferably between 9.5 and 11.
  • the term 'premix for hydrolysis' is defined as the premix containing water and further additives prior to addition of the aluminium alcoholate, i.e. the mixture wherein hydrolysis takes place, once the alcoholate is added.
  • the alkaline pH value of the premix for hydrolysis can be reached by adding suitable substances, such as ammonia, alkaline solution, or pH adjusters.
  • the monohydrates (boehmites) modified according to this invention which are thus novel, show surprising high ⁇ -Al 2 O 3 conversion temperatures and large pore volumes with defined pore radii. High purity of the products is ensured by the manufacturing process, namely the hydrolysis of aluminium alcoholate and mixtures thereof.
  • Example 1 Comparative Example, PURAL ® 200
  • Example 2 2-Hydroxypropionic Acid (Lactic Acid) Example 1 was repeated using the following quantities: 475 g of water 3.9 g of ammonia solution (25%) 1.33 g of lactic acid (90%ig) in the premix
  • Example 1 was repeated using the following quantities: 475 g of water 3.9 g of ammonia solution (25%) 1.2 g of pyruvic acid in the premix
  • Example 4 Hydroxybutanedioic Acid (DL( ⁇ )Malic Acid) Example 1 was repeated using the following quantities:
  • Example 1 was repeated using the following quantities: 475 g of water 3.9 g of ammonia solution (25%)
  • Example 1 was repeated using the following quantities: 475 g of water 3.9 g of ammonia solution (25%) 1.2 g of diammonium hydrogen citrate, calculated as citric acid, in the premix 400 g of aluminium hexanolate pH value prior to aging: 10.0
  • Example 1 was repeated using the following quantities: 475 g of water 3.9 g of ammonia solution (25%) 1.2 g of L-aspartic acid in the premix 400 g of aluminium hexanolate pH value prior to aging: 9.6
  • Example 8 L-Serine Example 1 was repeated using the following quantities: 475 g of water 3.9 g of ammonia solution (25%)
  • Example 1 was repeated using the following quantities: 475 g of water 3.9 g of ammonia solution (25%)
  • Example 1 was repeated using the following quantities: 470 g of water 3.9 g of ammonia solution (25%)
  • the ⁇ -Al 2 O 3 conversion temperature was determined by simultaneous thermal analysis (STA). The heating rate was 10 K/min with air purge. STA comprises differential thermo analysis and thermogravimetry. The surface area of the aluminas was measured by N sorption analysis according to BET (DIN 66131). Pore volume and average pore radius were determined by mercury penetration (DIN 66133, contact angle 131°). Table 1

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Abstract

The present invention relates to a process for preparing boehmitic aluminas by hydrolysis of aluminium alcoholates in aqueous, alkaline solution. It further relates to aluminas or alumina hydrates prepared by this process and their uses.

Description

PROCESS FOR PREPARING BOEHMITIC ALUMINAS HAVING A HIGH α-CONVERSION TEMPERATURE
The present invention relates to a process for preparing boehmitic aluminas by hydrolysis of aluminium alcoholates in aqueous, alkaline solution. It further relates to boehmitic aluminas prepared by this process or aluminas obtained by calcination and to their uses.
The usefulness of an alumina-based catalyst carrier in general and for car exhaust gas catalysis in particular is characterised by physical properties, such as specific surface areas, pore volumes, and high surface stability. The intensity of the α-Al2O3 conversion temperature, i.e. the temperature at which conversion into the alpha phase of the Al2O3 takes place, is a measure of high surface stability. With conventional boehmitic aluminas (aluminium monohydrates) this temperature can be as high as approx. 1150°C and rarely max. 1300°C. The conversion temperature and thus the surface stability can be increased to a certain extent for example by doping with foreign metals, which, however, would result in contamination of the catalyst carrier and restrict its uses.
It is, therefore, an object of this invention to provide a process for preparing boehmitic aluminas having α-Al2O3 conversion temperatures above 1200°C and large pore volumes and surface areas, which does not result in contamination with foreign metals or foreign anions.
