Method for the removal of nitrous oxide from off gas in presence of catalyst comprising an Fe-AEI zeolite material essentially free of alkali metal

Abstract

A method for the removal of nitrous oxide from off gas by direct decomposition or by selective catalytic reduction in presence of a reducing agent, comprising the steps of contacting the gas directly or together with the reducing agent or a precursor thereof with a catalyst comprising an Fe-AEI zeolite material essentially free of alkali metal ions (Alk) and having the following molar compositions:
SiO.sub.2: oAl.sub.2O.sub.3: pFe: qAlk wherein o is in the range from 0.001 to 0.2; wherein p is in the range from 0.001 to 0.2; wherein Alk is one or more of alkali ions and wherein q is less than 0.02.

Claims

1. A method for the removal of nitrous oxide from off gas, which comprises removing nitrous oxide from the off gas by contacting the off gas directly or together with a reducing agent or a precursor thereof with a catalyst comprising an Fe-AEI zeolite material essentially free of alkali metal ions (Alk), said Fe-AEI zeolite material having the following molar compositions:
SiO.sub.2: oAl.sub.2O.sub.3: pFe: qAlk wherein o is in the range from 0.001 to 0.2; wherein p is in the range from 0.001 to 0.2; and wherein Alk is one or more of alkali ions and wherein q is less than 0.02, via (a) direct decomposition wherein the catalyst is located inside an ammonia burner, or (b) selective catalytic reduction wherein the catalyst is located after the production of nitric acid wherein nitrogen dioxide is adsorbed in water and results in nitrous oxide.

2. The method according to claim 1, wherein o is in the range from 0.005 to 0.1, p is in the range from 0.005 to 0.1 and q is below 0.005.

3. The method according to claim 1, wherein o is in the range from 0.02 to 0.07, p is in the range from 0.01 to 0.07 and q is below 0.001.

4. The catalyst of claim 1, wherein Alk is sodium.

5. The method of claim 1, wherein the off gas contains more than 0.05% steam.

6. The method of claim 1, wherein the off gas is at a temperature of above 200° C.

7. The method of claim 1, wherein the catalyst is coated within or on a substrate.

8. The method of claim 7, wherein the substrate is a metallic substrate or an extruded ceramic substrate or a corrugated ceramic substrate.

9. The method of claim 7, wherein the substrate is in form of a flow-through monolith, a flow-through honeycomb, or a wall-flow filter.

10. The method of claim 7, wherein the catalyst is coated in an amount of between 10 and 600 g/L calculated on the weight of catalyst material per volume of the total substrate including the catalytic material.

11. The method of claim 10, wherein the amount is between 100 and 300 g/L.

12. The method of claim 7, wherein the catalyst is coated in or on the porous substrate in form of a wash coat comprising the catalyst and a binder comprising TiO.sub.2, SiO.sub.2, Al.sub.2O.sub.3, ZrO.sub.2, CeO.sub.2, and combinations thereof.

13. The method of claim 7, wherein the catalyst is coated as a layer on the substrate and wherein the substrate comprises one or more other layers comprising a catalyst with a different catalytic activity or on other zeolite catalysts.

14. The method of claim 7, wherein the catalyst is zone coated on the substrate.

15. The method of claim 1, wherein the off gas is formed during production of nitric acid or adipic acid.

16. The method according to claim 1, wherein the reducing agent is selected from the group consisting of ammonia, hydrocarbon, nitrogen monoxide, and mixtures thereof.

17. The method according to claim 1, wherein the reducing agent is added in controlled amounts to the off gas upstream of the catalyst.

18. The method according to claim 1, wherein nitrogen oxides are reduced by an additional catalyst unit, by the addition of a reducing agent, or both.

19. The method of claim 18, wherein the additional catalyst unit comprises the Fe-AEI zeolite material essentially free of alkali metal ions (Alk).

20. The method according to claim 18, wherein the reducing agent is selected from the group consisting of ammonia and hydrocarbons.

21. The method according to claim 20, wherein the catalyst comprising an Fe-AEI zeolite material essentially free of alkali metal ions (Alk) is combined with a second catalyst composition active in the selective reduction of nitrogen oxides.

