JPH06312139A - Exhaust gas purifying catalyst and purifying process for nitrogen oxide - Google Patents

Abstract

PURPOSE:To provide the purifying process for removing effectively a nitrogen oxide from a combustion exhaust gas containing oxygen and also for removing efficiently nitrogen oxide even in the presence of a sulfur oxide. CONSTITUTION:The exhaust gas purifying catalyst is prepared by making the ratio of alkali metal contained in zeolite 0.2 equivalent weight or less to aluminum composing zeolite of 1 equivalent weight and SiO2/Al2O3 containing indium 10 molar ratio or more to the zeolite. The purifying process for a nitrogen oxide is characterized by purifying the nitrogen oxide from a combustion exhaust gas containing oxygen by using a hydrocarbon in the presence of the above-said exhaust gas purifying catalyst.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst and a method for purifying nitrogen oxides, and more particularly to a method for efficiently purifying nitrogen oxides, which are air pollutants, from combustion exhaust gas containing oxygen. .

[0002]

2. Description of the Related Art From the viewpoint of environmental protection, purification of air pollutants is a major social issue. In particular, purification of combustion exhaust gas accompanying the expansion of industrial activities is an urgent issue at present.

Nitrogen oxides contained in combustion exhaust gas discharged from a factory which is a fixed generation source and an automobile which is a mobile generation source is a gas which is said to be a cause of photochemical smog and which is harmful to the human body, particularly nitric oxide. Since (NO) is difficult to purify, it is the most important subject for consideration. Several methods for purifying nitrogen oxides in combustion exhaust gas have been proposed so far. For example, a method called a catalytic reduction method uses a reducing agent such as ammonia or hydrogen to convert NO into N ₂ and H ₂ O on a catalyst.
It is a method of purifying. However, since this method uses a dangerous reducing agent, it is necessary to take measures for recovery and leakage of the reducing agent, which is effective for large-scale fixed sources, but is not suitable for mobile sources such as automobiles.

On the other hand, many catalysts have been developed and are generally used for purification of exhaust gas of a gasoline engine whose exhaust gas is a reducing gas. However, these catalysts cannot be used in the coexistence of oxygen.

By the way, catalytic decomposition of NO, that is, NO
The method of directly decomposing Pd into N ₂ and O ₂ only needs to pass the exhaust gas to the catalyst layer and is extremely simple, so that it has a wide range of applications. Various catalysts have heretofore been found for this as well. Although Pt, Cu, and Co-based catalysts have an effect on NO decomposing activity, they all have a problem that they are poisoned by generated oxygen. Since exhaust gas from a diesel engine and exhaust gas from a lean-burn gasoline engine usually contain oxygen, conventional catalysts cannot handle it, and development of a new method is desired.

Several catalysts have been proposed for such problems. For example, in (A) U.S. Pat. No. 4,297,328 and Japanese Patent Laid-Open No. 63-283727, nitrogen oxides in a combustion gas containing oxygen are purified by a zeolite catalyst containing copper or cobalt in the presence of hydrocarbons. The method of doing is proposed. On the other hand, recently (B) Chemistry Letters P.10
25-1026 (1992) shows that ZSM-5 type zeolite ion-exchanged with gallium or indium has a high purification rate of nitrogen oxides under a high oxygen concentration of 10%.

[0007]

However, in the known catalysts such as the above (A), when the oxygen concentration becomes high, the combustion reaction of hydrocarbons with oxygen increases, and the ability to purify nitrogen oxides remarkably deteriorates. There are a lot of problems to be realized. Further, in the above (B), since the hydrocarbon concentration in the automobile exhaust gas is not sufficiently present with respect to the nitrogen oxides, it is necessary to efficiently purify the nitrogen oxides with a slight amount of hydrocarbons. However, these catalysts have a problem that the ability to purify nitrogen oxides decreases as the hydrocarbon concentration decreases. Therefore, in order to put it into practical use, a catalyst having a further improved nitrogen oxide purification capability is required. In addition, it is known that sulfur oxides are present in actual exhaust gas, which poisons the catalyst and greatly reduces the activity. Therefore, for practical use, it is also an important issue that the catalyst has sulfur oxide resistance.

An object of the present invention is to purify nitrogen oxides from combustion exhaust gas containing oxygen with hydrocarbons at a practical level.

