SCR-Active Material Having Enhanced Thermal Stability

Abstract

The invention relates to an SCR-active material, comprising a small-pore zeolite of the structure type levyne (LEV), aluminum oxide, and copper, characterized in that, based on the total material, the material contains 4 to 25 wt % of aluminum oxide.

Claims

1. An SCR-active material comprising (i) a small-pore zeolite of the levyne (LEV) structure type, (ii) aluminum oxide, and (iii) copper, wherein the copper is present in a first concentration on the aluminum oxide and in a second concentration on the small-pore zeolite, wherein it contains 4 to 25 wt % aluminum oxide, relative to the total SCR-active material.

2. The SCR-active material according to claim 1, wherein it contains 6 to 16 wt % aluminum oxide, relative to the total SCR-active material.

3. The SCR-active material according to claim 1, wherein the total amount of copper, calculated as CuO and relative to the total SCR-active material, is 0.5 to 15 wt %.

4. The SCR-active material according to claim 1, wherein the small-pore zeolite of the levyne (LEV) structure type is an aluminosilicate.

5. The SCR-active material according to claim 4, wherein the small-pore zeolite of the levyne (LEV) structure type has an SAR value of 5 to 50.

6. The SCR-active material according to claim 1, wherein the small-pore zeolite of the levyne (LEV) structure type is a silicoaluminosilicate or an aluminophosphate.

7. The SCR-active material according to claim 1, wherein the atomic ratio of copper exchanged in the zeolite to skeleton aluminum in the zeolite is 0.25 to 0.6.

8. The SCR-active material according to claim 1, wherein the average crystallite size (d.sub.50) of the small-pore zeolite of the levyne (LEV) structure type is 0.1 to 20 m.

9. The SCR-active material according to claim 1, wherein the small-pore zeolite of the levyne (LEV) structure type forms a core, and the aluminum oxide forms a shell surrounding this core.

10. The SCR-active material according to claim 1, wherein its specific surface area, determined according to ISO 9277, after calcination at 950 C. for 5 hours is above 400 m.sup.2/g.

11. The SCR-active material according to claim 1, wherein the first concentration is higher than the second concentration.

12. The SCR-active material according to claim 1, wherein the first concentration is at least 1.5 times higher than the second concentration.

13. The SCR-active material according to claim 1, wherein it is present in the form of a coating on a carrier substrate or that it was extruded by means of a matrix component to form a substrate.

14. A method for purifying exhaust gas of lean-operated combustion engines, wherein the exhaust gas is passed over an SCR-active material according to claim 1.

15. A device for purifying exhaust gas from lean-operated combustion engines, wherein it comprises an SCR-active material according to claim 1, as well as a means for providing a reducing agent.

16. The device according to claim 15, wherein the means for providing a reducing agent is an injector for aqueous urea solution.

17. The device according to claim 15 and/or 16, wherein it comprises an oxidation catalyst.

18. The device according to claim 15, wherein the means for providing a reducing agent is a nitrogen oxide storage catalyst.

19. A method for producing the SCR-active material according to claim 1, wherein an aqueous suspension of a small-pore zeolite of the levyne (LEV) structure type, copper salt, and aluminum oxide or a precursor compound of aluminum oxide is dried and subsequently calcined.

20. The method according to claim 19, wherein the drying is spray drying.

21. The method according to claim 19, wherein the calcination takes place in air or in an air/water atmosphere at temperatures between 700 C. and 900 C.

Description

EXAMPLE 1: PREPARATION OF A CATALYST EK1 ACCORDING TO THE INVENTION ON A FILTER SUBSTRATE

[0074] An aqueous suspension of copper-exchanged levyne (Cu-LEV, calcined at 850 C. for 2 h) with a SiO.sub.2/Al.sub.2O.sub.3 ratio of 32 and a Cu content of 3.5 wt %, calculated as CuO relative to the zeolite, and a boehmite sol with a content of 20 wt % Al.sub.2O.sub.3 is produced so that the weight percentage of the cooper-exchanged levyne (LEV) is 88% and the weight percentage of Al.sub.2O.sub.3 is 12% in the dried material. The suspension is applied to a commercially available filter substrate in such a way that its loading after drying at 90 C. and calcination at 550 C. with dried material is 110 g/L of substrate volume.

COMPARATIVE EXAMPLE 1: PREPARATION OF A COMPARATIVE CATALYST VK1 ON A FILTER SUBSTRATE

[0075] An aqueous suspension of copper-exchanged levyne (Cu-LEV, calcined at 850 C. for 2 h) with a SiO.sub.2/Al.sub.2O.sub.3 ratio of 32 and a Cu content of 3.5 wt %, calculated as CuO relative to the zeolite, is produced. The weight percentage of the cooper-exchanged levyne (LEV) is 100%. The suspension is applied to a commercially available filter substrate in such a way that its loading after drying at 90 C. and calcination at 550 C. with dried material is 110 g/L of substrate volume.