According to the present invention, the problem is solved by a process for preparing boehmitic aluminas by hydrolysis of aluminium alcoholates in aqueous, alkaline solution, wherein the hydrolysis is carried out at pH values above 8.5, preferably from 9 to 11 and the hydrolysis and/or the aging of the mixture resulting from the hydrolysis, preferably at least the hydrolysis, is carried out in the presence of substituted carboxylic acids, the salts thereof or their derivatives which during hydrolysis and/or the hydrothermal aging are at least partially converted into the free carboxylic acid or the dissociated form thereof having at least one additional substituent (in addition to the carboxy group of the carboxylic acid) selected from the group comprised of carboxy-, hydroxy-, oxo-, and amino groups. Preferably the aging is conducted for at least 30 min and more preferred as hydrothermal aging for at least 30 min and most preferred by employing stirring / mixing. The dried products have α-Al2O3 conversion temperatures above 1200°C.
In particular, the substituted carboxylic acid or its salt is added to the aqueous premix for hydrolysis in quantities of from 0.1 to 0.5 wt.%, preferably 0.2 to 0.4 wt.%, calculated as free acid and referring to the total mass, and preferably comprises independently hereof 2 to 12 carbon atoms, most preferably 2 to 8.
Examples of substituted carboxylic acids according to this invention include carboxylic acids which furthermore have one or more carboxy-, hydroxy-, oxo-, or amino group(s) or a combination thereof, particularly di- or tricarboxylic acids, hydroxycarboxylic acids, hydroxydicarboxylic acids, hydroxytricarboxylic acids, dihydroxydicarboxylic acids, oxocarboxylic acids, and amino acids. Hydroxydicarboxylic acids, hydroxytricarboxylic acids, dihydroxydicarboxylic acids, oxocarboxylic acids, and amino acids are preferred.
When the carboxylic acids used according to this invention are present as salts, it is preferable that ammonium salts, including for example alkanol ammonium salts, be employed. Also suitable are derivatives of the carboxylic acid employed according to this invention, which at least partially set free in the premix for hydrolysis the free acid or the dissociated form thereof.
Examples of useful substituted carboxylic acids within the meaning of the present invention include 2-hydroxypropionic acid, 2-oxopropanoic acid, hydroxybutanedi- carboxylic acid, dihydroxybutanedicarboxylic acid, 2-hydro xypropane- 1,2,3 - tricarboxylic acid (citric acid), L-aspartic acid, L-serine, glycine, L-leucine, L-tyro- sine, or L-tryptophane. Included amongst the particularly preferred substances are hydroxybutanedicarboxylic acid, dihydroxybutanedicarboxylic acid, 2-hydr- oxypropane- 1,2, 3 -tricarboxylic acid (citric acid), L-aspartic acid, L-serine, glycine, or L-leucine.
In accordance with another embodiment of the present invention, the boehmitic aluminas prepared according to the invention can be subjected to additional hydro- thermal aging. The aged products then have a conversion temperature above 1350°C, preferably above 1400°C. The aging step is carried out at temperatures ranging from 80°C to 250°C, preferably from 120°C to 220°C, most preferably from 200°C to
220°C. Aging normally takes place from more than 1 hour or more than 2 hours to max. 20 hours for example, preferably 4 to 6 hours, and is preferably performed in a slurry having a solids content of preferably 2 to 17 wt.% prior to aging, most preferably 5 to 10 wt.%, referring to the total mass and calculated as Al2O3. The term 'slurry' as used herein is defined as a heterogeneous suspension of solid alumina hydrate in water.
The present invention also relates to high-purity boehmitic aluminas prepared according to the process of the invention, which comprise for example less than 40 ppm of sodium and less than 50 ppm of sulfate. Said boehmitic aluminas preferably have a lamellar or acicular crystal structure, depending on the type of carboxylic acid employed. The lamellar crystal structure is a structure wherein the crystals form plates. According to the acicular crystal structure crystals in the form of needles are build.
