Method for Preparing an Alumina Supported Perovskite Type Oxide Composition, Alumina Supported Perovskite Type Oxide Composition and Its use

Abstract

The present invention relates to a method for preparing an alumina supported perovskite type oxide composition, to an alumina supported perovskite type oxide composition and to the use of such an alumina supported perovskite type oxide composition in catalytic systems in emission control applications.

Claims

1. A method of preparing an alumina supported perovskite type oxide composition, the method comprising the steps of: i) providing a doped alumina, the doped alumina comprising: a. alumina and a rare-earth oxide, or b. alumina and an alkaline earth oxide, or c. alumina and a mixture of a rare-earth oxide and an alkaline earth oxide, wherein the doped alumina is provided by a method comprising at least the following steps: A) preparing a boehmite suspension, the boehmite suspension comprising a boehmite; B) preparing an aqueous salt solution, the aqueous salt solution comprising a. a rare-earth salt, or b. an alkaline earth salt, or c. a mixture of a rare-earth salt and an alkaline earth salt; C) combining the boehmite suspension with the aqueous salt solution to form a boehmite salt mixture; D) drying the boehmite salt mixture to form a dried boehmite salt mixture; and E) calcining the dried boehmite salt mixture to form a doped alumina; ii) impregnating the doped alumina with an impregnation aqueous solution, the impregnation aqueous solution comprising one or a mixture of water soluble rare-earth salts, water soluble alkaline earth salts, water soluble alkali salts, water soluble salts of Pb.sup.2+, water soluble salts of Bi.sup.3+, and water soluble transition metal salts to form an impregnated doped alumina; and iii) calcining the impregnated doped alumina.

2. The method of claim 1 wherein the boehmite suspension further comprises silica, titania, water soluble salts of alkaline earth metals, water soluble salts of rare-earth metals, zirconium or mixtures thereof.

3. The method of claim 1 wherein the aqueous salt solution comprises at least water and a water-soluble rare-earth salt, a water soluble alkaline earth salt, or mixtures thereof.

4. The method of claim 1 wherein the impregnation of the doped alumina comprises incipient wetness impregnation

5. The method of claim 1 wherein between 80 and 100% of the pore volume of the doped alumina is impregnated with the impregnation aqueous solution.

6. The method of claim 1 wherein the water soluble salts comprise mixtures of at least one of a), b) and c) with at least one of d): a) acetates or nitrates of the rare-earth elements, b) acetates or nitrates of alkaline earth elements, c) one or more of acetates or nitrates of Pb.sup.2+ and Bi.sup.3+ and d) one or more water soluble salts of transition metals comprising ammonium-iron-citrate, ammonium-titanium-lactate, zirconium acetate or mixtures thereof.

7. The method of claim 1 wherein the impregnated doped alumina is calcined at a temperature of between 500° C. and 1100° C., each for a period of at least 0.5 hours.

8. An alumina supported perovskite type oxide composition prepared according to the method of claim 1.

9. An alumina supported perovskite type oxide composition composite comprising the following features: i) at least 50 wt. % of a doped alumina; and ii) between 5 and 50 wt. % of a perovskite type oxide of formula:
ABO.sub.3, wherein: A comprises a rare-earth element, an alkaline earth element, an alkali element, Pb.sup.2+, Bi.sup.3+ or mixtures thereof and B comprises transition metals including mixtures of transition metals; and ABO.sub.3 being characterized by having a crystallite size of less than 5 nm after calcination at 850° C. for 3 hours and having a crystallite size of less than 2 nm after calcination at 700° C. for 4 hours.

10. The alumina supported perovskite type oxide composition of claim 8 having a weighted intensity ratio R of less than 10, calculated from the reflections at about 2θ=32° and 2θ=46° of an X-ray diffraction pattern of the composition obtained by Copper K-alpha emission having a wavelength of 1.54 Å and equation 1:
R=[(I.sub.32/I.sub.46)]/m.sub.p  (equation 1) I.sub.32: Intensity of the reflection around 32° I.sub.46: Intensity of the reflection around 46° m.sub.p: mass of perovskite/(mass of perovskite (calculated as ABO.sub.3)+mass of alumina).

11. The alumina supported perovskite type oxide composition of claim 8 wherein A comprises a mixture of one alkaline earth element and one rare-earth element.

12. The alumina supported perovskite type oxide composition of claim 8 wherein B comprises a mixture of two distinct transition metals.

13. The alumina supported perovskite type oxide composition of claim 8 further characterized by comprising a specific surface area between 50 and 300 m.sup.2/g, and a pore volume of between 0.1 and 1.5 ml/g.

