A METHOD OF MAKING A CATALYTIC COMPOSITION

Abstract

The present invention relates to a method for making a catalytic composition and, in particular, to a composition for treating a NOx-containing exhaust gas. The composition comprises a small-pore zeolite having a SAR of 9-30 and one or more rare earth metals. The method achieves the introduction of higher levels of rare earth (RE) metals into a zeolite than can be achieved with conventional wash-coating approaches.

Claims

1. A method of making a catalytic composition for treating a NOx-containing exhaust gas, wherein the composition comprises a small-pore zeolite having a SAR of 9-30 and one or more rare earth metals, the method comprising: i) providing a large-pore precursor zeolite; ii) introducing one or more rare earth metals into the precursor zeolite by ion exchange and calcination to form a rare earth metal-substituted precursor zeolite; iii) converting the rare earth metal-substituted precursor zeolite into a small pore zeolite in the presence of a structure directing agent.

2. The method according to claim 1, wherein step (iii) comprises forming a synthesis gel comprising the rare earth metal-substituted precursor zeolite and the structure directing agent and then heating the synthesis gel to form the small-pore zeolite.

3. The method according to claim 1, wherein the small pore zeolite comprises the one or more rare earth metals in a total amount of 0.05 to 3.5 wt %, preferably 0.05 to 2 wt %.

4. The method according to claim 1, wherein the small pore zeolite further comprises Cu and/or Fe, preferably in a total amount of 0.1 to 6 wt %, more preferably in a total amount of 1 to 3 wt %.

5. The method according to claim 1, wherein the large-pore precursor zeolite has a USY framework structure type.

6. The method according to claim 1, wherein the small-pore zeolite has a CHA or AEI framework structure type.

7. The method according to claim 1, wherein the structure directing agent is N,N,N, trimethyladamantylammonium hydroxide or a salt thereof.

8. The method according to claim 1, wherein the rare earth metals are selected from Y and Ce and mixtures thereof.

9. A catalytic composition for treating a NOx-containing exhaust gas, wherein the catalytic composition is produced according to the method of claim 1, wherein the small pore zeolite of the catalytic composition has a SAR of 9-30 and one or more rare earth metals.

10. Use of the A method comprising contacting a NOx-containing exhaust gas with the catalytic composition according to claim 9.

Description

[0063] The invention will now be discussed further in relation to the following non-limiting figures, in which:

[0064] FIG. 1 shows XRD patterns of the as-synthesized Y-containing CHA structure made in Example 1, Example 2, Example 3, Example 4, Example 5, Example 6, Example 7, Example 8 and Example 9.

[0065] FIG. 2 shows XRD patterns of the as-synthesized Ce-containing CHA structure made in Example 10, Example 11, Example 12, Example 13, Example 14, Example 15, and Example 16.

[0066] FIG. 3 shows XRD patterns of the activated Y-containing CHA structure made in Example 1, Example 2, Example 3, Example 4, Example 5, Example 6, Example 7, Example 8 and Example 9.

[0067] FIG. 4 shows XRD patterns of the activated Ce-containing CHA structure made in Example 10, Example 11, Example 12, Example 13, Example 14, Example 15, and Example 16.

[0068] FIG. 5 shows XRD patterns of the activated Y-containing CHA structure made in Example 5, Example 6, Example 8, Example 9 and Comparative examples C1 and C2.

[0069] FIG. 6 shows XRD patterns of the activated Ce-containing CHA structure made in Example 12, Example 14, Example 16, and Comparative examples C1, C3 and C4.

[0070] FIG. 7 shows microscopic images of the as-synthesized Y-CHA made in Example 2.

[0071] FIG. 8 shows microscopic images of the as-synthesized Ce-CHA made in Example 10.

[0072] FIGS. 9(a) and(b) are graphs demonstrating the NOx conversion activity and N.sub.2O selectivity, respectively, of the fresh and aged catalysts of example 15 and comparative example C1 tested at temperatures of 150 to 500 C. with a ramp rate of 5 C. per minute.

[0073] FIGS. 10(a) and(b) are graphs demonstrating the NOx conversion activity and N.sub.2O selectivity, respectively, of the fresh and aged catalysts of example 5 and comparative example C1 tested at temperatures of 150 to 500 C. with a ramp rate of 5 C. per minute.

[0074] FIGS. 11(a) and(b) are graphs demonstrating the NOx conversion activity and N.sub.2O selectivity, respectively, of the aged catalysts of examples 5, 6 and comparative example C1 tested at temperatures of 150 to 500 C. with a ramp rate of 5 C. per minute.

[0075] It was noted that some of the as-synthesized form of Y-CHA samples from Examples 1-9 have a non-CHA shoulder peak at 2-theta 12.65 (FIG. 1) but all of the activated forms of the same Y-CHA do not have such a shoulder peak (FIG. 3). This shoulder peak appears and grows with the increase of yttrium content in Y-CHA.

[0076] Unlike Y-CHA, as-synthesized Ce-CHA exhibits CHA-only XRD peaks (FIG. 2).

[0077] The overlaying XRD patterns of activated Y-CHA (Example 5, 6, 8, 9) and Ce-CHA (Examples 12, 14, 16) and RE-free CHA with similar SAR (Comparative Example C1, C2, C3, C4) show very well matching diffractograms in terms of peak broadening and peak positions (FIGS. 5 and 6).

[0078] The positions of RE atom in CHA structure can be either in framework as isomorphous substitution of T-atoms or off framework as extra framework species inside cages. Framework RE can also be expelled from a framework position to extra-framework position in post synthesis processing steps such as calcination treatment.

[0079] The well-shaped cube-like crystals with uniform size 0.5-1.0 m of RE-CHA from Example 2 and 10 are similar to RE-free CHA made from comparable syntheses (FIGS. 7 and 8).

[0080] As shown in FIGS. 9(a) and(b), catalysts formed according to the method of the present invention and containing 0.41 wt % of ceria with a SAR of 13.7 (Example 15) demonstrated similar fresh NOx conversion and N.sub.2O selectivity as the comparative example C1 which employ a CuCHA zeolite with approximately the same SAR of 13.

[0081] However, as shown in the same FIGS. 9(a) and(b), catalysts formed according to the method of the present invention and containing 0.41 wt % of ceria with a SAR of between 13.7 (Examples 15) demonstrate significantly improved aged NOx conversion and N.sub.2O selectivity over temperatures of 150 to 500 C.

[0082] As shown in FIGS. 10(a) and(b), catalysts formed according to the method of the present invention and containing 0.24 wt % Y with a SAR of 13.9 (Example 5) demonstrated the same fresh activity of the comparative example C1. However, after ageing, Example 5 shows significantly improved NOx conversion and N.sub.2O selectivity over temperatures of 150 to 500 C. compared to comparative example C1, which employs a CuCHA zeolite. It is noted that the amount of copper for both of these catalysts is the same at 2.75 wt % and so the improvement in activity is attributed to the 0.24% wt Y loading achieved by forming the zeolite according to the method of the present invention.

[0083] A similar effect is achieved where the Y loading for the catalysts is 0.11 wt %, as shown in FIGS. 11(a) and(b), which demonstrate the aged NOx conversion and N.sub.2O selectivity of Examples 5, 6 and comparative example C1.

[0084] Therefore, FIGS. 9 to 11 demonstrate that inclusion of rare earth metals, such as ceria and yttria, in a zeolite by forming the zeolite according to the method of the present invention, achieves improved aged NOx conversion and N.sub.2O selectivity.

[0085] Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the scope of the invention or of the appended claims.