JPH0760064A - Nitrogen oxide removal method - Google Patents

Abstract

(57) [Summary] [Purpose] Efficiently purifies exhaust gas discharged from internal combustion engines such as automobiles, especially in excess of oxygen, without using reducing agents such as ammonia, and at high temperatures where steam is present. There is provided a method for purifying exhaust gas using an exhaust gas purifying catalyst having excellent durability. [Structure] SiO treated with silicon tetrachloride in an organic solvent
Using a catalyst comprising a zeolite having a ₂ / Al ₂ O ₃ molar ratio of at least 15 or more and one or more active metals,
Removing nitrogen oxides from oxygen-rich exhaust gas containing nitrogen oxides and hydrocarbons.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating an exhaust gas containing nitrogen oxides, which is discharged from a boiler, an automobile engine, etc., in excess of oxygen using a catalyst. The present invention relates to a method for removing nitrogen oxides using a very excellent catalyst.

[0002]

2. Description of the Related Art As a method for removing nitrogen oxides in exhaust gas discharged from a boiler, an automobile engine, etc., a selective catalytic reduction method using ammonia in the presence of a catalyst, or a method of passing an exhaust gas through a catalyst and unburned A non-selective catalytic reduction method of reducing with carbon monoxide and hydrocarbon has been put to practical use.

JP-A-60-125250 proposes a copper ion-exchanged zeolite as a catalyst capable of directly catalytically decomposing nitrogen oxides in the absence of a reducing agent.

Further, for purification of exhaust gas of a diesel engine and a lean-burn engine for the purpose of reducing fuel consumption, nitrogen oxides are selectively removed by reducing components such as unburned carbon monoxide and hydrocarbons even in the presence of excess oxygen. As a catalyst that can be reduced, a catalyst in which a base metal is contained in zeolite or the like has been proposed (JP-A-63-100919).

However, these proposed catalysts have problems in durability, especially at high temperatures, and have not been put into practical use.

Therefore, Al near the surface of the zeolite is replaced by S.
Attempts have been made to improve the durability of zeolite in the presence of high temperature steam by substituting it with i to make the outer surface of the zeolite hydrophobic (JP-A-4-271843). However, in this proposed method, since Si substitution is carried out in an aqueous solution using ammonium silicofluoride, zeolite pores are generated by hydrofluoric acid produced by hydrolysis of part of the ammonium silicofluoride in the aqueous solution. However, sufficient Al durability was not obtained due to the removal of Al inside.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to efficiently purify exhaust gas discharged from an internal combustion engine such as an automobile, particularly in excess of oxygen, without using a reducing agent such as ammonia, and It is intended to provide a method for purifying exhaust gas by using an exhaust gas purifying catalyst having excellent durability at high temperature in the presence of water vapor.

[0008]

Means for Solving the Problems As a result of diligent studies on the above problems, the present inventors have found that by using a catalyst obtained by treating zeolite with silicon tetrachloride in an organic solvent, exhaust gas can be efficiently discharged even after being used at high temperature. They found that they could be purified, and completed the present invention.

That is, the present invention uses a catalyst composed of a zeolite which is treated with silicon tetrachloride in an organic solvent and has a SiO ₂ / Al ₂ O ₃ molar ratio of at least 15 and one or more active metals to obtain nitrogen. A method for removing nitrogen oxides from an oxygen-excess exhaust gas containing oxides and hydrocarbons.

Although the reason why the durability of the catalyst is improved by treating with silicon tetrachloride in an organic solvent is not clear,
Removal of hydroxyl groups on the zeolite surface by this treatment or by coating Bronsted acid points in the vicinity of the surface, Al in the vicinity of the zeolite surface that causes zeolite destruction in the presence of high temperature steam is protected, and further by zeolite surface hydrophobicization, It is presumed that the penetration of water vapor into the pores, which causes the catalyst deterioration, was suppressed, and the durability of the catalyst was improved.

The present invention will be described in more detail below.

The catalyst used in the present invention comprises zeolite treated with silicon tetrachloride in an organic solvent and having a SiO ₂ / Al ₂ O ₃ molar ratio of at least 15, and one or more active metals.

The raw material zeolite used in the present invention may have a SiO ₂ / Al ₂ O ₃ molar ratio of 15 or more after treated with silicon tetrachloride in an organic solvent. Also, SiO
The upper limit of the ₂ / Al ₂ O ₃ molar ratio is not limited. If the SiO ₂ / Al ₂ O ₃ molar ratio is less than 15, sufficient durability cannot be obtained.

The type of zeolite is not particularly limited, and examples thereof include mordenite, ferrierite and ZSM-.
5, ZSM-8, ZSM-11, ZSM-12, ZSM
Zeolites such as -20 and ZSM-35 can be used,
Among them, ZSM-5 is preferably used. Further, the method for producing these zeolites is not limited. Alternatively, the zeolite such as zeolite Y or zeolite L may be dealuminated.

As the raw material zeolite, a synthetic product or a calcined product thereof is used, but it is also possible to treat the ions such as Na in the raw material zeolite with an ammonium salt or a mineral acid and use it as an H type or an ammonium type. Further, K, Cs, Ba or the like can be used after ion exchange.

The starting zeolite is treated with silicon tetrachloride in an organic solvent, and one or more active metals are introduced.

