Method for producing an alumina gel having a high dispersibility and a specific crystallite size

Abstract

Process for preparing alumina gel in a single precipitation step consisting of dissolving an aluminium precursor, aluminium chloride, in water, at a temperature of 10° C. to 90° C. such that the pH of the solution is from 0.5 to 5, for a period of 2 to 60 minutes, then adjusting the pH to 7.5 to 9.5 by adding a basic precursor, sodium hydroxide, to the solution obtained to obtain a suspension, at a temperature of 5° C. to 35° C., and for 5 minutes to 5 hours, followed by a filtration step, said process not comprising any washing steps. Also, novel alumina gel having a high dispersibility index, in particular a dispersibility index of more than 80%, a crystallite dimension of 0.5 to 10 nm, a chlorine content of 0.001% to 2% by weight and a sodium content of 0.001% to 2% by weight, the percentages by weight being expressed with respect to the total weight of the alumina gel.

Claims

1. A process for the preparation of an alumina gel, wherein the alumina gel has a dispersibility index of more than 80%, a crystallite dimension, obtained by the Scherrer X ray diffraction formula along the crystallographic directions [020] and [120], respectively, in the range of 0.5 to 10 nm and in the range of 0.5 to 15 nm, a chlorine content in the range 0.001% to 2% by weight and a sodium content in the range 0.001% to 2% by weight, the percentages by weight being expressed with respect to the total weight of the alumina gel, the process comprising: a single precipitation step (a) consisting of dissolving an acidic aluminium precursor, aluminium chloride, AlCl.sub.3, in water, at a temperature in the range 10° C. to 90° C. and for a period in the range of 2 to 60 minutes, to obtain a solution, in a manner such that the pH of the solution is in the range of 0.5 to 5, then adjusting the pH to a pH in the range of 7.5 to 9.5 by adding a basic precursor, sodium hydroxide, to the solution obtained in order to obtain a suspension, at a temperature in the range 5° C. to 35° C. and for a period in the range of 5 minutes to 5 hours, and a filtration step b) of the suspension obtained at the end of step a) to obtain a precipitate, said process not comprising any steps for washing the precipitate obtained at the end of the filtration step b).

2. The process as claimed in claim 1, in which the acidic aluminium precursor, aluminium chloride, AlCl.sub.3, is dissolved in water at a temperature in the range 10° C. to 75° C.

3. The process as claimed in claim 1, in which the pH of the solution of aluminium chloride, AlCl.sub.3, in water, is in the range 1 to 4.

4. The process as claimed in claim 1, in which the pH is adjusted at a temperature in the range 10° C. to 30° C.

5. The process as claimed in claim 4, in which the pH is adjusted at a temperature in the range 10° C. to 25° C.

6. The process as claimed in claim 1, in which the pH is adjusted to a pH in the range 7.7 to 8.8.

7. The process as claimed in claim 1, in which the preparation process also comprises a drying step for the filtered suspension obtained at the end of the precipitation step in order to obtain a powder, said drying step being carried out by drying at a temperature of 100° C. or more or by spray drying.

8. The process as claimed in claim 7, in which the preparation process also comprises a shaping step of the powder obtained in order to obtain a green material.

9. The process as claimed in claim 8, in which the shaping step is carried out by mixing-extrusion or by oil-drop.

10. The process as claimed in claim 8, in which the shaped green material then undergoes a heat treatment step at a temperature in the range 500° C. to 1000° C., for a period in the range 2 to 10 h, in the presence or absence of a stream of air containing up to 60% by volume of water.

11. The process as claimed in claim 1, wherein the alumina gel has a dispersibility index in the range of 85% to 100%.

12. The process as claimed in claim 11, wherein the alumina gel has a dispersibility index in the range of 88% to 100%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the XRD of the precipitate obtained in Example 2 confirming that it is a precipitate of boehmite and is highly crystalline.

(2) FIG. 2 shows the XRD of the precipitate obtained in Example 3 confirming that it is a precipitate of boehmite and is highly crystalline.