22. The method according to claim 1, wherein the catalyst is located after an ammonia oxidation catalyst or after an ammonia oxidizer.

Description

BRIEF DESCRIPT OF THE DRAWINGS

(1) FIG. 1 is a Powder X-ray diffraction pattern of as-prepared silicoaluminate AEI zeolite synthesized according to Example 1;

(2) FIG. 2 is a Powder X-ray diffraction pattern of as-prepared direct synthesis of Fe- and Na-containing silicoaluminate AEI zeolite synthesized according to the Example 2;

(3) FIG. 3 is a NO.sub.x conversion over Fe-AEI zeolite catalyst with and without Na present;

(4) FIG. 4 is a NO.sub.x conversion over Fe-AEI zeolite catalyst with and without Na present after accelerated hydrothermal aging (conditions given in Example 9);

(5) FIG. 5 is a NO.sub.x conversion over Na-free Fe-AEI compared to state-of-the-art Fe-CHA and Fe-Beta zeolites (also Na-free) after accelerated hydrothermal aging (conditions given in Example 9);

(6) FIG. 6 is a NO.sub.x conversion over Na-free Fe-AEI compared to state-of-the-art Na-free Fe-CHA after severe accelerated hydrothermal aging at 600° C. with 100% H.sub.2O aging; and

(7) FIG. 7 is a SEM image of the Fe-AEI material synthesized according to Example 2.

DETAILED DESCRIPTION OF THE INVENTION

(8) The catalyst for use in the method according to the invention can be prepared by a method, comprising the following steps: (i) preparation of a mixture containing water, a high-silica zeolite as main source of silica and alumina, an alkyl-substituted cyclic ammonium cation as organic structure directing agent (OSDA), a source of iron, and a source of an alkali metal cation [Alk], to obtain a final synthesis mixture having the following molar composition:
SiO.sub.2: aAl.sub.2O.sub.3: bFe: cOSDA: dAlk: eH.sub.2O wherein a is in the range from 0.001 to 0.2; more preferably in the range from 0.005 to 0.1, and more preferably in the range from 0.02 to 0.07. wherein b is in the range from 0.001 to 0.2; more preferably in the range from 0.005 to 0.1, and more preferably in the range from 0.01 to 0.07. wherein c is in the range from 0.01 to 2; more preferably in the range from 0.1 to 1, and more preferably in the range from 0.1 to 0.6. wherein d is in the range from 0.001 to 2; more preferably in the range from 0.05 to 1, and more preferably in the range from 0.1 to 0.8. wherein e is in the range from 1 to 200; more preferably in the range from 1 to 50, and more preferably in the range from 2 to 20. (ii) crystallization of the mixture achieved in (i) in a reactor. (iii) recovery of the crystalline material achieved in (ii); (iv) removal of the OSDA occluded in the zeolite structure by calcination of the crystalline material from step (iii); (v) ion exchange of the alkali metal cation present in the crystalline material after step (iv), with ammonium or proton cations to obtain a final crystalline zeolite catalyst material with a low alkali content

(9) Preferably, the high-silica zeolite structure used as a main source of silica and alumina has a Si/Al ratio above 5. Even more preferable the high silica zeolite has the FAU structure, e.g. Zeolite-Y.

(10) The iron source can be selected from iron oxides or iron salts, such as chlorides and other halides, acetates, nitrates or sulfates, among others, and combinations of them. The iron source can be introduced directly in the mixture of (i), or previously combined with the crystalline source of Si and Al.

(11) Any alkyl-substituted cyclic ammonium cation can be used as OSDA. Preferred are N,N-dimethyl-3,5-dimethylpiperidinium (DMDMP), N,N-diethyl-2,6-dimethylpiperidinium, N,N-dimethyl-2,6-dimethylpiperidinium, N-ethyl-N-methyl-2,6-dimethylpiperidinium, and combinations of them.

(12) In step (i) any alkali cation can be used, such as sodium, potassium, lithium, and cesium and combinations of them.