It is most preferable that the high-temperature boehmitic aluminas have large pore volumes and high specific surface areas.
The aluminas (including alumina hydrates) prepared according to the process of the invention are suitable as catalyst carriers.
According to the present invention, aluminium alcoholates comprising per Aluminium atom at least one alcoholate group are used for preparing high-purity boehmitic aluminas. The aluminium alcoholates can be prepared for example by the Ziegler process, wherein preferably one purification step is filtration. The aluminium alcoholates can be prepared for example from C\- to C24-alcohols or mixtures thereof.
The process of the invention yields high-purity boehmitic aluminas having particularly regular crystallite structures and significantly higher α-Al2O3 conversion temperatures, especially after the slurry aging step.
The term 'α-Al2O3 conversion temperature' as used herein refers to the temperature at which during the thermal degradation of aluminium hydroxides the α-Al2O3 (also termed corundum) is formed. The final step of the known exothermal crystal lattice remodelling resulting in the energetically most favourable structure takes place at this temperature. However, lattice remodelling is concomitant with a drastic reduction of surface area and pore volume. A high conversion temperature has the advantage of providing a larger surface area and pore volume even at high temperatures and prior to conversion into α-Al2O3. It has surprisingly been found that the boehmitic aluminas prepared according to the process of the invention excel by unusually regular crystal structures and the special physical properties attributable thereto. Having access to the boehmitic aluminas of the invention and their special physical properties is of great importance with respect to the further development of alumina-based catalyst carriers. In addition to the process for hydrolysing aluminium alcoholates, this invention therefore also relates to the aging and calcination of follow-up products obtained by hydrolysis.
The dried products obtained from unaged slurry are outstanding in that their conversion into α-Al2O3 takes place at above 1200°C. The dried products obtained for example from slurry aged at 210°C for 5 hours excel by even higher α-Al2O conversion temperatures of about 1400°C and much higher.
Pore volume and surface area of hydrothermally aged products are considerably higher in comparison with a product which was hydrolysed without additives. Aging can be done in closed apparatuses under the pressure generated thereby.
When using short-chain hydroxycarboxylic or oxocarboxylic acids for the hydrolysis, the α-Al2O3 conversion temperatures of the dried products obtained from slurries aged at 210°C increase to nearly 1400°C. When using hydroxydicarboxylic- and dihydroxydicarboxylic acids, the α-Al2O3 conversion temperatures of the dried products obtained from slurries aged at 210°C increase to above 1400°C.
At high temperatures, i.e. after calcination at 1300°C for 3 hours, pore volume and specific surface area increase overproportionately with narrow and monomodal pore radii distribution. Both α-Al2O3 conversion temperature and pore volume can be further increased with defined pore radii, when using hydroxytricarboxylic acids.
When employing amino acids, it is also possible to raise the α-Al2O3 conversion temperature above 1400°C.
Furthermore, the -Al2O conversion temperature can be influenced by the type of carboxylic acid as well as by the amount of acid added. A larger quantity of substituted carboxylic acid in the premix for hydrolysis may lower the -Al2O3 conversion temperature due to strong hindrance of the crystallite growth. A smaller quantity of substituted carboxylic acid would support precipitation of a conventional boehmitic alumina (not incorporated in the present invention). For example, when using citric acid, the optimum amount in the premix for hydrolysis would be 0.1 to 0.5 wt.% in order to reach the highest possible α-Al2O3 conversion temperature.
The hydrolysis temperature preferably ranges from 50°C to 95°C, particularly from 70°C to 95°C. It is essential that the pH value of the premix for hydrolysis be in the alkaline range, preferably above 8.5, more preferably above 9, most preferably between 9.5 and 11. For the purpose of the present invention the term 'premix for hydrolysis' is defined as the premix containing water and further additives prior to addition of the aluminium alcoholate, i.e. the mixture wherein hydrolysis takes place, once the alcoholate is added. The alkaline pH value of the premix for hydrolysis can be reached by adding suitable substances, such as ammonia, alkaline solution, or pH adjusters.
