KR0146879B1 - Mordenite containing zeolite catalyst for nox removal - Google Patents

Abstract

The present invention relates to a zeolite catalyst using hydrocarbon as a reducing agent and having excellent durability against moisture.

This catalyst is ion-exchanged with 0.1-7.0 wt% of transition metals such as iron, cobalt, nickel, copper, chromium, zinc, and manganese, 3.5-15 Si / Al ratio, and the pore size of catalyst is well developed at 100-1,000Å. The cation which compensates the negative charge of Al before ion exchange is a zeolite catalyst which remains in a small amount even after ion exchange.

This catalyst is particularly excellent in water resistance.

Description

Mordenite-containing natural zeolite catalyst for nitrogen oxide removal

The present invention relates to a mordenite-containing natural zeolite catalyst for removing nitrogen oxides, and more particularly, to a mordenite-containing natural zeolite catalyst having hydrocarbons as a reducing agent and having excellent durability against moisture.

In the process of removing NO _x , which is an air pollutant discharged to the atmosphere by Selective Catalytic Reduction, ammonia or hydrocarbon is used as a reducing agent, and among these, selective catalytic reduction using hydrocarbon as a reducing agent ( SCR) is attracting attention because it avoids secondary pollution problems, is easy to transport and store, and does not cause corrosion problems, compared to ammonia as a reducing agent.

In addition, this method is applicable not only to fixed nitrogen oxide emission sources, but also to mobile sources such as lean-burning vehicles to purify the nitrogen oxides released without supplying a reducing agent to diesel or gasoline engines that produce large amounts of unburned hydrocarbons. There are advantages to it.

As catalysts for removing nitrogen oxides using a hydrocarbon as a reducing agent, those developed so far include zeolite-based catalysts in which transition metals are ion-exchanged, and transition metal catalysts supported on solid oxide-based and solid oxides.

Zeolite catalysts with ion-exchanged double transition metals have excellent nitrogen oxide removal capabilities, especially mordenite and ZSM-5 system zeolites with the best removal efficiency.

However, the above-mentioned problems show a sudden decrease in activity when water is present in the exhaust gas containing nitrogen oxide to be removed, and thus cannot be applied to the actual exhaust gas.

In this regard, Japanese Patent No. 3,217,218 (1991) describes a method for effectively removing nitrogen oxides contained in an exhaust gas at 200-500 ° C. in which hydrocarbons coexist using a zeolite catalyst exchanged with copper. Cu-ZSM-5, the catalyst used in the patent, showed a nitrogen oxide removal rate of 40-75% when no water was present at an oxygen concentration of 1%. However, in the nitrogen oxide removal method using hydrocarbon as a reducing agent, 1% oxygen concentration has no meaning in actual application, and most of exhaust gas has 2-18% of water, but the above patent has no effect and effect. It has not been investigated. Until now, all catalysts developed as a catalyst for removing nitrogen oxides using hydrocarbon as a reducing agent have been observed to have a 50% or more reduction in activity in the presence of water. Therefore, the effect of water must be considered in such a method. et al., J. Catal., 142 (1993) 561, T. Miyadera, Appl. Catal., B: Environ., 2 (1993) 199, G. Mabilon et al., Catal. Today, 17 (1993) 285, W. Held et al., SAE Paper No. 900496 (1990) 13, C.H. Bartholomew et al., AIChE 1992 Annual Meeting, P. 240a, M. Iwamoto et al., J. Auto. Eng., 207 (1993) 23, K.C.C. Kharas, Appl. Catal., B: Environ., 2 (1993) 207, T.J. Truex et al., Platinum Metals Rev., 36 (1992) 2, Y. Li et al., Appl. Catal., B: Environ., 3 (1993) Ll].