(13) In the crystallization step (ii), hydrothermal treatment is performed in an autoclave, under static or dynamic conditions. The preferred temperature is in the range of between 100 and 200° C., more preferably in the range of 130 to 175° C.

(14) The preferred crystallization time is ranged from 6 hours to 50 days, more preferably in the range of 1 to 20 days, and more preferably in the range of 1 to 7 days. It should be taken into consideration that the components of the synthesis mixture may come from different sources, and depending on them, times and crystallization conditions may vary.

(15) In order to facilitate the synthesis, crystals of AEI can be added as seeds, in quantities up to 25% by weight respect to the total of oxides, to the synthesis mixture. These can be added before or during the crystallization process.

(16) After the crystallization stage described in (ii), the resultant solids are separated from the mother liquor. The solids can be washed and separated from the mother liquor in (iii) by decantation, filtration, ultrafiltration, centrifugation, or any other solid-liquid separation technique.

(17) The method comprises a stage of elimination of the organic occluded inside the material, which can be performed by extraction and/or thermal treatment at temperatures over 25° C., preferentially between 400 and 750° C., during a period of time between 2 minutes and 25 hours.

(18) The material essentially free of occluded organic molecules obtained in step (iv) is ion exchanged with ammonium or hydrogen to selectively remove the alkali metal cations by cation exchange procedures. The resulting exchanged AEI material can be calcined with air and/or nitrogen at temperatures between 200 and 700° C.

(19) The catalyst for use in the method according to the invention can also be prepared by first synthesizing an AEI zeolite SSZ-39 according to known methods as described in U.S. Pat. No. 5,958,370. After synthesis the occluded organic material must be removed as described above. Afterwards the material essentially free of occluded organic molecules is ion exchanged with ammonium or hydrogen ions to selectively remove the alkali metal cations by cation exchange procedures. Instead of including iron compounds in the synthesis mixture, iron can be introduced into the cation exchanged material after step (v) by exchange, impregnation or solid-state procedures to yield a zeolite with the AEI framework containing iron species and essentially free of alkali metals.

(20) The Fe-AEI zeolite catalyst for use in the method according to the invention is particular useful in heterogeneous catalytic converter systems, such as when the solid catalyst catalyzes the reaction of molecules in the gas phase. To improve the applicability of the catalyst it can be applied into or onto a substrate that improves contact area, diffusion, fluid and flow characteristics of the gas stream wherein the present invention is applied.

(21) The substrate can be a metal substrate, an extruded substrate or a corrugated substrate made of ceramic paper. The substrate can be designed for the gas as a flow-through design or a wall-flow design. In the latter case the gas should flow through the walls of the substrate and in this way contribute with an additional filtering effect.

(22) In the method according to the invention, the Fe-AEI zeolite catalyst is preferably present on or in the substrate in amounts between 10 and 600 g/L, preferably 100 and 300 g/L, as measured by the weight of the zeolite material per volume of the total catalyst article

(23) In the method according to the invention, the Fe-AEI zeolite catalyst can be coated onto the substrate using known wash-coating techniques. In this approach the zeolite powder is suspended in a liquid media together with binder(s) and stabilizer(s) whereafter the washcoat can be applied onto the surfaces and walls of the substrate.

(24) The washcoat containing the Fe-AEI zeolite catalyst contains optionally binders based on TiO.sub.2, SiO.sub.2, Al.sub.2O.sub.3, ZrO.sub.2, CeO.sub.2 and combinations thereof.

(25) The Fe-AEI zeolite catalyst can also be applied as a single or multiple layers on the substrate in combination with other catalytic functionalities or other zeolite catalysts. One specific combination is a layer with a catalytic oxidation functionality containing for example platinum or palladium or combinations thereof.

(26) The Fe-AEI zeolite catalyst can be additionally applied in limited zones along the gas-flow-direction of the substrate.

(27) In one embodiment of the invention, the catalyst capable of removing nitrous oxide can be located in combination with a nitric acid production loop and to facilitate nitrous oxide removal by functioning in either a secondary or a tertiary abatement setup.

