Method for the production of an SCR-active zeolite catalyst, and SCR-active zeolite catalyst

Abstract

An SCR-active zeolite catalyst and a method for producing same. To produce the catalyst, an Fe ion-exchanged zeolite is initially subjected to a first temperature treatment within a range of 300 to 600 C. in a reducing hydrocarbon atmosphere such that the oxidation state of the Fe ions decreases and/or the dispersity of the Fe ions on the zeolite increases, whereupon the reduced zeolite is subjected to a second temperature treatment within a range of 300 to 600 C. in an oxidizing atmosphere such that hydrocarbon residues or carbon residues are oxidatively removed, the zeolite being calcined to obtain a catalyst material during the two temperature treatments. Iron contained in the zeolite is stabilized in an oxidation state of less than +3 and/or the dispersity of the Fe ions on the zeolite is permanently increased such that a high SCR activity is achieved within a temperature range of less than 300 C.

Claims

1. A process for producing an SCR-active zeolite catalyst, characterized in that an Fe ion-exchanged zeolite is first subjected in a reducing hydrocarbon atmosphere to a first thermal treatment within a range between 300 C. and 600 C., which at least one of (i) reduces the oxidation state of the Fe ions and (ii) increases the dispersity of the Fe ions on the zeolite, then the reduced zeolite is subjected in an oxidizing atmosphere to a second thermal treatment between 300 C. and 600 C., which oxidatively removes at least one of (i) hydrocarbon residues and (ii) carbon residues, wherein the zeolite is calcined during the first and second thermal treatments to produce the catalyst.

2. The process as claimed in claim 1, characterized in that a temperature of 500 C. is exceeded for a period of more than 50 minutes during the first thermal treatment.

3. The process as claimed in claim 1, characterized in that the zeolite at the end of the first thermal treatment is cooled to a temperature below 400 C.

4. The process as claimed in claim 1, characterized in that the hydrocarbon atmosphere contains less than 1% by volume of oxygen.

5. The process as claimed in claim 1, characterized in that the first thermal treatment is performed in an inert gas atmosphere to which hydrocarbons are supplied for reduction.

6. The process as claimed in claim 5, characterized in that the hydrocarbons supplied are at least one of (i) organic polymers and (ii) biopolymers which are converted to gaseous decomposition products during the first thermal treatment.

7. The process as claimed in claim 6, characterized in that the hydrocarbons supplied are at least one of polyethylenes, polyglycols and cellulose.

8. The process as claimed in claim 1, characterized in that the zeolite is preliminarily processed with addition of an organic plasticizer to give a free-flowing composition, and the first thermal treatment is performed in an inert gas atmosphere, the organic plasticizer releasing hydrocarbons into the inert gas atmosphere as a result of pyrolysis.

9. The process as claimed in claim 8, characterized in that the plasticizer used is at least one of a polyethylene glycol, a polyethylene oxide and cellulose.

10. The process as claimed in claim 8, characterized in that the free-flowing composition is extruded to give an unsupported catalyst, and the unsupported catalyst is subjected to the first and second thermal treatments.

11. The process as claimed in claim 8, characterized in that a support body is coated with the free-flowing composition, and the coated support body is subjected to the first and second thermal treatments.

12. The process as claimed in claim 1, characterized in that the second thermal treatment is performed under air.

13. The process as claimed in claim 1, characterized in that the first and second thermal treatments are performed in immediate succession with exchange of gas.

14. The process as claimed in claim 1, characterized in that the zeolite used is a zeolite of the beta or MFI type.

15. The process as claimed in claim 1, characterized in that the zeolite contains between 3 and 7% by weight of iron.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention are illustrated in detail with reference to a drawing and the examples which follow. The drawings show:

(2) FIG. 1 a schematic flow diagram for the production of an SCR-active zeolite catalyst and

(3) FIG. 2 in a graph for various catalysts, a comparison of the temperature-dependent conversion rate for NO in the presence of ammonia.