[0076] Unlike example 1, no boehmite sol is added in comparative example 1.

COMPARATIVE EXAMPLE 2: PREPARATION OF A COMPARATIVE CATALYST VK2 ON A FILTER SUBSTRATE

[0077] An aqueous suspension of copper-exchanged chabazite (Cu-CHA) with a SiO.sub.2/Al.sub.2O.sub.3 ratio of 30 and a Cu content of 4.0 wt %, calculated as CuO relative to the zeolite, is produced. Added thereto is a boehmite sol with a content of 20 wt % Al.sub.2O.sub.3 so that the weight percentage of the cooper-exchanged chabazite (CHA) is 92.6% and the weight percentage of Al.sub.2O.sub.3 is 7.4% in the dried material. The suspension is applied to a commercially available filter substrate in such a way that its loading after drying at 90 C. and calcination at 550 C. with dried material is 110 g/L of substrate volume.

EXAMPLE 2: VARIATION OF THE ALUMINUM OXIDE CONTENT IN THE CATALYSTS ACCORDING TO THE INVENTION (EK2 TO EK5) AND PREPARATION ON A FLOW-THROUGH SUBSTRATE

[0078] Produced are four aqueous suspensions of cooper-exchanged levyne (Cu-LEV, calcined for 2 hours at 850 C.) with a SiO.sub.2/Al.sub.2O.sub.3 ratio of 32 and a Cu content of 3.5 wt %, calculated as CuO relative to the zeolite, and a boehmite sol with a content of 20 wt % Al.sub.2O.sub.3, such that the weight percentage of the cooper-exchanged levyne (Cu-LEV) X and the weight percentage of Al.sub.2O.sub.3 Y vary in the dried materials according to Table 1. The suspensions are each applied to a commercially available flow-through substrate so that the loading of the flow-through substrates after drying at 90 C. and calcination at 550 C. with dried material corresponds to the variable Z in g/L substrate volume. This is a coating with equivalent mass with respect to the cooper-exchanged levyne (Cu-LEV).

TABLE-US-00001 TABLE 1 Designations of the catalysts according to the invention, as well as values of the variables X, Y, and Z X Cu-LEV Y Al.sub.2O.sub.3 Z loading Designation [wt %] [wt %] [g/L] EK2 88 12 112 EK3 90 10 110 EK4 92 8 108 EK5 94 6 106

EXAMPLE 3: VARIATION OF THE AL.SUB.2.O.SUB.3 .ADDITION TO FORM THE MATERIAL ACCORDING TO THE INVENTION (EK6)

[0079] A levyne (LEV) with a SiO.sub.2/Al.sub.2O.sub.3 ratio of 32 is dispersed in an aqueous copper acetate solution and, after 3 h at 80 C. and cooling to room temperature, a boehmite sol with a content of 20 wt % Al.sub.2O.sub.3 is added. In this case, the amounts of reactant used are selected in such a way that, in the dried material, a Cu content of 3.5 wt %, calculated as CuO relative to the amount of levyne (LEV), is present, and the Al.sub.2O.sub.3 weight percentage, relative to the oxidic proportion of the total material, is 4%. With the material obtained after drying and calcining for 2 h at 850 C., an aqueous suspension is produced, with the addition of a boehmite sol with a content of 20 wt % Al.sub.2O.sub.3, so that the weight percentage of Al.sub.2O.sub.3 in the dried material according to the invention is 8%. The suspension is applied to a commercially available flow-through substrate in such a way that its loading after drying at 90 C. and calcining at 550 C. with dried material is 108 g/L substrate volume. This is thus the same loading as in the case of EK4. In contrast to EK4, the same total amount of Al.sub.2O.sub.3 is thus introduced into the material according to the invention in two steps.

EXAMPLE 4: PREPARATION OF EK7 AND EK8 FOR SPECIFIC SURFACE AREA DETERMINATION ACCORDING TO THE BET METHOD

[0080] A levyne (LEV) with a SiO.sub.2/Al.sub.2O.sub.3 ratio of 32 is dispersed in an aqueous copper acetate solution and, after 3 h at 80 C. and cooling to room temperature, a boehmite sol with a content of 20 wt % Al.sub.2O.sub.3 is added, and the mixture is dried.

[0081] In this case, the amounts of reactant used are selected in such a way that a Cu content of 3.5 wt %, calculated as CuO and relative to the amount of levyne (LEV), is present in the dried material, and the Al.sub.2O.sub.3 weight percentage, relative to the oxidic proportion of the total material, is 4% (EK7) or 8% (EK8).