U. S. Patent No. 5,141906 (1992) discloses a method for preparing a catalyst that can effectively remove nitrogen oxides in automobile exhaust gas in which hydrocarbons coexist on a transition metal ion-exchanged ZSM-5. In this patent, the conventional ion exchange method and the coating method on the support are used as they are, but after the ion exchange with a transition metal such as Cu, Co, etc., to the ZSM-5, the mother catalyst, a sulfur compound such as H _By using an additional process of treatment with ₂ S or washing with glycol (Ethylene glycol), transition metal oxides, such as copper oxides and cobalt oxides, which may be formed in the zeolite pores in the conventional manufacturing method, are in the form of sulfides. By converting or washing the sulfide form thus formed with a glycol solution, it was possible to inhibit the direct oxidation of hydrocarbons in the exhaust gas, promote the removal of nitrogen oxides, and at the same time stabilize the zeolite structure.

However, in the conventional ion exchange method, oxides of ion-exchanging metals may be formed in the zeolite pores, but if sufficient washing is performed with distilled water under vacuum, these substances in the pores can be removed, and a catalytic amount of 0.7 liter The effect of inhibiting the oxidation reaction of hydrocarbons on the catalyst prepared by the above method was observed, but there was no change in the removal rate of nitrogen oxides. Of course, the use of 1.7 liters of catalytic amount can lead to a significant increase in the removal rate of nitrogen oxides. However, since the nitrogen oxide removal reaction mechanism using hydrocarbons as the reducing agent varies in a very complicated manner depending on various reaction variables, more detailed investigation is needed. Will be needed. In particular, the close relationship between the oxidation of hydrocarbons and the removal of nitrogen oxides is known. Hamada et al., Appl. Catal., 64 (1990) L1, Y. Kintaichi et al., Catal. Lett., 6 (1990) 239, has also been observed to be more effective in the case of excess exchange of more than 100% for the selective removal of nitrogen oxides with hydrocarbon as the reducing agent on Cu-ZSM-5. Excess ion exchange is known to produce [Cu-O-Cu] ²⁺ or clusters in the zeolite pores. [KCC Kharas, Appl. Catal., B: Environ., 2 (1993) 207].

European Patent 459,396 (1991) found that when removing nitrogen oxides using propane or propylene at an oxygen concentration of 10%, hydrogen type zeolites and alkali metal ion exchanged zeolites are superior to ion exchange with transition metals. It was. These catalysts include zeolites such as ZSM-5, mordenite, zeolite Y, zeolite X, zeolite L and silicates and alumina, titanium oxide, zirconium oxide, silica-alumina, silica-magnesia, silica-zirconia and alumina-titania. In the latter case, the acid metal oxide was used as a catalyst or impregnated with a noble or basic metal.

However, the catalytic activity investigated in this patent is also contrary to the fact that the effect on water, which is important in practical applications, has not been investigated and that zeolites exchanged with transition metals such as copper ions are more effective in nitrogen oxide removal. [M. Iwamoto et al., J. Auto. Eng., 207 (1993) 23].

U.S. Patent No. 5,041,272 (1991) is capable of removing nearly 100% of nitrogen oxides from zeolites impregnated with transition metals by using simulated exhaust gas with 7.3% water and a propane concentration of 4-5 times the concentration of nitrogen oxides. In this case, synthetic carriers (Y, L, ferrierite, mordenite, ZSM-5, offetite-erionite) zeolites and natural (mordenite-containing or clinopthiolite-containing) zeolites, which are used as carriers, were prepared in a hydrogen type.

According to US Pat. No. 5,017,538 (19991), a catalyst capable of effectively removing nitrogen oxides present in exhaust gas under lean combustion conditions in which excess residual oxygen remains, is a site where ion exchange is carried out in various types of zeolites when the transition metal is ion exchanged. It can be realized by adjusting the structure. That is, transition metals such as copper ions in zeolites such as X, Y, ZSM-5, ferrierite, opretite, erionite, A, and chabazite may be zeolites. When exchanged into a rectangular structure and location by lattice oxygens on the surface of the supercage, it is possible to purify lean combustion exhaust gases to a high level. In this case, in order to obtain a catalyst having such catalytic properties, a method of increasing the volume of anion, limiting acid strength, and increasing ion exchange rate was used in addition to the existing ion exchange method.