[0009]

Means for Solving the Problems The present invention is to solve the above-mentioned problems. The inventors of the present invention described above (B) have a method in which an alkali metal is present in the zeolite even if In is ion-exchanged. It was found that a considerable amount of the alkali metal remained, and this alkali metal adversely affected the catalytic activity. That is, the amount of alkali metal in the zeolite is 0.2 equivalent or less, more preferably 0.05 or less, relative to 1 equivalent of aluminum constituting the zeolite, and in the presence of a catalyst composed of zeolite containing indium, in a combustion exhaust gas containing oxygen. It has been found that the nitrogen oxides of can be efficiently purified by hydrocarbons. Further, even 50,000 h ^-1 or more 100,000 h ^-1 or more high gas space velocities required for automobile exhaust gas purifying catalyst according to the catalyst of the present invention (GHSV), yet even in the presence of sulfur oxides long It has been found that it exhibits a high nitrogen oxide purification capacity.

That is, in the present invention, the alkali metal contained in the zeolite is 0.2 equivalent or less, more preferably 0.05 equivalent to 1 equivalent of aluminum constituting the zeolite.
SiO ₂ / Al ₂ containing the following and containing indium
Purification catalyst comprising zeolite having an O ₃ ratio of 10 or more in molar ratio and purification method of nitrogen oxides from combustion exhaust gas containing oxygen using hydrocarbons in the presence of such catalysts Is.

The zeolite referred to in the present invention is a crystalline aluminosilicate, and its composition is generally represented by the formula (1). _{xM 2 / n O · Al 2} O 3 · ySiO 2 · zH 2 O ...... (1) ( wherein, n valence of the cation M, x is 0.8 to 2.0
, Y is a number of 2.0 or more, and z is a number of 0 or more. )

The basic structure of zeolite is one in which Si, Al, and O are regularly bonded in a three-dimensional manner, and has various crystal structures depending on the difference in structural units. Many kinds of zeolites are known, but they are characterized by X-ray diffraction and have different names depending on their crystal structure. For example, as natural products, mordenite, erionite, ferrierite,
There are chabazite and the like. As synthetic products, synthetic products of these natural products, X-type, Y-type, MFI, beta-type and the like are known.

The zeolite used in the present invention has a molar ratio of SiO ₂ / Al ₂ O ₃ of 1 from the viewpoint of heat resistance and steam resistance.
It must be 0 or more. The zeolite structure is not particularly limited, but MFI, mordenite, ferrierite and beta are preferable. Either a natural product or a synthetic product may be used, but in the former case, a synthetic product is preferably used since it contains impurities and requires purification.

Generally speaking, the zeolite synthesis method is as follows:
A suitable silica source, alumina source, alkali source, or optionally a metal compound (eg, F 2) in place of the alumina source.
e, Ga, etc.) and are mixed together and crystallized under hydrothermal conditions of about 100 to 250 ° C. A method of adding an organic substance called a template to the above mixture has also been proposed. Zeolites are generally commercially available, and they may be used.

In the present invention, the alkali metal means a metal belonging to Group Ia of the periodic table, and examples thereof include lithium, sodium, potassium, rubidium and cesium.

In the present invention, the alkali metal contained in the zeolite is aluminum 1 which constitutes the zeolite.
It is important that the amount is 0.2 equivalent or less with respect to the equivalent.

In the present invention, as a method for reducing the alkali metal contained in zeolite to 0.2 equivalent or less relative to 1 equivalent of aluminum constituting the zeolite, is it necessary to carry out ion exchange treatment with an aqueous solution containing an indium salt many times? Alternatively, a method of previously exchanging with a cation that is easily exchanged with an alkali metal ion can be mentioned. Examples of the ions used in the latter method include ammonium ions and metal cations other than alkali metals, for example, Group Ib and IIa of the periodic table or transition metals.

The method of introducing indium metal is not particularly limited. Examples of the introduction method include an ion exchange method for exchanging cations and indium cations in zeolite, and an impregnation method for immersing zeolite in a solution containing a target metal. In the case of the ion exchange method, a method in which zeolite is dispersed in a solution of an indium metal salt and an alkaline solution such as aqueous ammonia is added to adjust the pH is also preferably used.

The raw material compound of indium used in the present invention can be used in any form as long as it is a water-soluble salt. For example,
Sulfates, hydrochlorides, nitrates and the like can be mentioned.

The indium content of the zeolite used in the present invention is 0.4 to 12% by weight, preferably 1 to 8% by weight.

