Metal doped zeolite and process for its preparation

Abstract

A metal-doped or metal-exchanged zeolite is disclosed, wherein the doping metal is present in the zeolite in the form of individual atoms i.e. as monomeric and/or dimeric species. Further, a process for the preparation of such a metal-doped or metal-exchanged zeolite is disclosed. The metal-doped zeolites are useful, in particular, as catalysts for the reduction of nitrogen oxides.

Claims

1. A process for the preparation of a metal-exchanged zeolite comprising providing an aqueous suspension of a zeolite having a zeolite skeleton in a sealable reaction vessel and then: a) increasing the pH of the suspension to a value in the range from 8 to 10 using NH.sub.4OH, and setting the oxygen level of the reaction vessel to a value of <10%, b) reducing the pH to a value in the range from 1.5 to 6, c) adding a metal salt including a replacement metal and reacting the suspension over a period of 1 to 15 hours to prepare a metal-exchanged zeolite, and d) filtering off and washing the metal-exchanged zeolite.

2. The process according to claim 1, wherein the suspension of step a) contains 5 to 25 wt.-% zeolite.

3. The process according to claim 1, wherein a molar ratio of ammonia to zeolite at the end of step a) has a value of 0.01 to 0.1.

4. The process according to claim 1, wherein the pH is reduced in step b) by adding a mineral acid.

5. The process according to claim 4, wherein the pH in step b) is set to a value in the range from 1.5 to 3.

6. The process according to claim 4, wherein, after reducing the pH in step b), the suspension is heated to a temperature in the range from 80 to 100 C.

7. The process according to claim 6, wherein, during the exchange reaction in step c), the oxygen level in the reaction vessel is less than 5%.

8. The process according to claim 7, wherein the zeolite obtained after step d) is dried at a temperature of greater than 100 C.

9. The process according to claim 7, wherein the process is carried out several times.

10. The process according to claim 8, wherein the dried zeolite is calcined at a temperature of 400 to 600 C.

11. The process according to claim 10, wherein the calcination takes place under inert gas.

12. The process according to claim 1, wherein the zeolite is selected from the group consisting of AEL, BEA, CHA, EUO, FAO, FER, KFI, LTA, LTL, MAZ, MOR, MEL, MTW, LEV, OFF, TON, ERI, and MFI.

13. The process according to claim 12, wherein the zeolite is selected from the group consisting of BEA, MFI, FER, MOR, MTW, and ERI.

14. The process according to claim 1, wherein a pore size of the zeolites is from 0.4 to 1.5 nm.

15. The process according to claim 14, wherein the replacement metal is catalytically active.

16. The process according to claim 15, wherein the replacement metal is selected from the group consisting of Fe, Co, Ni, Ag, Cu, V, Rh, Pd, Pt, and Ir.

17. The process according to claim 16, wherein the replacement metal is selected from the group consisting of Fe, Co, Ni, and Cu.

18. The process according to claim 16, wherein the replacement metal is provided in step c) in a quantity of 1 to 5 wt.-% calculated as metal oxide relative to the total weight of the zeolite.

19. The process according to claim 18, wherein 70% to 90% of the exchangeable sites of the zeolite obtained in step d) are occupied by the replacement metal.

20. A process for the preparation of a metal-exchanged zeolite comprising providing an aqueous suspension of a zeolite having a zeolite skeleton in a sealable reaction vessel and then: a) increasing the pH of the suspension to a value in the range from 8 to 10 using a base, and setting the oxygen level of the reaction vessel to a value of <10%, b) reducing the pH to a value in the range from 1.5 to 6, c) adding a metal salt including a replacement metal and reacting the suspension over a period of 1 to 15 hours to prepare a metal-exchanged zeolite, and d) filtering off and washing the metal-exchanged zeolite.

Description

Example 1

(1) Preparation of an Iron-Exchanged (or Doped) Zeolite

(2) 2 g NH.sub.4-ZSM5 (alternatively H-ZSM5 or Na-ZSM5 was also used) was suspended in an aqueous solution in a quantity of 10 to 15 wt.-% relative to the aqueous solution, and stirred at room temperature. Ammonia in the form of ammonia water was then added in a ratio of NH.sub.3 to zeolite of 0.04, with the result that a pH of >9 was set. The pH was at most 10.

(3) The reaction vessel was then closed, flushed with inert gas and the atmosphere inside the vessel set to an oxygen level of <5%, followed by a wait of 20 min.