14. The alumina supported perovskite type oxide composition of claim 9 obtainable according to a method comprising the steps of: i) providing a doped alumina, the doped alumina comprising: a. alumina and a rare-earth oxide, or b. alumina and an alkaline earth oxide, or c. alumina and a mixture of a rare-earth oxide and an alkaline earth oxide, wherein the doped alumina is provided by a method comprising at least the following steps: D) preparing a boehmite suspension, the boehmite suspension comprising a boehmite; E) preparing an aqueous salt solution, the aqueous salt solution comprising a. a rare-earth salt, or b. an alkaline earth salt, or c. a mixture of a rare-earth salt and an alkaline earth salt; F) combining the boehmite suspension with the aqueous salt solution to form a boehmite salt mixture; D) drying the boehmite salt mixture to form a dried boehmite salt mixture; and E) calcining the dried boehmite salt mixture to form a doped alumina; ii) impregnating the doped alumina with an impregnation aqueous solution, the impregnation aqueous solution comprising one or a mixture of water soluble rare-earth salts, water soluble alkaline earth salts, water soluble alkali salts, water soluble salts of Pb.sup.2+, water soluble salts of Bi.sup.3+, and water soluble transition metal salts to form an impregnated doped alumina; and iii) calcining the impregnated doped alumina.

15. The alumina supported perovskite type oxide composition of claim 8 wherein the doped alumina comprises no LaAlO.sub.3.

16. (canceled)

17. The alumina supported perovskite type oxide composition of claim 9 having a weighted intensity ratio R of less than 10, calculated from the reflections at about 2θ=32° and 2θ=46° of an X-ray diffraction pattern of the composition obtained by Copper K-alpha emission having a wavelength of 1.54 Å and equation 1:
R=[(I.sub.32/I.sub.46)]/m.sub.p  (equation 1) I.sub.32: Intensity of the reflection around 32° I.sub.46: Intensity of the reflection around 46° m.sub.p: mass of perovskite/(mass of perovskite (calculated as ABO.sub.3)+mass of alumina).

18. The alumina supported perovskite type oxide composition of claim 9 wherein A comprises a mixture of one alkaline earth element and one rare-earth element.

19. The alumina supported perovskite type oxide composition of claim 9 wherein B comprises a mixture of two distinct transition metals.

20. The alumina supported perovskite type oxide composition of claim 9 further characterized by comprising a specific surface area between 50 and 300 m.sup.2/g, and a pore volume of between 0.1 and 1.5 ml/g.

21. The alumina supported perovskite type oxide composition of claim 9 wherein the doped alumina comprises no LaAlO.sub.3.

Description

[0067] The invention will now be described with reference to the following non-limiting examples and Figures in which:

[0068] FIG. 1 represents the X-ray diffraction pattern of Example 1 after calcination at 700° C. for 4 hours and after calcination at 850° C. for 3 hours where the reflection marked with an asterisk (*) shows a reflection of the X-ray pattern of a perovskite type oxide;

[0069] FIG. 2 represents the X-ray diffraction pattern of Example 2 after calcination at 700° C. for 4 hours and after calcination at 850° C. for 3 hours where the reflection marked with an asterisk (*) shows the reflection of the X-ray pattern of perovskite type oxide;

[0070] FIG. 3 represents the X-ray diffraction pattern of Example 2 and Comparative Example 1 after calcination at 700° C. for 4 hours and after calcination at 850° C. for 3 hours;

[0071] FIG. 4 represents the X-ray diffraction pattern of Example 2 and Comparative Example 2 after calcination at 700° C. for 4 hours and after calcination at 850° C. for 3 hours where the reflection marked with an asterisk (*) shows the reflection of the X-ray pattern of perovskite type oxide and the reflection marked with a hash (#) the reflection of the X-ray pattern of SrAl.sub.2O.sub.4 and the reflections marked with plus (+) the reflections of the X-ray pattern of SrCO.sub.3;

[0072] FIG. 5 represents the X-ray diffraction pattern of Example 3 and Comparative Example 3 after calcination at a temperature of 700° C. for 4 hours where the reflections marked with an asterisk (*) show the X-ray pattern of perovskite type oxide.

[0073] Homogeneity is measured by scanning-electron-microscope (SEM) cross-section imaging, optionally together with EDX (Energy Dispersive X-ray Analysis) element mapping revealing the domain sizes of the doped alumina and perovskite type oxide.

[0074] The crystal size of the perovskite type oxide is determined by using the Debye-Scherrer method analyzing the (022)-reflection (in space group Fm-3c). It is less than 5 nm when determined after a calcination at 850° C. for 3 hours and less than 2 nm when determined after a calcination at 700° C. for 3 hours.