(3) FIG. 3 shows the XRD of the precipitate obtained in Example 4 confirming that it is a precipitate of boehmite and has low crystallinity.

(4) FIG. 4 shows the XRD of the precipitate obtained in Example 5 confirming that it is a precipitate of boehmite and has low crystallinity.

EXAMPLES

Example 1: (Comparative, Sol-Gel)

(5) A commercial alumina gel powder, Pural SB3, was prepared using a sol-gel route, by hydrolysis-polycondensation of an aluminium alkoxide.

(6) The characteristics of the Pural SB3 boehmite gel are summarized in Table 1.

(7) TABLE-US-00001 TABLE 1 Characteristics of Pural SB3. Commercial alumina gel Dispersibility index (%) 98 Dimension [020] (nm) 3.1 Dimension [120] (nm) 4.7 Sodium Na (% by wt) <detection limit Chlorine Cl (% by wt) <detection limit

(8) The chlorine content was measured using the X ray fluorescence method, and the sodium content was measured by ICP, or inductively coupled plasma spectrometry, and were below the detection limits for these measurement methods.

(9) The alumina gel obtained had a very good dispersibility index but large crystallite dimensions.

Example 2 (Comparative, High Temperature without Washing)

(10) An alumina gel was prepared in accordance with a synthesis process which was not in accordance with the invention, in that the temperature at which precipitation of the alumina gel was carried out was a temperature which was higher than 35° C.

(11) Precipitation of Boehmite, AlOOH

(12) In a beaker cooled in an ice bath, a solution was prepared containing 326 mL of deionized water and 135.6 g of aluminium chloride hexahydrate (AlCl.sub.3.6H.sub.2O) at a temperature of 55° C., in a manner such as to obtain a solution with a pH of 0.5, over a period of 5 minutes.

(13) Next, with magnetic stirring, 67.5 g of sodium hydroxide (NaOH) was added over 30 minutes in a manner such as to adjust the pH. The pH reached at the end of the synthesis was 8. The temperature was maintained at 55° C. throughout the duration of the step. The solid was then filtered.

(14) A sample of the precipitate obtained was removed from the reaction medium. The XRD (FIG. 1) of the precipitate showed that the precipitate obtained in Example 2 was in fact a precipitate of boehmite. The precipitate of boehmite obtained in Example 2 was highly crystalline.

(15) The size of the crystallites of boehmite obtained were measured using Scherrer's method.

(16) The characteristics of the gel obtained in accordance with Example 2 are summarized in Table 2.

(17) TABLE-US-00002 TABLE 2 Characteristics of gel obtained in accordance with Example 2 Example Dispersibility index (%) 25 Dimension [020] (nm) 2.9 Dimension [120] (nm) 3.6 Sodium Na (% by wt) 1.4 Chlorine Cl (% by wt) 1.9

(18) A high precipitation temperature had resulted in an alumina gel with large crystallite dimensions and a mediocre dispersibility index.

Example 3 (Comparative, High pH without Washing)

(19) An alumina gel was prepared in accordance with a synthesis process which was not in accordance with the invention, in that the pH following precipitation of the alumina gel was a pH higher than 9.5.

(20) Precipitation of Boehmite, AlOOH

(21) In a beaker cooled in an ice bath, a solution was prepared containing 326 mL of deionized water and 135.6 g of aluminium chloride hexahydrate (AlCl.sub.3.6H.sub.2O) at a temperature of 20° C., in a manner such as to obtain a solution with a pH of 0.5, over a period of 5 minutes.

(22) Next, with magnetic stirring, 67.5 g of sodium hydroxide (NaOH) was added over 30 minutes in a manner such as to adjust the pH. The pH reached at the end of the synthesis was 10.5. The temperature was maintained at 20° C. throughout the duration of the step. The solid was then filtered.

(23) A sample of the precipitate obtained was removed from the reaction medium. The XRD (FIG. 2) of the precipitate showed that the precipitate obtained in Example 3 was in fact a precipitate of boehmite. The boehmite precipitate obtained in Example 3 was highly crystalline.

