System for the purification of exhaust gas from an internal combustion engine

Abstract

An exhaust gas cleaning system, comprising an active regenerable particulate filter and an SCR catalyst comprising a hydrothermally microporous stable zeolite having the AEI type framework and being promoted with copper, where the zeolite is SSZ-39.

Claims

1. An exhaust gas cleaning system, comprising an active regenerable particulate filter and an SCR catalyst comprising a hydrothermally microporous stable zeolite with the AEI type framework and being promoted with copper, wherein the microporous stable zeolite of the AEI type framework is SSZ-39, wherein the SCR catalyst retains 80% of the initial reduction of nitrogen oxides at 250 C. after the catalyst has been exposed to a temperature of 750 C. and a water vapor content of 100% in the exhaust gas for 13 hours.

2. The exhaust gas cleaning system of claim 1, wherein the SCR catalyst is integrated into the particulate filter.

3. The exhaust gas cleaning system of claim 1, wherein the atomic copper to aluminium ratio in the zeolite is between about 0.01 and about 1.

4. The exhaust gas cleaning system of claim 1, wherein the atomic ratio of silicon to aluminium in the SCR catalyst is between 5 and 50 for the zeolite.

5. The exhaust gas cleaning system of claim 1, wherein the SCR catalyst is deposited on a monolithic support structure.

6. An exhaust gas cleaning system, comprising an active regenerable particulate filter and an SCR catalyst comprising a hydrothermally microporous stable zeolite with the AEI type framework and being promoted with copper, wherein the microporous stable zeolite of the AEI type framework is SSZ-39, wherein the SCR catalyst retains at least 80% to 90% of the initial microporosity after aging at 600 C., and at least 30% to 40% of the initial microporosity after aging at 750 C.

7. The exhaust gas cleaning system of claim 6, wherein the SCR catalyst is integrated into the particulate filter.

8. The exhaust gas cleaning system of claim 6, wherein the atomic copper to aluminium ratio in the zeolite is between about 0.01 and about 1.

9. The exhaust gas cleaning system of claim 6, wherein the atomic ratio of silicon to aluminium in the SCR catalyst is between 5 and 50 for the zeolite.

10. The exhaust gas cleaning system of claim 6, wherein the SCR catalyst is deposited on a monolithic support structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A shows the powder X-ray diffraction (PXRD) pattern of treated Cu-SSZ-39 samples.

(2) FIG. 1B shows the powder X-ray diffraction (PXRD) pattern of treated CHA samples.

(3) FIG. 2 is a summary of the results of Examples 1-4 with the Cu-SSZ-39 and CHA catalysts.

(4) FIG. 3 shows the results of Example 5 with the Cu-SSZ-39 catalyst.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) Hydrothermally stable means that the zeolite and zeotype catalyst have the ability to retain at least 80 to 90% of initial surface area and 80 to 90% microporous volume after exposure to temperatures of at least 600 C. and a water vapour content up to 100 volume % for 13 hours, and at least 30 to 40% of initial surface area and micropore volume after exposure to temperatures of at least 750 C. and a water vapour content up to 100 volume % for 13 hours.

(6) Preferably, the hydrothermally stable zeolite or zeotype with an AEI type framework has an atomic ratio of silicon to aluminium between 5 and 50 for the zeolite or between 0.02 and 0.5 for the zeotype.

(7) The most preferred zeolite or zeotype catalysts for use in the invention are zeolite SSZ-39 and zeotype SAPO-18 both having the AEI framework structures, in which copper is introduced by impregnation, liquid ion exchange or solid ion exchange.

(8) The atomic copper to aluminium ratio is preferred to be between about 0.01 and about 1 for the zeolite. For the zeotype the preferred atomic copper to silicon ratio is correspondingly between 0.01 and about 1.

(9) By means of the above catalysts employed in the invention, 80% of the initial NOx reduction is maintained at 250 C. after aging at 750 C. as compared to 20% for a Cu-CHA catalyst.

(10) Thus, in an embodiment of the invention, 80% of the initial reduction of nitrogen oxides at 250 C. is maintained after the catalyst has been exposed to a temperature of 750 C. and a water vapour content of 100% in the exhaust gas for 13 hours.

(11) In further an embodiment of the exhaust gas cleaning system according to the invention, the SCR catalyst is integrated into the particulate filter.

(12) In the above embodiments, the SCR catalyst can be deposited on a monolithic support structure.

(13) The Cu-SSZ-39 catalyst system has shown an improved performance compared to the typical state-of-the-art Cu-SSZ-13 when similar Si/A1 ratios are compared.