[0075] Specific surface area and pore volume are measured with N.sub.2 physisorption using typical volumetric devices like the Quadrasorb from Quantachrome at the temperature of liquid nitrogen. The specific surface area is determined using BET theory (DIN ISO 9277) while the pore volume is determined according to DIN 66131.

EXAMPLES

Example 1—Composite with 20 wt. % of Perovskite La.SUB.0.5.Sr.SUB.0.5.Fe.SUB.0.5.Ti.SUB.0.5.O.SUB.3

[0076] A gamma alumina containing 10 wt. % La.sub.2O.sub.3 was prepared by mixing an aqueous solution of Lanthanum acetate with a suspension of 5 wt. % boehmite in water. The mixture was subsequently spray dried and calcined at 500° C. for 1 h.

[0077] The La doped alumina was impregnated by incipient wetness impregnation with a mixed solution of Sr-acetate, Ammonium-Iron-Citrate and Tyzor LA (titanium solution) to achieve a loading of 4.8 wt. % SrO, 3.8 wt. % Fe.sub.2O.sub.3 and 3.8 wt. % TiO.sub.2 after calcination. The product was calcined at 850° C. for 3 h and 700° C. for 4h, respectively.

[0078] FIG. 1 shows the X-ray diffraction pattern of the material obtained by Example 1 after calcination at 850° C. for 3 hours and at 700° C. for 4 hours.

Example 2—Composite with 20 wt. % of Perovskite La.SUB.0.5.Sr.SUB.0.5.Fe.SUB.0.5.Zr.SUB.0.5.O.SUB.3

[0079] A gamma alumina containing 7.8 wt. % La.sub.2O.sub.3 was prepared by mixing an aqueous solution of Lanthanum acetate with a suspension of 5 wt. % boehmite in water. The mixture was subsequently spray dried and calcined at 500° C. for 1 h.

[0080] The doped alumina was impregnated by incipient wetness impregnation with a mixed solution of Ammonium-Iron-Citrate, Zr-acetate and Sr-acetate to achieve a loading of 3.4 wt. % Fe.sub.2O.sub.3, 5.3 wt. % ZrO.sub.2 and 4.4% SrO. The product was calcined at 850° C. for 3 h and 700° C. for 4 h, respectively.

[0081] The X-ray diffraction pattern of the material obtained after calcination at 850° C. for 3 hours and at 700° C. for 4 hours calcination is shown in FIG. 2.

Comparative Example 1—Composite with 20 wt. % of Perovskite La.SUB.0.5.Sr.SUB.0.5.Fe.SUB.0.5.Ti.SUB.0.5.O.SUB.3

[0082] The composite was prepared according to Example 5 of U.S. Pat. No. 4,921,829.

[0083] LaAlO.sub.3 powder was first synthesized by adding 100.9 g of gamma alumina to 400 ml of an aqueous solution of 425 g of lanthanum nitrate hexahydrate. The resultant mixture was evaporated and dried. Thereafter, the mixture was calcined in air at 600° C. for 3 hours and further at 900° C. for 8 hours to obtain the LaAlO.sub.3 powder.

[0084] In a second step the LaAlO.sub.3 powder was mixed with an aqueous solution of nitrates of lanthanum, strontium, iron and zirconium in quantities to achieve a loading of 3.4 wt. % Fe.sub.2O.sub.3, 5.3 wt. % ZrO.sub.2, 4.4 wt. % SrO and additional 6.9 wt. % La.sub.2O.sub.3, in the calcined composite. The resultant mixture was dried in air at 110° C. for 10 hours and calcined at 700° C. for 4 h and 850° C. for 3 h, respectively.

[0085] The X-ray diffraction pattern of the material obtained after calcination at 850° C. for 3 h and at 700° C. for 4 h calcination is shown in FIG. 3.

[0086] The results show that the obtained product differs from the compositions of the present invention in the way that no alumina is present, the perovskite crystal size is higher, and the specific surface area is substantially lower.

Comparative Example 2— Composite with 20 wt. % of Perovskite La.SUB.0.5.Sr.SUB.0.5.Fe.SUB.0.5.Ti.SUB.0.5.O.SUB.3

[0087] The composite was prepared according to Example 6 of U.S. Pat. No. 5,882,616.