DETAILED DESCRIPTION

EXAMPLE 1a

(4) FIG. 1 shows, by a schematic flow diagram, the production of an SCR-active zeolite catalyst according to an illustrative embodiment. In a first step, 1, a pulverulent Fe ion-exchanged synthetic MFI zeolite with a proportion of 3% by weight of iron is processed to give a plastic and free-flowing composition. For this purpose, the pulverulent MFI zeolite is mixed with glass fibers and likewise pulverulent boehmite and, with addition of cellulose, a commercial plasticizer and polyethylene oxide as an organic assistant, processed in an aqueous acidic solution having a pH of <5 to give the plastic and free-flowing mixture. The plastic mixture is subsequently extruded to give a honeycomb catalyst body permeated by channels and having a round cross section and a cell density of 300 cpsi (cells per square inch). Subsequently, the catalyst body is dried. The unsupported catalyst has a contact area with a diameter of about 2.5 cm (1 inch) and a flow length of about 7 cm.

(5) Subsequently, the unsupported catalyst produced in this way is subjected to a calcination step 2. For this purpose, the catalyst body is subjected in a furnace under an N.sub.2 atmosphere to a first thermal treatment 3. The catalyst body is heated to a temperature of 600 C. and kept there for a period of 1.5 hours. Subsequently, the catalyst body is cooled and withdrawn from the furnace at room temperature.

(6) In the course of the first thermal treatment 3 in an inert gas atmosphere, the cellulose and the polyethylene oxide introduced as an organic plasticizer decompose as a result of pyrolysis to gaseous hydrocarbons, as a result of which a reducing hydrocarbon atmosphere forms in the furnace. Under these conditions, iron present in the MFI zeolite is reduced gradually to iron with an oxidation state of +2, or at least to a fractional oxidation state of less than +3, and/or the dispersity of the iron on the zeolite is increased. The temperature profile selected achieves the effect that iron in the reduced form or the high dispersity of the iron is stabilized, which is accomplished crucially through the influence of the hydrocarbons present in the atmosphere. The thermal treatment additionally drives water out of the catalyst body and achieves solidification of the catalyst composition. During the reducing calcination, the catalyst composition at the same time takes on a pale yellow to beige base color which indicates presence of iron in the +2 oxidation state and/or an increase in the dispersity. This color may be masked by the decomposition products of the organic polymers, such that the catalyst body appears black overall.

(7) Subsequently, the reduced zeolite catalyst, for performance of a second thermal treatment 4, is introduced into a second furnace in which the atmosphere used is air. The catalyst body is again brought therein to a temperature of about 600 C. and kept there for a period of about 50 minutes. During this time, the decomposition products present in the catalyst and the original organic polymers of the cellulose and of the plasticizer still present are oxidized and finally removed. The catalyst composition solidifies further. At the end of the second thermal treatment 4, the zeolite catalyst now visibly has the pale yellow to beige base color mentioned.

(8) Subsequently, the zeolite catalyst is cooled and removed from the second furnace. The result is the finished zeolite catalyst, which is in the form of a honeycomb unsupported catalyst 5, and is prepared for the use thereof for degradation of nitrogen oxides by the SCR process.

EXAMPLE 1b

(9) In an alternative variant, thermal treatments 3 and 4 are performed in a common furnace. In this case, on completion of the first thermal treatment 3, the catalyst body is cooled to a temperature of below 400 C., then the reducing hydrocarbon atmosphere is drawn off and air is let into the furnace. This is then followed immediately by the second thermal treatment 4, which is run through analogously to example 1a.

EXAMPLE 1c

(10) According to example 1a, a plastic and free-flowing composition is produced. A catalyst body composed of cordierite of the same dimensions and the same cell density as an inert support body is coated with the plastic composition. Subsequently, the coated support body is subjected to the further process steps 3 and 4 according to example 1a. The result is a coated catalyst body 5.

(11) In an experiment, the unsupported catalysts 5 and 5 produced according to examples 1a, 1b and 1c are heated to 900 C. and subjected to air flow at a space velocity of 10 000 1/h for 2 hours. The unsupported catalysts 5 and 5 do not exhibit any color change in the course of this. They retain the inherent pale yellow to beige base color. In other words, iron of the +2 oxidation state and/or the high dispersity of the iron is stabilized permanently by the reducing calcination according to the first thermal treatment 3. Even under an oxidizing atmosphere, such as air, no oxidation of the iron of the +2 oxidation state to iron of the +3 oxidation state takes place, and no large Fe clusters form. Any oxidation would lead immediately to a color change to a rust-red base color, as is typical of iron in the +3 oxidation state. This color is typical of rust, iron being present principally in the form of an Fe.sub.2O.sub.3. The same applies to iron oxide clusters present.