The monohydrates (boehmites) modified according to this invention, which are thus novel, show surprising high α-Al2O3 conversion temperatures and large pore volumes with defined pore radii. High purity of the products is ensured by the manufacturing process, namely the hydrolysis of aluminium alcoholate and mixtures thereof.
Examples
Example 1: Comparative Example, PURAL® 200
In a 2-litre three-neck flask 475 grams of water and 3.9 grams of a 25% ammonia solution were heated to 90 °C. In this premix 400 grams of aluminium hexanolate were hydrolysed in three steps during 30 minutes while stirring and maintaining the temperature. Hydrolysis took 45 minutes in total yielding two immiscible phases: a supernatant alcohol phase and an alumina/ water phase. After removal of the water- dissolved alcohol and measurement of the pH value ranging from 9.5 to 10.5, the resultant alumina suspension was aged at 210°C for 5 hours under pressure (about 25 bar) and with stirring, followed by spraydrying.
Example 2: 2-Hydroxypropionic Acid (Lactic Acid) Example 1 was repeated using the following quantities: 475 g of water 3.9 g of ammonia solution (25%) 1.33 g of lactic acid (90%ig) in the premix
400 g of aluminium hexanolate pH value prior to aging: 9.4 Example 3: 2-Oxopropanoic Acid (Pyruvic Acid)
Example 1 was repeated using the following quantities: 475 g of water 3.9 g of ammonia solution (25%) 1.2 g of pyruvic acid in the premix
400 g of aluminium hexanolate pH value prior to aging: 9.3
Example 4 : Hydroxybutanedioic Acid (DL(±)Malic Acid) Example 1 was repeated using the following quantities:
475 g of water
3.9 g of ammonia solution (25%)
1.2 g DL<±>malic acid in the premix
400 g of aluminium hexanolate pH value prior to aging: 9.5
Example 5: Dihydroxybutanedioic Acid (L(+)-Tartaric Acid)
Example 1 was repeated using the following quantities: 475 g of water 3.9 g of ammonia solution (25%)
1.2 g L(+)-tartaric acid in the premix
400 g of aluminium hexanolate pH value prior to aging: 9.5
Example 6: 2-Hydroxypropane-l,2,3-tricarboxylic Acid (Citric Acid)
Example 1 was repeated using the following quantities: 475 g of water 3.9 g of ammonia solution (25%) 1.2 g of diammonium hydrogen citrate, calculated as citric acid, in the premix 400 g of aluminium hexanolate pH value prior to aging: 10.0
Example 7: L-Aspartic Acid
Example 1 was repeated using the following quantities: 475 g of water 3.9 g of ammonia solution (25%) 1.2 g of L-aspartic acid in the premix 400 g of aluminium hexanolate pH value prior to aging: 9.6
Example 8: L-Serine Example 1 was repeated using the following quantities: 475 g of water 3.9 g of ammonia solution (25%)
1.2 g of L-serine in the premix
400 g of aluminium hexanolate pH value prior to aging: 9.3
Example 9: L-Leucine
Example 1 was repeated using the following quantities: 475 g of water 3.9 g of ammonia solution (25%)
1.2 g of L-leucine in the premix
400 g of aluminium hexanolate pH value prior to aging: 9.5
Example 10: Citrate Premix
Example 1 was repeated using the following quantities: 470 g of water 3.9 g of ammonia solution (25%)
4.8 g of diammonium hydrogen citrate, calculated as citric acid, in the premix 400 g of aluminium hexanolate pH value prior to aging: 9.3
The products prepared in this way were analysed to determine α-Al2O3 conversion temperature, surface area, pore volume, and average pore radius. The analytical results of Examples 1 through 10 have been compiled in Table 1.