U.S. Patent 5,260,043 (1991) discloses a catalyst that can selectively remove carbon monoxide and nitrogen oxides in combustion gas using methane as a reducing agent. As a result of investigating the removal rate of nitrogen oxide on the zeolite exchanged with transition metal and the catalyst supported with niobium (Nb) and aluminum (A1) under various reaction conditions, the catalyst composition containing Co-ZSM-5 was the most It was observed to be effective. However, the catalytic activity investigated in this patent is the result of the absence of water, especially when 2% of water is present on the same catalyst, a decrease of 5-30% is observed at a reaction temperature of 250-500 ° C. [ Y. Li et al., J. Catal., 142 (1993) 561], with the co-FER catalysts investigated together, 10-35% activity when 2% water is present at a reaction temperature of 500-600 Degradation occurs. [Y. Li et al., Appl. Catal., B: Environ., 3 (1993) L1]. In the presence of 2% water on Ga-H-ZSM-5, 30% and 25% reduction in activity was observed at 500 and 550 ° C, respectively. [Y. Li et al., J. Catal., 145 (1994) 1.

On the other hand, US Patent No. 5,270,024 (1993) discloses a method for improving durability at high temperatures, decreasing stability of copper ions and low activity at low temperatures below 200 ° C, which are problems of zeolite catalysts containing copper ions effective for nitrogen oxide removal reactions. Is mentioned. In this patent, four kinds of catalyst production methods are proposed.

First, in order to secure the durability of the catalyst and the stability of copper ions, the rare earth metals (La, Ce, Nd, Y. Pr, Sm) were exchanged with 5-80% of copper ions based on the aluminum atoms present in the zeolite. The exchange rate of -10% improved the durability of the catalyst and the stability of the ion-exchanged copper ions and prevented the formation of coke than zeolites containing only copper ions.

In addition, the problem that existing zeolites containing only copper ions have low activity in the reaction of 200 ° C. or less has a problem of absorbing nitrogen dioxide by preparing a catalyst additionally containing 0.1-10% of alkaline earth metals (Mg, Ca, Sr, Ba). Increased the performance. However, when the heat treatment was carried out at 700 ° C. for 5 hours on the former catalyst, the activity was decreased by about 25%. The latter improvement method was about 2 times better than the catalyst containing only copper ions. The effectiveness varies depending on the properties, and especially when calcium or strontium is further exchanged at 200-300 ° C., it is known that the activity is hardly improved or even decreased. [Y. Teraoka et al., Catal. Lett., 12 (1992) 361].

Accordingly, an object of the present invention is to provide an improved catalyst for removing nitrogen oxides that can solve the problems of the conventional catalyst as described above.

Furthermore, an object of the present invention is to provide a catalyst having high durability against water while selectively removing nitrogen oxide using hydrocarbon as a reducing agent on a zeolite catalyst exchanged with a transition metal such as copper ions.

According to the present invention, the transition metal is 0.1-7.0% ion-exchanged based on the weight of the zeolite base material, the Si / Al ratio is 3.5-15, and the cation that compensates the negative charge of aluminum before ion exchange is 0.01-1.5% by weight even after ion exchange. There is provided a mordenite-containing natural zeolite catalyst for nitrogen oxide removal using a hydrocarbon selected from the group consisting of methane, ethane, ethylene, propylene, butane and butylene as the reducing agent.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

In the present invention, both a synthetic zeolite and a natural zeolite can be used as a mother catalyst, but a natural zeolite is more effective in order to secure durability against water under actual exhaust gas conditions.

The zeolite used in the present invention is collected as a gemstone, wherein the gemstone is a crystalline compound in which mordenite, clinoptilolite and helandite are mixed according to X-ray diffraction analysis. These zeolites remove acid and thermally stable components such as ratelandite and some impurities, and result in zeolites having structural stability. At this time, the Si / Al ratio of the zeolite is about 3.5-15, about 5-10 is more efficient.

The catalyst of the present invention compensates the negative charge of aluminum by sodium, calcium, potassium, magnesium, iron, manganese, etc. before ion exchange. The mother catalyst may be acid-treated and treated with an ammonium salt solution to form a hydrogen type, which may prepare a zeolite ion-exchanged with a transition metal in the transition metal salt solution. At this time, the transition metal may be iron, cobalt, nickel, copper, chromium, zinc, manganese, and the like. In particular, zeolite exchanged with copper ions is more effective in removing nitrogen oxides in the exhaust gas in which hydrocarbons are present.