After introducing indium into zeolite, the catalyst of the present invention may be molded into an appropriate shape such as spherical, columnar or honeycomb by using an inorganic oxide such as silica or alumina or clay as a binder. Alternatively, it may be coated on a molded body such as a honeycomb made of alumina, cordierite or the like. Alternatively, a binder may be added to the zeolite before the indium is introduced, and the zeolite may be molded, and then the indium may be introduced. In any case, it is not particularly limited.

The hydrocarbon used in the present invention is a compound composed of carbon and hydrogen, and is usually so-called olefins, paraffins, cyclic compounds or hydrocarbons containing these compounds. It is preferably volatile and gaseous at the processing temperature of the present invention. More preferably, at least 1 selected from olefins having 1 to 6 carbon atoms, paraffins and naphthenes.
It is a species of hydrocarbon. Specific examples of preferable hydrocarbons include, for example, ethylene, propylene, butylene, pentene, hexene, methane, ethane, propylene, butylene, pentene, hexene, methane, ethane, propane,
Examples include butane, pentane, hexane, cyclopropane, cyclobutane, cyclopentane, cyclohexane and cyclohexene. It goes without saying that unburned hydrocarbons contained in the combustion exhaust gas are also preferably used. The hydrocarbons present on the catalyst are 0.10 in terms of methane with respect to nitrogen oxides contained in the combustion exhaust gas.
It is preferred to be present in a 2 to 5 molar ratio, more preferably 0.4 to 4 molar ratio. If it is less than 0.2 mol ratio, the purification rate of nitrogen oxides will be low, while if it is more than 5 mol ratio, excess hydrocarbon will be present and a new hydrocarbon purification device will be required, which is not preferable.

The flue gas used in the present invention contains oxygen, and preferably contains 0.1% by volume or more of oxygen. This combustion exhaust gas is emitted from a normal internal combustion engine, boiler, or the like. The present invention is particularly effective for exhaust gas containing a large amount of oxygen, such as combustion exhaust gas from a diesel engine or a lean-burn gasoline engine. The oxygen concentration in the combustion exhaust gas of the diesel engine varies depending on the operating conditions, but is typically 5 to 16%, and is 3 to 8% in the lean-burn gasoline engine.

The nitrogen oxide purification temperature according to the present invention is a catalyst layer temperature of preferably 200 to 800 ° C, more preferably 250 to 600 ° C. If this purification temperature is low, the purification of nitrogen oxides will be insufficient, and if the purification temperature is too high, the coexisting hydrocarbons will burn and the purification of nitrogen oxides will decrease, which is not preferable.

In the conventional method, if the flue gas treatment rate per catalyst volume, that is, the gas space velocity is increased, for example, 5
Ten thousand h ^-1 or more, there is even more far from the practical level for reduces the purification performance of nitrogen oxides when 100,000 h ^-1 or more mobile sources such as automobiles. Further, the sulfur oxides contained in the exhaust gas markedly reduced the ability of the catalyst to purify nitrogen oxides. Therefore, according to the method of the present invention,
It exhibits sufficient nitrogen oxide purification capacity even at high gas space velocities such as automobile exhaust gas, and even in the presence of sulfur oxides.

It is needless to say that the hydrocarbons contained in the combustion exhaust gas can be used to carry out the removal method of the present invention, but in order to increase the hydrocarbon concentration in the combustion exhaust gas, the existing fuel as hydrocarbons is used. It is possible to use a part of fuel oil such as light oil or gasoline in a tank and directly add it to the catalyst layer provided on the exhaust gas outlet side through the bypass line without sending it to the engine. A reforming section may be provided, and a part of fuel oil such as light oil or gasoline may be subjected to a reforming treatment or the like and then added to the catalyst layer.

Further, in the case of a diesel engine, by delaying the fuel injection timing in the engine, it is possible to change the explosive combustion conditions and increase the hydrocarbon concentration in the exhaust gas. Further, in order to maintain the temperature of the catalyst layer in an appropriate range, combustion exhaust gas that has been brought to a predetermined temperature by a cooler or the like may be introduced, and if the combustion exhaust gas does not reach the predetermined temperature, the catalyst layer may be heated. Good.

[0028]

EXAMPLES The present invention will be described below with reference to examples.