The α-Al2O3 conversion temperature was determined by simultaneous thermal analysis (STA). The heating rate was 10 K/min with air purge. STA comprises differential thermo analysis and thermogravimetry. The surface area of the aluminas was measured by N sorption analysis according to BET (DIN 66131). Pore volume and average pore radius were determined by mercury penetration (DIN 66133, contact angle 131°). Table 1
Figure imgf000009_0001
Legend: * Activation temperature: 550°C, 3 hours # Activation temperature: 1300°C, 3 hours

Claims

Claims:
1. A process for preparing boehmitic aluminas by hydrolysis of aluminium alcoholates in aqueous, alkaline solution, optionally followed by aging, characterised in that - the hydrolysis is carried out at pH values above 8.5 and - the hydrolysis and/or the aging of the mixture resulting from the hydrolysis is carried out in the presence of substituted carboxylic acids, the salts thereof or their derivatives which during hydrolysis and/or the hydrothermal aging are at least partially converted into the free carboxylic acid or the dissociated form thereof, wherein at least one of the additional substituents is selected from the group consisting of carboxy-, hydroxy-, oxo- and amino groups.
2. The process according to claim 1, characterised in that the substituted carboxylic acid, their derivatives or the salt thereof is added in quantities of from 0.1 to 0.5 wt.%, preferably 0.2 to 0.4 wt.%, referring to the total mass of aqueous premix for hydrolysis and/or the aging composition and calculated as substituted carboxylic acid.
3. The process according to any one of the preceding claims, characterised in that the substituted carboxylic acid, their derivatives or salt thereof are selected from the group comprised of di- or tricarboxylic acids, hydroxycarboxylic acids, hydroxydicarboxylic acids, hydroxytri- carboxylic acids, dihydroxydicarboxylic acids, oxocarboxylic acids and amino acids.
4. The process according to any one of the preceding claims, characterised in that the hydrolysis is carried out at 50 to 95°C, preferably above 60 to 95°C.
5. The process according to any one of the preceding claims, characterised in that the boehmitic aluminas are subsequently subjected to aging.
6. The process according to claim 5, characterised in that the aging step is carried out at temperatures ranging from 80°C to 250°C, preferably 130°C to 220°C, most preferably 205°C to 215°C for at least 1 hour, preferably at least 2 hours.
7. The process according to claim 5 or 6, characterised in that the aging step is carried out in an aqueous environment with a solid matter con- centration (as Al2O3) at the beginning of the aging step ranging from 2 to 17 wt.%, preferably 5 to 10 wt.%, referring to the total mass of composition subjected to aging.
8. Boehmitic aluminas which can be manufactured by the process according to any one of the preceding claims, preferably according to claim 5 to 7, and which convert to the α-phase only at temperatures of above 1350°C.
9. The boehmitic aluminas according to claim 8, characterised in that the aluminas have a lamellar (plate type) or needle shaped (acicular) crystal structure, preferably an acicular one, depending on the carboxylic acid used.
10. The boehmitic aluminas according to claim 8 or 9 or the alumina prepared therefrom by calcination, characterised in that before and after calcination the boehmitic aluminas or the alumina are dispersible even at neutral pH values in aqueous or organic media, particularly Cj- to C - alcohols, in quantities above 1 wt.%, preferably above 7 wt.%, most preferably above 10 wt.%, calculated as Al2O3 and referring to the total composition.
11. An alumina prepared according to any one of claims 1 to 7 followed by calcination, characterised in that the alumina when treated with temperatures of above 1200°C remains to have a pore volume of above 0.5 ml/g, based on pore radii from 2 to 100 nm, and a surface area above 20 m /g, measured in accordance with DIN 66131.
12. An alumina prepared according to any one of claims 1 to 7 followed by calcination, characterised in that calcination is carried out at above 450 °C and the alumina has a particle size ranging from 10 to 50 nm in aqueous suspension or dispersion.
13. Use of the aluminas obtained by calcination of the boehmitic aluminas according to any one of claims 1 to 7 as catalyst carriers, particularly for motor car exhaust gas catalysis.
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