In the present invention, the pore size of the zeolite exchanged with copper ions is preferably in the range of 3-5,000 kPa, and 100-1,000 kPa is more efficient in removing nitrogen oxides. In the zeolite ion-exchanged with the transition metal, the amount of metal ion-exchanged is about 0.1-7.0% by weight, and when less than 0.1%, the effect of metal ion is not shown. Nitrogen oxide removal technology using hydrocarbon as a reducing agent in the presence of oxygen should be greater than 0.1% in order to obtain a desired level of removal rate since the transition metal ions ion-exchanged in the zeolite are adsorption points of nitrogen oxides. However, the presence of more than 7.0% of transition metal ions promotes the direct oxidation of hydrocarbons due to the excessive oxidizing power of the catalyst surface, and indirectly oxidizes the activator, which is an intermediate of the reaction. . Due to the characteristics of the removal mechanism, the amount of transition metal ion exchanged in the zeolite is 1.1-2.5% more effective, especially 100% ion exchange when the ion exchange with about 2.0% by weight of copper ions.

In particular, in the zeolite exchanged with the transition metal, calcium, iron, manganese, zinc, etc. contained in the gemstone are present in the zeolite even after the ion exchange, thereby causing a positive effect on the stability and reactivity of the ion exchanged copper ions. As for the remainder, 0.01-1.5 weight% is appropriate.

When the zeolites of the present invention, in particular copper zeolites, are used as catalysts for the removal of nitrogen oxides, their form is not limited in the present invention. That is, the prepared catalyst may be used as a fine powder, it may be used in the form of a particle size or pellets. On the other hand, such a zeolite can be prepared by using a honeycomb-type reactor or coated on a support made of cordiolite or the like.

The zeolite catalyst of the present invention can be applied to purify the nitrogen oxides of the exhaust system in which hydrocarbons can be used as reducing agents in the presence of oxygen, the application of which is not limited in the present invention. In this case, the hydrocarbon used as the reducing agent may be methane, ethane, ethylene, propane, propylene, butane, butylene, isobutylene, isobutane, etc., but most of the hydrocarbons may be used. Especially in these reducing agents, unsaturated hydrocarbons are effectively superior to propylene or ethylene. In the zeolite catalyst of the present invention, the removal rate of nitrogen oxide varies depending on the type of reducing agent, the supply ratio of nitrogen oxide and hydrocarbon, oxygen concentration and copper ions, and especially water according to the type of reducing agent used, oxygen concentration and copper ion content. The durability for is different. Since 2-18% of water is usually present in the exhaust gas, the durability of the catalyst to the denitrification catalyst process using hydrocarbon as a reducing agent is as important as the selectivity to oxygen concentration. As described above, the water resistance of previously developed catalysts did not meet expectations under simulated conditions or actual exhaust gas conditions. In the case of the present invention, the durability against water was increased by exchanging copper ions with hydrogen zeolite, and particularly, the copper zeolite catalyst prepared with a natural zeolite as a mother catalyst has excellent durability against water and is weak when propylene is used as a reducing agent. Only about 5% of activity was observed even when 16% of water was present.

Since the catalyst of the present invention is mainly composed of thermally stable mordenite and clinopthiolite, it can be used in a high temperature flue gas system, and is usually suitable in the range of 200-850 ° C. The nitrogen oxide adsorbed at the adsorption point effective for the nitrogen oxide removal reaction below 200 ° C cannot be effectively activated, and the hydrocarbon is insufficient to adsorb to the acid point in the zeolite to form the activator which is the reaction intermediate. Above 850 ℃, the structural stability of zeolite used as a carrier of transition metal is destroyed and ion exchanged copper ions move or agglomerate during the reaction, which is not effective for adsorption of nitrogen oxide, but below 850 ℃, calcium present in gemstone Since iron, manganese and zinc coexist in the range of 0.01-1.5% by weight, it is appropriate because copper ion migration or aggregation is prevented. More effectively, the reaction temperature of 200-600 ° C. is a preferred range in terms of suppressing direct oxidation of hydrocarbons.