[0082] After drying, the materials EK7 and EK8 produced were calcined for 5 h at 950 C. in air, and the specific surface area was measured according to ISO 9277. The results are presented in Table 2.

TABLE-US-00002 TABLE 2 Specific surface areas of EK7 and EK8 after calcination for 5 h at 950 C. Material Specific surface areas [m.sup.2/g] EK8 514 10 EK7 528 10

COMPARATIVE EXPERIMENTS: DETERMINATION OF THE NOX CONVERSION OF EK1, VK1, VK2, EK2 TO EK6

[0083] EK1 and VK1 were measured after preparation (fresh) and after aging in a hydrothermal atmosphere (10% H.sub.2O, 10% O.sub.2, remainder N.sub.2). VK2 and EK2 to EK6 were measured only after preparation after aging in a hydrothermal atmosphere (10% H.sub.2O, 10% O.sub.2, remainder N.sub.2). The holding times and aging temperatures for EK1, VK1, and VK2 were 4 h at 900 C. and 1 h at 950 C. EK2 to EK5 were aged only for 1 h at 950 C. in hydrothermal atmosphere.

[0084] The NOx conversion of the catalysts EK1, VK1, VK2, and EK2 to EK5 as a function of the temperature upstream of the catalyst was determined in a model gas reactor in the so-called NOx conversion test.

[0085] This NOx conversion test consists of a test procedure that comprises a pre-treatment and a test cycle that is run through for various target temperatures. The applied gas mixtures are noted in Table 3.

[0086] Test Procedure: [0087] 1. Preconditioning at 600 C. in N.sub.2 for 10 min [0088] 2. Test cycle repeated for the target temperatures [0089] a. Approaching the target temperature in gas mixture 1 [0090] b. Addition of NO.sub.x (gas mixture 2) [0091] c. Addition of NH.sub.3 (gas mixture 3), wait until NH.sub.3 breakthrough >20 ppm, or a maximum of 30 min. in duration [0092] d. Temperature-programmed desorption up to 500 C. (gas mixture 3)

TABLE-US-00003 TABLE 3 Gas mixtures of the NOx conversion test. Gas mixture 1 2 3 N.sub.2 Balance Balance Balance O.sub.2 10 vol % 10 vol % 10 vol % NOx 0 ppm 500 ppm 500 ppm NO.sub.2 0 ppm 0 ppm 0 ppm NH.sub.3 0 ppm 0 ppm 750 ppm CO 350 ppm 350 ppm 350 ppm C.sub.3H.sub.6 100 ppm 100 ppm 100 ppm H.sub.2O 5 vol % 5 vol % 5 vol %

[0093] The space velocity in the case of the measurements of EK2 to EK6 was at a space velocity (GHSV) of 60,000 h.sup.1. In the case of EK1, VK1, and VK2, the NOx conversion was determined at 500 C. at a space velocity (GHSV) of 60,000 h.sup.1. From 500 C., the space velocity (GHSV) was 100,000 h.sup.1.

[0094] For each temperature point below 500 C., the conversion with an NH.sub.3 slip of 20 ppm is determined for test procedure range 2c. For each temperature point above 500 C., the conversion in a state of equilibrium is determined in the test procedure range 2c. Plotting this NOx conversion for the various temperature points results in a plot as shown in FIGS. 1, 3, and 4.

[0095] Comparison of the Catalytic Activity of EK1 and VK1, as Well as VK2:

[0096] FIG. 1 shows that EK1, in comparison to VK1, has significantly improved NOx conversions over the temperature range under consideration after hydrothermal aging for 4 h at 900 C. and, particularly clearly, after hydrothermal aging for 1 h at 950 C. This is due to the material according to the invention produced by adding Al.sub.2O.sub.3.

[0097] FIG. 4 shows that the NOx conversions of VK2 after both aging conditions are substantially below those of EK1 and EK4 (after hydrothermal aging for 1 h at 950).

[0098] Comparison of the Catalytic Activity of EK2 to EK5:

[0099] FIG. 2 shows that, after hydrothermal aging for 1 h at 950 C., stabilization of the NOx conversion at 650 C., with increasing Al.sub.2O.sub.3 weight proportion of EK5 to EK2 in the formed materials according to the invention, takes place.

[0100] Comparison of the Catalytic Activity of EK4 and EK6:

[0101] FIG. 3 shows that, after hydrothermal aging for 1 h at 950 C., a further stabilization of the NOx conversion of the material according to the invention in EK4 is obtained in the temperature range above 350 C., when the addition of Al.sub.2O.sub.3, as shown with the material according to the invention in EK6, takes place in the steps described.