(28) In a certain embodiment of the invention, the catalyst is applied in a secondary nitrous oxide abatement setup, where the catalyst is located inside an ammonia oxidizer or ammonia burner, immediately after the ammonia oxidation catalyst. In such a setup the catalyst is exposed to high temperatures and catalyst performance can therefore only be achieved using a highly stable material as described herein.

(29) In another certain embodiment of the invention, the catalyst is applied in a tertiary nitrous oxide abatement setup. In this case the catalytic article is located downstream from the ammonia oxidizer or ammonia burner after an absorption loop of the nitrogen dioxide to produce the nitric acid. In this embodiment the catalytic article is part of a two-step process and located up-stream from a catalyst for removal of nitrogen oxides. The catalytic article of the present invention will remove the nitrous oxide either by direct decomposition or assisted by nitrogen oxides (NOx) also present in the gas stream or assisted by the presence of hydrocarbons (HC). The highly stable material described herein will result in long lifetime of a catalyst in such an application or enable a higher operating temperature which results in a faster reaction rate and a smaller catalyst volume.

(30) In a certain embodiment of the invention where the catalyst is applied in a tertiary setup, the catalyst for removal of nitrous oxide and the catalyst for removal of nitrogen oxides located downstream are both the Fe-AEI zeolite catalyst essentially free of alkali obtained by one of the realizations described herein. In this case a reducing agent will be added upstream the catalyst for removal of nitrogen oxides. In a particular embodiment, the catalytic step for removal of nitrogen oxides will also remove an amount of nitrous oxide which was not removed in the first catalytic step. This way, the volume of the first catalytic step can be reduced, resulting in lower costs.

(31) In a certain embodiment of the invention where the catalyst is applied in a tertiary setup, the catalyst for removal of nitrous oxide and the catalyst for removal of nitrogen oxides are both the Fe-AEI zeolite catalyst essentially free of alkali obtained by one of the inventive features described herein, where the catalytic step for removal of nitrogen oxides are located upstream the catalytic step of removing nitrous oxide. In this case a reducing agent will be added upstream the catalyst for removal of nitrogen oxides and a reducing agent is added upstream the catalyst for removal of nitrous oxide. In a particular embodiment, the catalytic step for removal of nitrogen oxides will also remove an amount of nitrous oxide.

(32) The two catalytic functions (deN.sub.2O and deNO.sub.x) may be located in separate reactors or inside the same reactor, where a reducing agent is added before the catalytic step of removing nitrogen oxides.

(33) The two catalytic functions (deN.sub.2O and deNO.sub.x) may also be combined into a one-step catalytic conversion. In such a converter the Fe-AEI zeolite catalyst essentially free of alkali can be used as the only catalytically active phase or it can be applied in combinations with other nitrous oxide removal catalysts or SCR catalysts.

(34) In another certain embodiment, a hydrocarbon can be used as reducing agent. In one particular embodiment the hydrocarbon is methane.

(35) In another certain embodiment, ammonia is used as reducing agent.

(36) The Fe-AEI catalyst can also be combined in zones or layers with other catalytic materials. For example, the catalyst can be combined with other zeolites or other materials with a different functionality.

(37) In all application of the method according to the invention mentioned and described above, the Fe-AEI zeolite catalyst can be applied in or on a substrate such as a monolithic structure or it can be shaped into pellets depending on the requirements of the application.

EXAMPLES

Example 1

Synthesis of AEI Zeolite (Na-Containing Material)

(38) 4.48 g of a 7.4% wt aqueous solution of N,N-dimethyl-3,5-dimethylpiperidinium hydroxide was mixed with 0.34 g of a 20% wt aqueous solution of sodium hydroxide (NaOH granulated, Scharlab). The mixture was maintained under stirring 10 minutes for homogenization. Afterwards, 0.386 g of FAU zeolite (FAU, Zeolyst CBV-720 with SiO.sub.2/Al.sub.2O.sub.3=21) was added in the synthesis mixture, and maintained under stirring the required time to evaporate the excess of water until achieving the desired gel concentration. The final gel composition was SiO.sub.2:0.047 Al.sub.2O.sub.3:0.4 DMDMP:0.2 NaOH:15H.sub.2O. The resultant gel was charged into a stainless steel autoclave with a Teflon liner. The crystallization was then conducted at 135° C. for 7 days under static conditions. The solid product was filtered, washed with abundant amounts of water, dried at 100° C. and, finally, calcined in air at 550° C. for 4 h.