EXAMPLE 1

Cu-SSZ-39 Catalyst Preparation

(14) The zeolite SSZ-39 with the framework type code AEI was synthesized in a similar way as given in U.S. Pat. No. 5,958,370 using 1,1,3,5-tetramethylpiperidinium as the organic template. A gel with the following composition: 30 Si:1.0 Al:0.51 NaOH:5.1 OSDA:600 H.sub.2O, was autoclaved at 135 C. for 7 days, the product filtered, washed with water, dried and calcined in air. The final SSZ-39 had a Si/Al=9.1 measured by ICP-AES.

(15) To obtain the Cu-SSZ-39 the calcined zeolite was ion exchanged with Cu(CH.sub.3COO).sub.2 to obtain the final catalyst with a Cu/Al=0.52 after calcination.

(16) The powder X-ray diffraction (PXRD) pattern of Cu-SSZ-39 after calcination is shown in FIG. 1A.

EXAMPLE 2

Catalytic Testing

(17) The activity of the samples for the selective catalytic reduction of NO.sub.x was tested in a fixed bed reactor to simulate an engine exhaust stream using a total flow rate of 300 mL/min consisting of 500 ppm NO, 533 ppm NH.sub.3, 7% O.sub.2, 5% H.sub.2O in N.sub.2 in which 40 mg catalyst was tested.

(18) The NO.sub.x present in the outlet gases from the reactor were analyzed continuously and the conversion is shown in FIG. 2.

EXAMPLE 3

Test of Hydrothermal Durability

(19) In order to test the hydrothermal stability of the zeolites, steaming treatments were done to the samples. They were exposed to a water feed (2.2 mL/min) at 600 or 750 C. during 13 hours in a conventional oven and afterwards tested similarly to Example 2.

(20) The catalytic results can also be seen in FIG. 2. The samples that underwent a hydrothermal treatment have been marked with 600 or 700 C., depending on the temperature used during the hydrothermal treatment.

(21) Additional characterization has also been performed to all treated samples. PXRD patterns after hydrothermal treatments are shown in FIG. 1A, and BET surface areas, micropore areas, and micropore volumes of treated samples are summarized in Table 1 below.

EXAMPLE 4

Comparative Example with Cu-CHA (Cu-SSZ-13)

(22) A Cu-CHA zeolite was prepared from a gel with the molar composition: SiO.sub.2:0.033 Al.sub.2O.sub.3:0.50 OSDA:0.50 HF:3 H.sub.2O, where the OSDA is N,N,N-trimethyl-1-adamantamonium hydroxide.

(23) The gel was autoclaved at 150 C. for 3 days under tumbling to give a final zeolite product with a Si/Al=12.7 after washing, drying and calcination.

(24) To obtain the Cu-CHA the calcined zeolite was ion exchanged with Cu(CH.sub.3COO).sub.2 to obtain the final catalyst with a Cu/Al=0.54.

(25) The powder X-ray diffraction (PXRD) pattern of Cu-CHA after calcination is shown in FIG. 1B.

(26) This catalyst was also tested according to example 2, and the hydrothermal durability evaluated similarly to example 3. The catalytic results are summarized in FIG. 2 of the drawings. PXRD patterns of treated-CHA samples are shown in FIG. 1B, and textural properties (BET surface area, micropore volume, and micropore area) are summarized on Table 1.

(27) TABLE-US-00001 TABLE 1 Volume mi- BET surface Micropore cropore Sample area (m.sup.2/g) area (m.sup.2/g) (cm.sup.3/g) SSZ-39_Calc 571 568 0.28 SSZ-39_600 C. 554 551 0.28 SSZ-39_750 C. 565 563 0.28 Cu-SSZ-39_600 C. 465 463 0.24 Cu-SSZ-39_750 C. 158 152 0.09 CHA_calc 675 637 0.32 CHA_600 C. 687 645 0.32 CHA_750 C. 674 623 0.31 Cu-CHA_600 C. 633 585 0.29 Cu-CHA_750 C. 50 35 0.02

EXAMPLE 5

Cu-SAPO-18

(28) Silicoaluminophosphate SAPO-18 with the framework type code AEI was synthesized according to [J. Chen, J. M. Thomas, P. A. Wright, R. P. Townsend, Catal. Lett. 28 (1994) [241-248] and impregnated with 2 wt. % Cu. The final Cu-SAPO-18 catalyst was hydrothermally treated in 10% H.sub.2O and 10% O.sub.2 at 750 C. and tested under the same conditions as given in Example 2. The results are shown in FIG. 3 of the drawings.