(24) The characteristics of the gel obtained in accordance with Example 3 are summarized in Table 3.

(25) TABLE-US-00003 TABLE 3 Characteristics of gel obtained in accordance with Example 3 Example Dispersibility index Ta = 10% (%) 48 Dimension [020] (nm) 2.4 Dimension [120] (nm) 3.1 Sodium Na (% by wt) 1.4 Chlorine Cl (% by wt) 1.85

(26) A high precipitation pH had resulted in large crystallite dimensions and the gel obtained had a mediocre dispersibility index.

Example 4 (Comparative, pH and T in Accordance but Intense Washing)

(27) An alumina gel was prepared in accordance with a synthesis process which was not in accordance with the invention, in that the precipitate was washed following the filtration step.

(28) Precipitation of Boehmite, AlOOH

(29) In a beaker cooled in an ice bath, a solution was prepared containing 326 mL of deionized water and 135.6 g of aluminium chloride hexahydrate (AlCl.sub.3.6H.sub.2O) at a temperature of 25° C., in a manner such as to obtain a solution with a pH of 0.5, over a period of 5 minutes.

(30) Next, with magnetic stirring, 67.5 g of sodium hydroxide (NaOH) was added over 30 minutes in a manner such as to adjust the pH. The pH reached at the end of the synthesis was 8. The temperature was maintained at 20° C. throughout the duration of the step. The solid was then filtered and washed with 3 L of demineralized water.

(31) A sample of the precipitate obtained was removed from the reaction medium. The XRD (FIG. 3) of the precipitate showed that the precipitate obtained in Example 4 was in fact a precipitate of boehmite. The boehmite precipitate obtained had low crystallinity.

(32) The characteristics of the gel obtained in accordance with Example 4 are summarized in Table 4.

(33) TABLE-US-00004 TABLE 4 Characteristics of gel obtained in accordance with Example 4 Example Dispersibility index Ta = 10% (%) 30 Dimension [020] (nm) 2.1 Dimension [120] (nm) 2.8 Sodium Na (% by wt) <0.01 Chlorine Cl (% by wt) <0.02

(34) Due to the intense washing of the precipitate obtained following the precipitation step, the crystallites had a large size and the gel obtained had a mediocre dispersibility index.

Example 5 (in Accordance with the Invention, pH and T in Accordance and No Washing)

(35) An alumina gel was prepared in accordance with a synthesis process which was in accordance with the invention.

(36) Precipitation of Boehmite, AlOOH

(37) In a beaker cooled in an ice bath, a solution was prepared containing 326 mL of deionized water and 135.6 g of aluminium chloride hexahydrate (AlCl.sub.3.6H.sub.2O) at a temperature of 25° C., in a manner such as to obtain a solution with a pH of 0.5, over a period of 5 minutes.

(38) Next, with magnetic stirring, 67.5 g of sodium hydroxide (NaOH) was added over 30 minutes in a manner such as to adjust the pH. The pH reached at the end of the synthesis was 8. The temperature was maintained at 20° C. throughout the duration of the precipitation step. The precipitate was then filtered, but no washing followed this step.

(39) A sample of the precipitate obtained was removed from the reaction medium. The XRD (FIG. 4) of the precipitate showed that the precipitate obtained in Example 5 was in fact a precipitate of boehmite. The boehmite precipitate obtained in Example 5 was of low crystallinity.

(40) The characteristics of the gel obtained in accordance with Example 5 are summarized in Table 5.

(41) TABLE-US-00005 TABLE 5 Characteristics of gel obtained in accordance with Example 5 Example Dispersibility index (%) 100 Dimension [020] (nm) 0.6 Dimension [120] (nm) 1.4 Sodium Na (%) 1.3 Chlorine Cl (% by wt) 1.75

(42) The preparation process in accordance with the invention can be used to obtain a gel which is 100% dispersible and is also less expensive than in conventional prior art alumina preparation processes such as, for example, sol-gel type preparation processes using Pural SB3 as described in Example 1. Furthermore, the crystallite dimension is lower than that obtained by any other mode for the preparation of alumina gels known in the literature.