(39) The solid was characterized by Powder X-ray Diffraction, obtaining the characteristic peaks of the AEI structure (see FIG. 1). The chemical analysis of the sample indicates a Si/Al ratio of 9.0.

Example 2

Direct Synthesis of the Fe-Containing AEI Structure (Na-Containing Material)

(40) 1.98 g of a 7.0% wt aqueous solution of N,N-dimethyl-3,5-dimethylpiperidinium hydroxide was mixed with 0.24 g of a 20% wt aqueous solution of sodium hydroxide (NaOH granulated, Scharlab). The mixture was maintained under stirring 10 minutes for homogenization. Afterwards, 0.303 g of FAU zeolite (FAU, Zeolyst CBV-720 with SiO.sub.2/Al2O.sub.3=21) was added in the synthesis mixture. Finally, 0.11 g of a 20% wt aqueous solution of iron (III) nitrate [Fe(NO.sub.3).sub.3, Sigma Aldrich, 98%] was added, and the synthesis mixture was maintained under stirring the required time to evaporate the excess of water until achieving the desired gel concentration. The final gel composition was SiO.sub.2:0.047 Al.sub.2O.sub.3:0.01 Fe:0.2 DMDMP:0.2 NaOH:15H.sub.2O. The resultant gel was charged into a stainless steel autoclave with a Teflon liner. The crystallization was then conducted at 140° C. for 7 days under static conditions. The solid product was filtered, washed with abundant water, and dried at 100° C. The solid was characterized by Powder X-ray Diffraction, obtaining the characteristic peaks of the AEI structure (see FIG. 2). Finally, the as-prepared solid was calcined in air at 550° C. for 4 h. The solid yield achieved was above 85% (without taking into account the organic moieties). The chemical analysis of the sample indicates a Si/Al ratio of 8.0, an iron content of 1.1% wt and a sodium content of 3.3% wt.

Example 3

Synthesis of Fe-Containing Na-Free AEI Zeolite by Post-Synthetic Ion Exchange

(41) The Na-containing AEI material from Example 1 was first exchanged with a 0.1 M solution of ammonium nitrate (NH.sub.4NO.sub.3, Fluka, 99 wt %) at 80° C. Then, 0.1 g of ammonium-exchanged AEI zeolite was dispersed in 10 ml of deionized water with pH adjusted to 3 using 0.1 M HNO.sub.3. The suspension was heated to 80° C. under nitrogen atmosphere, 0.0002 moles of FeSO.sub.4.7H.sub.2O was then added, and the resultant suspension maintained under stirring at 80° C. for 1 h. Finally, the sample was filtered, washed and calcined at 550° C. for 4 h. The final iron content in the sample was 0.9 wt % and the Na content was below 0.0% wt.

Example 4

Removal of Na from the Direct Synthesis of the Fe-Containing AEI Material from Example 2

(42) 200 mg of the calcined Fe-containing AEI material synthesized according to the Example 2, was mixed with 2 ml of a 1 M aqueous solution of ammonium chloride (Sigma-Aldrich, 98% wt), and the mixture was maintained under stirring at 80° C. for 2 h. The solid product was filtered, washed with abundant water, and dried at 100° C. Finally, the solid was calcined in air at 500° C. for 4 h. The chemical analysis of the sample indicates a Si/Al ratio of 8.0, an iron content of 1.1% wt and sodium content below 0.0% wt.