(4) Sulphuric acid in the form of dilute sulphuric acid concentration (25 vol. %) was then added, wherein the ratio of sulphuric acid to zeolite was 0.05. The oxygen level was left at <5% and a pH of 4.0 set. The acidified suspension was then heated to a temperature of 90 and thereafter immediately solid FeSO.sub.47 H.sub.2O added in a weight ratio of FeSO.sub.4:7 H.sub.2O to zeolite of 0.2. The pH was 3 and the reaction was carried out in the reaction vessel over 8 hours accompanied by stirring at an oxygen level of less than 5%.

(5) The exchanged zeolite, which had a virtually white colour, was then filtered and washed three times with distilled water and dried at 150 C. Calcination took place under inert gas at 500 C. for 3 hours.

(6) The resulting product contained 1.5 wt.-% Fe.sub.2O.sub.3 relative to the total mass of zeolite.

(7) UV/VIS Spectrum:

(8) The thus-obtained iron-exchanged zeolite showed no bands in the wavelength range of 10-25,000 cm.sup.1 assigned to the polynuclear Fe clusters (i.e. more than 3 Fe atoms) which are only slightly, if at all, active during the SCR reaction.

(9) On the other hand, the iron-exchanged zeolite according to the invention shows thick bands in the wavelength range of 25,000 to 30,000 cm.sup.1, which are allocated to iron-oxide dimers (FeOFe) which are highly active during the SCR reaction. Likewise, the zeolite obtained in example 1 showed bands in the wavelength range between 30,000 and 50,000 cm.sup.1, which can be allocated to monomers, iron species arranged in the zeolite lattice, or in particular monomeric FeOH species which are likewise highly active for the SCR reaction.

(10) A catalyst prepared by means of the iron zeolite according to the invention obtained in example 1 thus does not have polynuclear catalytically inactive iron clusters.

Example 2

(11) Preparation of a Cobalt-Exchanged Zeolite

(12) The reaction took place as in example 1, except that in place of iron sulphate (FeSO.sub.47 H2O), a corresponding quantity of Co(NO.sub.3).sub.2 or alternatively Co(acac).sub.2((acac)=acetylacetonate) was used. Instead of sulphuric acid, the corresponding quantity of 0.01 M HNO.sub.3 was used.

(13) UV/VIS: 38-45,000 cm.sup.1 (m) (SCR active monomers and dimeric Co-centres), no bands at 10 to 15,000 cm.sup.1 (cluster species).

Example 3

(14) Preparation of a Copper-Exchanged Zeolite

(15) Synthesis took place as in example 1, except that a corresponding quantity of copper-acetylacetonate solution was used as metal salt and again 0.01 M HNO.sub.3 as acid.

(16) UV/VIS: 37-45,500 cm.sup.1 (m) (SCR active monomers and dimeric Cu-centres), no bands at 10 to 15,000 cm.sup.1 (cluster species).

Example 4

(17) Preparation of a Silver-Exchanged Zeolite

(18) Synthesis took place as in example 1, except that AgNO.sub.3 was used as metal salt and 0.01 M HNO.sub.3 as acid.

Example 5

(19) Preparation of a Nickel-Exchanged Zeolite

(20) Synthesis took place as in example 1, except that Ni(NO.sub.3).sub.2 was used as metal salt and 0.01 M HNO.sub.3 as acid.

Example 6

(21) The catalyst obtained in example 1 was tested during the reduction of NO to N.sub.2.

(22) The catalyst obtained according to the invention was hydrothermally aged at 800 C. for 12 hours in an atmosphere with 10% water vapour, then pressed into shaped bodies and sieved to a size of 0.4 to 0.8 mm.

(23) Likewise, a comparison catalyst which was obtained according to the example of EP 955 080 B1 was hydrothermally aged in the same manner. This comparison catalyst was obtained by solid-state ion exchange.

(24) The exhaust-gas composition for the comparison test was:

(25) NO: 500 ppm

(26) NH.sub.3: 500 ppm

(27) H.sub.2O: 50 vol. %

(28) SV (space velocity): 80,000

(29) Remainder N.sub.2.

(30) The test was carried out under customary test conditions.

(31) TABLE-US-00001 TABLE 1 Conversion of NO to N.sub.2 Fe zeolite Comparison Fe Temperature example 1 zeolite 200 C. 35% 24% 350 C. 95% 88% 500 C. 92% 88%

(32) As can be seen from Table 1, the conversion of NO to N.sub.2 using a catalyst obtained according to the invention is clearly higher at 350 C. than when using a comparison catalyst from the state of the art. The temperature of 350 C. is approximately the optimum temperature for the above-named DeNO.sub.x processes.

(33) The increase in activity at 350 C. was thus approx. 58%.

(34) It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.