EMBODIMENT OF THE INVENTION Hereinafter, the Example of this invention is described.

Example 1

Manufacture of catalyst by ion exchange method

The natural zeolite, which is present as a natural stone, was collected and crushed into particles of 20-50 mesh size, which was named NZ. 256 ml of 35% hydrochloric acid solution was added to a 2 liter flask, distilled water was poured to prepare a 3N hydrochloric acid solution, and the prepared NZ was stirred while maintaining the solution at 90 ° C. in dihydrochloric acid solution. Was removed. In this pickling process, the zeolite component which is unstable to acid or heat was removed. This pickling was carried out twice, and the supernatant was discarded, followed by filtration using a Class B manure paper, followed by rinsing with a continuous flow washing apparatus until the pH was 7.0. The zeolite after washing with water was dried at 110 ° C. for 20 hours. The zeolite thus dried was stirred at 90 ° C. in a 2N ammonium nitrate solution, ion-exchanged to hydrogen type for 24 hours, washed with water sufficiently and dried at 110 ° C. for 10 hours. After repeating this two-time exchange process for hydrogen type, it was heated to 2 ° C./min in an air atmosphere and calcined at 500 ° C. for 10 hours to obtain NZA.

In addition, before going through the calcination process, the mixture was stirred at 90 ° C in 0.5N copper nitrate solution for 20 hours, ion-exchanged with copper for 20 hours, washed sufficiently with water, and dried at 110 ° C for 10 hours. After repeating this process twice, the mixture was stirred again while maintaining it at 110 ° C. in 2N copper nitrate solution, ion-exchanged with copper for 20 hours, washed with water and dried, and then heated to 2 ° C./min in an air kiln. CuNZA was prepared by baking at 500 ° C. for 10 hours. The physical and chemical properties of the hydrogen-type and copper-type zeolites and natural zeolites thus prepared were examined, and the results are shown in Table 1 below.

Example 2

Durability survey for water

Reactivity was evaluated in a fixed-bed continuous flow reactor by mixing 500 ppm nitrogen oxide, 2,000 ppm propylene, and 4.2% oxygen with helium on the CuNZA (Cu 1.83 wt%) catalyst prepared in Example 1. The removal rate of nitrogen oxide at each water concentration at the reaction temperature of 400 ° C. was shown in Table 2 below. At this time, the space velocity was 13,200hr _-1 .

According to Table 2, when the nitrogen oxide is removed using the catalyst of the present invention, the removal rate is 85% even when the concentration of water is 16% or more.

According to the above, there is provided a durable mordenite-containing natural zeolite catalyst having excellent nitrogen removal rate when treating a reaction gas containing excess water in the reaction gas.

Claims (6)

Based on the weight of the zeolite base, 0.1-7.0% of the transition metal was ion-exchanged, and the cation, which compensated the negative charge of aluminum before ion exchange, with a Si / Al ratio of 3.5-15, remained 0.01-1.5% by weight in the catalyst after ion exchange. A mordenite-containing natural zeolite catalyst for nitrogen oxide removal using a hydrocarbon selected from the group consisting of ethane, ethylene, propylene, butane, butylene, isobutylene and isobutane as a reducing agent. The catalyst of claim 1 wherein the transition metal is selected from the group consisting of iron, cobalt, nickel, copper, chromium, zinc, manganese, and is 1.1-2.5% ion exchanged. The catalyst according to any one of claims 1 to 4, wherein the transition metal is copper. The catalyst of claim 1 wherein the pore size of the zeolite is in the range of 3-5,000 mm 3. 4. The catalyst of claim 3 wherein the pore size of the zeolite is in the range of 100-1,000 mm 3. The catalyst of claim 1, wherein the cation comprises sodium, calcium, potassium, magnesium, iron, and manganese. The catalyst of claim 1 wherein the Si / A1 ratio is 5-10.