[0088] 25 g of gamma-alumina beads were impregnated twice with an aqueous solution containing nitrates of Lanthanum, strontium, iron and zirconium in quantities to achieve a loading of 6.9 wt. % La.sub.2O.sub.3, 3.4 wt. % Fe.sub.2O.sub.3, 5.3 wt. % ZrO.sub.2 and 4.4 wt. % SrO in the calcined composite, 5 g ethanol and 10 g citric acid. The resultant material was dried under vacuum following the first impregnation (to remove the solution). After the second impregnation the product was calcined at 700° C. for 4 h and 850° C. for 3 h, respectively.

[0089] The X-ray diffraction pattern of the material obtained after calcination at 850° C. for 3 hours and at 700° C. for 4 hours calcination is shown in FIG. 4.

[0090] The powder X-ray diffraction pattern reveals phases that significantly differ from the compositions of the present invention. In detail, the strontium does not form part of the perovskite structure but is instead present in the form of SrCO.sub.3 after calcination at 700° C. and in the form SrAl.sub.2O.sub.4 after calcination at 850° C. Therefore, it can be concluded that this procedure is not suitable for forming the desired composition. The Results are included in Table 1 hereunder.

TABLE-US-00001 TABLE 1 Comparative Comparative Example 1 Example 2 Example 1 Example 2 Perovskite La0.5Sr0.5 La0.5Sr0.5 La0.5Sr0.5 La0.5Sr0.5 molar Fe0.5 Fe0.5 Fe0.5 Fe0.5 composition Ti0.5O3 Zr0.5O3 Ti0.5O3 Ti0.5O3 Perovskite 20 20 20 20 wt. % Other phase Al.sub.2O.sub.3 Al.sub.2O.sub.3 LaAlO.sub.3 SrAl.sub.2O.sub.4, present Al.sub.2O.sub.3 Crystal size 2.7 nm <2 nm 16 nm 7 nm (3 h 850.sup.o C.) Crystal size <2 nm <2 nm 6 nm n.a. (4 h 700° C.) Intensity 0.85 0.87 n.a. n.d. ratio I32/I46 Weighted 4.3 4.3 n.a. n.d. intensity ratio R (see equation 1) BET 115 m.sup.2/g 122 m.sup.2/g 22 n.d. PV 0.84 ml/g 0.91 ml/g 0.18 n.d. n.d. = not detected, n.a. = not applicable

Example 3—Composite with 10 wt. % of Perovskite LaFeO.SUB.3

[0091] A gamma alumina containing 11.7 wt. % La.sub.2O.sub.3 was prepared by mixing an aqueous solution of Lanthanum acetate with a suspension of 5 wt. % boehmite in water. The mixture was subsequently spray dried and calcined at 500° C. for 1 h.

[0092] The doped alumina was impregnated by incipient wetness impregnation with a solution of Ammonium-Iron-Citrate to achieve a loading of 3.3 wt. % Fe.sub.2O.sub.3. The product was calcined at 850° C. for 3 h and 700° C. for 4 h, respectively.

[0093] The X-ray diffraction pattern of the material obtained after 700° C., 4 h calcination is shown in FIG. 5.

Comparative Example 3—Composite with 10 wt. % of Perovskite LaFeO.SUB.3

[0094] The composite was prepared according to Example 3 of US2012/0046163A1.

[0095] A mixture of 2.2 g iron acetate in 75 ml water and 4.46 g lanthanum acetate in 75 ml water were mixed and added to a dispersion that was prepared by mixing 27 g of lanthanum doped alumina (commercially available as PURALOX TH100/150 L4) and 150 ml water. 11.2 g of 25% NH.sub.3 solution were added to this mixture to reach a pH of 10. After stirring for 1.5 h the precipitate was filtered and the obtained powder calcined 4 h at 700° C.

[0096] The X-ray diffraction pattern of the material obtained after 700° C., 4 h calcination is shown in FIG. 5.

[0097] The comparison of the X-ray diffraction pattern patterns of the materials as per Example 3 and Comparative Example 3 clearly indicates the difference in crystallinity of the perovskite phases.

[0098] A crystalline perovskite phase can be detected for Comparative Example 3 as indicated by the asterisk in FIG. 2, whereas the perovskite reflection is very weak. Therefore, it can be concluded that perovskite phase exists in a nearly X-ray amorphous state.

[0099] The results are included in Table 2 hereunder.

TABLE-US-00002 TABLE 2 Comparative Example 3 Example 3 Perovskite molar LaFeO.sub.3 LaFeO.sub.3 composition Perovskite wt. % 10 10 Crystal size (4 h 700° C.) <2 nm Intensity ratio I32/I46 0.73 1.7 Weighted intensity 7.3 17 ratio R (see equation 1) BET 123 m.sup.2/g 119 m.sup.2/g PV 0.96 ml/g 0.85 ml/g