Example 5

Direct Synthesis of the Fe-Containing CHA Structure (Na-Containing Material)

(43) 0.747 g of a 17.2% wt aqueous solution of trimethyl-1-adamantammonium hydroxide (TMAdaOH, Sigma-Aldrich) was mixed with 0.13 g of a 20% wt aqueous solution of sodium hydroxide (NaOH, Sigma-Aldrich). Then, 0.45 g of a colloidal suspension of silica in water (40% wt, LUDOX-AS, Sigma-Aldrich) and 23 mg of alumina (75% wt, Condea) were added, and the resultant mixture maintained under stirring for 15 minutes. Finally, 0.458 g of a 2.5% wt aqueous solution of iron (III) nitrate [Fe(NO.sub.3).sub.3, Sigma Aldrich, 98%] was added, and the synthesis mixture was maintained under stirring the required time to evaporate the excess of water until achieving the desired gel concentration. The final gel composition was SiO.sub.2:0.05 Al.sub.2O.sub.3:0.01 Fe:0.2 TMAdaOH:0.2 NaOH:20H.sub.2O. The resultant gel was charged into a stainless steel autoclave with a Teflon liner. The crystallization was then conducted at 160° C. for 10 days under static conditions. The solid product was filtered, washed with abundant water, and dried at 100° C. The solid was characterized by Powder X-ray Diffraction, obtaining the characteristic peaks of the CHA zeolite. Finally, the as-prepared solid was calcined in air at 550° C. for 4 h. The chemical analysis of the sample indicates a Si/Al ratio of 12.6, an iron content of 1.0% wt and a sodium content of 1.5% wt.

Example 6

Removal of Na from the Direct Synthesis of the Fe-Containing CHA Structure from Example 5

(44) 100 mg of the calcined Fe-containing CHA material was mixed with 1 ml of a 1 M aqueous solution of ammonium chloride (Sigma-Aldrich, 98% wt), and the mixture maintained under stirring at 80° C. for 2 h. The solid product was filtered, washed with abundant water, and dried at 100° C. Finally, the solid was calcined in air at 500° C. for 4 h. The chemical analysis of the sample indicates a Si/Al ratio of 12.6, an iron content of 1.10% wt and a sodium content of 0.0% wt.

Example 7

Direct Synthesis of the Fe-Containing Beta Structure (Na-Free Material)

(45) 0.40 g of a 35% wt aqueous solution of tetraethylammonium hydroxide (TEAOH, Sigma-Aldrich) was mixed with 0.34 g of a 50% wt aqueous solution of tetraethylammonium bromide (TEABr, Sigma-Aldrich). Then, 0.60 g of a colloidal suspension of silica in water (40% wt, LUDOX-AS, Sigma-Aldrich) and 18 mg of alumina (75% wt, Condea) were added, and the resultant mixture maintained under stirring for 15 minutes. Finally, 0.33 g of a 5% wt aqueous solution of iron (III) nitrate [Fe(NO.sub.3).sub.3, Sigma Aldrich, 98%] was added, and the synthesis mixture was maintained under stirring the required time to evaporate the excess of water until achieving the desired gel concentration. The final gel composition was SiO.sub.2:0.032 Al.sub.2O.sub.3:0.01 Fe:0.23 TEAOH:0.2 TEABr:20H.sub.2O. The resultant gel was charged into a stainless steel autoclave with a Teflon liner. The crystallization was then conducted at 140° C. for 7 days under static conditions. The solid product was filtered, washed with abundant water, and dried at 100° C. The solid was characterized by Powder X-ray Diffraction, obtaining the characteristic peaks of the Beta zeolite. Finally, the as-prepared solid was calcined in air at 550° C. for 4 h. The chemical analysis of the sample indicates a Si/Al ratio of 13.1, an iron content of 0.9% wt and a sodium content of 0.0% wt.

Example 8

Catalytic Test of Materials in the Selective Catalytic Reduction of Nitrogen Oxides Using Ammonia

(46) The activity of selected samples was evaluated in the catalytic reduction of NO.sub.x using NH.sub.3 in a fixed bed, quartz tubular reactor of 1.2 cm of diameter and 20 cm of length. The catalyst was tested using 40 mg with a sieve fraction of 0.25-0.42 mm. The catalyst was introduced in the reactor, heated up to 550° C. in a 300 NmL/min flow of nitrogen and maintained at this temperature for one hour. Afterwards 50 ppm NO, 60 ppm NH.sub.3, 10% O.sub.2 and 10% H.sub.2O was admitted over the catalyst while maintaining a flow of 300 mL/min. The temperature was then decreased stepwise between 550 and 250° C. The conversion of NO was measured under steady state conversion at each temperature using a chemiluminiscence detector (Thermo 62C).