Example 1 (Catalyst preparation) N with a SiO ₂ / Al ₂ O ₃ molar ratio of about 25
20 g of a-type MFI zeolite was dispersed in 1.0 liter of an aqueous solution containing 17.5 g of indium nitrate trihydrate, stirred at 80 ° C. for 5 hours, and then filtered. This was washed twice with 250 ml of water. Further, the ion exchange operation and the water washing operation were repeated 5 times. The supported amount of indium ion-exchanged with zeolite was 8.2 as metal.
The residual sodium content was 0.15 equivalent to 1 equivalent of aluminum constituting the zeolite.

Example 2 Na-type MFI having a SiO ₂ / Al ₂ O ₃ molar ratio of about 25
Type zeolite 20g 10% ammonium chloride aqueous solution 4
It was dispersed in 0 ml and stirred at 80 ° C. for 2 hours. After that, it was filtered and then washed twice with 250 ml of water. Further, the ion exchange operation and the water washing operation were repeated 6 times. 1.0% containing 17.5 g of indium nitrate trihydrate
Dispersed in 1 liter of aqueous solution, stirred overnight at room temperature and then filtered. This was washed twice with 250 ml of water. The amount of indium ion-exchanged with the zeolite was 3.8% by weight as a metal, and the amount of residual sodium was 0.01 equivalent or less (not measurable) with respect to 1 equivalent of aluminum constituting the zeolite.

Comparative Example 1 Na-type MFI having a SiO ₂ / Al ₂ O ₃ molar ratio of about 25.
Type zeolite 20g to 17.5g indium nitrate.3
It was dispersed in 1.0 liter of an aqueous solution containing a hydrate, stirred at room temperature overnight, and then filtered. Then, it was washed twice with 250 ml of water and then dried at 110 ° C. overnight. The amount of indium ion-exchanged with the zeolite was 6.5% by weight as a metal, and the amount of residual sodium was 0.37 equivalent to 1 equivalent of aluminum constituting the zeolite.

Examples 3 and 4 (Catalyst Evaluation) Using the catalysts obtained in Examples 1 and 2, Table 1
12% by volume of oxygen and 1000 of nitric oxide under the reaction conditions shown in
The initial activity was evaluated 3 hours after the start of the reaction at 450 ° C. in a gas containing ppm, ethylene as a hydrocarbon of 250 ppm, and sulfur dioxide of 50 ppm. Then, after treatment at 550 ° C. for 100 hours, the removal performance of nitric oxide at 450 ° C. was examined again. The conversion rate of NO was calculated from the conversion rate of NO to N ₂ . The results are shown in Table 2.

Comparative Example 2 Using the catalyst obtained in Comparative Example 1, under the reaction conditions shown in Table 1, a gas containing 12% by volume of oxygen, 1000 ppm of nitric oxide, 250 ppm of ethylene as a hydrocarbon, and 50 ppm of sulfur dioxide at 450 ° C. The initial activity was evaluated 3 hours after the start of the reaction. Then, after treating at 550 ° C for 100 hours, 4
The removal performance of nitric oxide at 50 ° C. was examined again. N
The conversion rate of O was determined from the conversion rate of NO to N ₂ . The results are shown in Table 2.

[0034]

[Table 1]

[0035]

[Table 2]

As is clear from the results of Table 2, SiO ₂ / Al ₂ O containing indium and containing alkali metal contained in the zeolite in an amount of 0.2 equivalents or less with respect to 1 equivalent of aluminum constituting the zeolite. If a catalyst made of zeolite with a molar ratio of ₃ or more is used, a trace amount of hydrocarbons can be used to efficiently purify nitrogen oxides from combustion exhaust gas containing oxygen, and the deterioration of catalyst performance over time is small. I understood. It was also found that the sulfur poison, which is a catalyst poisoning component, works effectively even if it is contained in the reaction gas.

[0037]

According to the present invention, nitrogen oxides can be efficiently removed from combustion exhaust gas containing oxygen. Further, it can be efficiently removed even in the presence of sulfur oxide.

Claims (2)

[Claims] 1. The alkali metal contained in the zeolite is 0. 1 to 1 equivalent of aluminum constituting the zeolite.
SiO containing 2 equivalents or less and containing indium
_An exhaust gas purification catalyst made of zeolite having a ₂ / Al ₂ O ₃ ratio of 10 or more in molar ratio. 2. A method for purifying nitrogen oxides, which comprises purifying nitrogen oxides from a combustion exhaust gas containing oxygen using hydrocarbons in the presence of the catalyst according to claim 1.