EXAMPLE 2

(12) For comparison with this, a zeolite catalyst produced identically according to process step 1 of example 1a is manufactured. This is then calcined according to conventional technology under air at temperatures above 500 C.

EXAMPLE 3

(13) According to example 1a, a catalyst body is again extruded with a round cross section, a cell density of 300 cpsi, with a contact area having a diameter of 2.5 cm and a flow length of about 7 cm. Instead of a zeolite of the MFI type, however, the zeolite used is an Fe ion-exchanged synthetic zeolite of the beta type. A zeolite of the beta type differs from a zeolite of the MFI type by a different characteristic three-dimensional structure.

(14) Subsequently, for catalysts 1a, 2 and 3, the catalytic activity for conversion of NO in the presence of ammonia is determined For this purpose, catalysts 1a, 2 and 3 are each subjected to a flow of a standard gas composed of nitrogen with a proportion of 600 ppm of NO at a standardized space velocity of 25 000 1/h. In each case, the proportion of NO before and after flow through the catalyst body is determined and this is used to determine the conversion based on the proportion of NO upstream of the catalyst Ammonia NH.sub.4 is supplied to the standard gas as a reducing agent with a stoichiometry factor of =0.9, i.e. in a slightly substoichiometric amount in relation to the proportion of NO. Subsequently, the respective conversion is determined for various temperatures below 300 C.

(15) The same experiment is repeated for an unsupported catalyst of the same geometry, which comprises a catalyst composition composed of titanium dioxide with additions of oxides of tungsten and vanadium. The catalyst is referred to hereinafter as TiMoV catalyst.

(16) The result of the studies is shown in FIG. 2. A graph 10 shows plots of each of the conversions of NO normalized to the comparative catalyst 2 against temperature 14. The graph 10 shows the measurements for catalyst 1a (MFI) according to curve 15, for catalyst 3 (beta) according to measurement curve 17 and for comparative catalyst 2 according to measurement curve 16. As a result of the normalization, the latter is calculated as a straight line of value 1.

(17) According to the graph 10 in FIG. 2, it is evident that the unsupported catalyst 1a obtained by reduced calcination, zeolite MFI (measurement curve 15), in the low-temperature range of below 300 C., displays a distinct improvement in catalytic activity with regard to the selective catalytic reduction of NO compared to a conventionally produced catalyst of the same composition. The conversion is increased in this case. The oxidized unsupported catalyst 3, zeolite beta (measurement curve 17), shows a still further-improved catalytic activity.

(18) In the temperature range above 300 C., the achieved conversions of NO and the catalytic activities of catalysts 1a and 3 approach those of comparative catalyst 2.

(19) FIG. 2 shows that, an unsupported catalyst produced according to embodiments of the process according to the invention has an excellent catalytic activity with regard to the selective catalytic reduction of nitrogen oxides in the low-temperature range below 300 C. The permanent stabilization of the iron in the +2 oxidation state or in a fractional oxidation state of less than +3 and/or the permanently high dispersity of the iron on the zeolite makes an SCR-active zeolite catalyst produced in such a way outstandingly suitable in this respect for use for nitrogen oxide reduction in the exhaust gases of internal combustion engines operated with excess air, as is the case particularly for a diesel engine. While the catalytic activity in the low-temperature range in the case of conventional zeolite catalysts depends crucially on the proportion of NO.sub.2, this is not the case for the zeolite catalyst specified in the present case. FIG. 2 shows an outstanding catalytic activity of the catalysts for degradation of NO, specifically in the absence of NO.sub.2. An oxidation catalyst which is usually connected upstream for conventional zeolite catalysts, more particularly also for Fe ion-exchanged zeolite catalysts, and increases the proportion of NO.sub.2 in the emitted nitrogen oxides within a low-temperature range can thus be dispensed with entirely for the zeolite catalyst specified in the present case. This means not just a saving of construction space. In fact, the invention gives a considerable cost advantage since the oxidation catalysts needed generally contain noble metals and are therefore expensive.

LIST OF REFERENCE NUMERALS

(20) 1 Production of extrudable composition 2 Calcination 3 First, reducing thermal treatment 4 Second, oxidizing thermal treatment 5 Unsupported catalyst, extruded 5 Unsupported catalyst, coated 10 Graph 12 NOx conversion 14 Temperature 15 Measurement curve for MFI catalyst 1a 16 Measurement curve for comparative catalyst 2 17 Measurement curve for beta catalyst 3