Example 9

Accelerated Hydrothermal Aging Treatment of Samples

(47) Selected samples were treated in a gas mixture containing 10% H.sub.2O, 10% O.sub.2 and N.sub.2 for 13 hours at 600° C. and afterwards their catalytic performance was evaluated according to Example 8.

Example 10

Influence of Na on Catalytic Performance of Fe-AEI Before Accelerated Aging

(48) The Fe-AEI zeolite containing Na as synthesized in Example 2 was tested according to Example 8. For comparison the Fe-AEI zeolite that was essentially free of Na, prepared according to Example 4, was also evaluated in the NH.sub.3—SCR reaction according to Example 8. The steady state-conversion of NO is shown as a function of temperature for the two catalysts in FIG. 3. The results clearly show the beneficial influence of removing the Na from the Fe-AEI zeolite as the NO.sub.x conversion increases at all temperatures.

Example 11

Influence of Na on Catalytic Performance of Fe-AEI after Accelerated Hydrothermal Aging

(49) The two zeolites that were tested in Example 10 (and prepared in Example 2 and Example 4) were aged under the accelerated aging conditions given in Example 9. The NO.sub.x conversion after aging is shown in FIG. 4.

Example 12

Catalytic Performance of Na-Free Fe-AEI Compared to State-Of the Art Fe-Beta and Fe-CHA Zeolites after Accelerated Hydrothermal Aging

(50) The NO.sub.x conversion over Na-free Fe-AEI, prepared according to Example 4, was evaluated in the NH.sub.3—SCR reaction after accelerated hydrothermal aging. For comparison Na-free Fe-CHA and Na-free Fe-Beta catalysts (prepared in Example 6 and Example 7, respectively), which represents state-of-the-art iron promoted zeolite catalysts, were also tested after accelerated hydrothermal aging. The measured NO.sub.x conversion is shown in FIG. 5. As can be seen the NOx conversion is higher over Na-free Fe-AEI compared to the other zeolites.

Example 13

Catalytic Performance of Na-free Fe-AEI Compared to State-Of the Art Fe-CHA Zeolites after Severe Accelerated Hydrothermal Aging

(51) A severe accelerated aging of Na-free Fe-AEI and Na-free Fe-CHA prepared in Example 4 and Example 6 respectively was performed by steaming the catalyst in a muffle furnace with 100% H.sub.2O for 13 h at 600° C. Afterwards the samples were evaluated according to Example 8. The NO.sub.x conversion in the NH.sub.3—SCR reaction over the two Fe-zeolites is shown in FIG. 6. As seen from FIG. 6 the improved stability of Fe-AEI is evident from the higher NO.sub.x seen at all temperatures.

Example 14

Determination of Crystal Size

(52) The Fe-containing AEI zeolite prepared in Example 2 was characterized using scanning electron microscopy to determine the size of the primary zeolite crystals. FIG. 7 shows an image of the obtained material that indicates primary crystallite sizes up to 400 nm.

Example 15

Measurement of Porosity Loss During Accelerated Hydrothermal Aging of Fe-AEI Zeolites

(53) The surface area and porosity of a sample prepared according to Example 4 and the same sample hydrothermally aged according to Example 9 using nitrogen adsorption. The results are given in Table 1. As seen the surface area and porosity of the Na-free Fe-AEI catalyst is decreased less than 25% after the accelerated hydrothermal aging treatment.

(54) TABLE-US-00001 TABLE 1 Surface area and porosity measurement of Na-free Fe-AEI before and after accelerated hydrothermal aging (according to Example 9). BET Micropore Micropore surface area* volume* Material area (m.sup.2/g) (m.sup.2/g) (cm.sup.3/g) Na-free Fe-AEI 516 505 0.25 HT AGED Na-free Fe-AEI 411 387 0.19 Percentage loss −20% −23% −24% *calculated using the t-plot method