The order of introducing the active metal and treating with silicon tetrachloride in an organic solvent is not particularly limited, and the active metal may be introduced after treating the starting zeolite with silicon tetrachloride in the organic solvent. It is also possible to perform silicon tetrachloride treatment in an organic solvent after introducing the metal.

The conditions of the treatment with silicon tetrachloride in an organic solvent are not particularly limited, and the raw material zeolite or the zeolite containing one or more active metals is added to the organic solvent containing silicon tetrachloride and treated in the liquid phase. Should be done.

The concentration of silicon tetrachloride in the organic solvent is not particularly limited, but is preferably 0.1 to 20%. As the organic solvent, any solvent in which silicon tetrachloride is stably present may be used, and hexane, cyclohexane, toluene, benzene and the like are used. When water is contained in the organic solvent, silicon tetrachloride is changed to an oxide or the like, and there is a possibility that selective removal of hydroxyl groups on the zeolite surface or coating of Bronsted acid points near the surface cannot be performed. It is desirable to remove water in the solvent as much as possible. Further, when water is used as a solvent, silicon tetrachloride is deposited as an oxide or the like, and a catalyst having excellent durability cannot be obtained.

The temperature for the treatment with silicon tetrachloride in an organic solvent is not particularly limited, but it may be any temperature as long as silicon tetrachloride is stably present and the reaction with hydroxyl groups on the outer surface of the zeolite proceeds, for example, room temperature. To the boiling point of the organic solvent may be used. Also, when treating with silicon tetrachloride in an organic solvent, in order to remove the water adsorbed in the raw material zeolite and to make the hydroxyl groups on the zeolite surface exist in an active state, under vacuum or with an inert gas or dry. Pretreatment is preferably performed at 300 to 800 ° C. in an air atmosphere.

Zeolites treated with silicon tetrachloride in an organic solvent are preferably subjected to solid-liquid separation in order to prevent excess silicon tetrachloride from adhering. Alternatively, the washing with an acid, an organic solvent or the like or the removal of silicon tetrachloride by decompression may be performed. In addition, after the treatment with silicon tetrachloride, in order to further stabilize the Si introduced on the surface of the zeolite, 100 to 8
The heat treatment may be performed at a temperature of 00 ° C. under vacuum or in an atmosphere of inert gas or air.

It is essential that the SiO ₂ / Al ₂ O ₃ molar ratio of the zeolite treated with silicon tetrachloride in an organic solvent is 15 or more. If the SiO ₂ / Al ₂ O ₃ molar ratio is less than 15, sufficient durability at high temperatures cannot be obtained. The upper limit is not particularly limited, but is preferably 200 or less in order to have sufficient catalytic activity.

When treated with silicon tetrachloride in an organic solvent, S
Since i is introduced to the zeolite surface, some SiO ₂ /
An increase in the Al ₂ O ₃ molar ratio occurs.

One or more active metals are introduced into the catalyst used in the present invention. The active metal may be any metal that is usually used for exhaust gas purification, such as Cu or A.
Ib group such as g, Au, Fe, Co, Ni, Ru, Rh,
VIII group such as Pd and Pt, VIa group such as Cr and Mo,
Alternatively, a Group VIIa such as Mn is used. Particularly preferred is Cu or Co.

The method of introducing the active metal is not particularly limited, and an impregnation-supporting method, an evaporation-drying method, an ion exchange method and the like can be used. The active metal shows high activity even when it is supported on the surface of zeolite, but when it is present at the ion exchange site of zeolite, it has higher activity and durability, so it is necessary to introduce the active metal by the ion exchange method. Is preferred.

The ion exchange is carried out by bringing the starting zeolite or the zeolite treated with silicon tetrachloride in an organic solvent into contact with an aqueous solution containing a salt of an active metal and washing it.

As the active metal salt, salts of active metal chlorides, nitrates, sulfates, acetates and the like are preferably used. Further, an ammine complex salt of an active metal or the like can also be preferably used.

The addition amount, concentration, exchange temperature, time, etc. of the active metal at the time of ion exchange are not particularly limited, and a commonly used method may be used. In order to have sufficient activity and durability, the amount of the active metal added should be A in the starting zeolite or zeolite treated with silicon tetrachloride in an organic solvent.
0.5-20 times equivalent is preferable with respect to 1. The ion exchange slurry concentration is preferably 5 to 50% which is usually performed. In addition, the ion exchange temperature and time are
Room temperature to 100 ° C for 5 minutes to ensure durability
It is preferably a time of 3 days. Further, the ion exchange operation can be repeated if necessary.

As described above, the exhaust gas purifying catalyst used in the present invention can be prepared.

The SiO ₂ / Al ₂ O ₃ molar ratio of the exhaust gas-purifying catalyst containing the active metal used in the present invention does not substantially change before and after the introduction of the active metal. Also, the crystal structure of the exhaust gas-purifying catalyst is not essentially different before and after the treatment with silicon tetrachloride in the organic solvent or before and after the ion exchange.

The exhaust gas-purifying catalyst used in the present invention is
It can also be used by mixing with a binder such as clay mineral and molding. It is also possible to preform a raw material zeolite or a zeolite that has been treated with silicon tetrachloride in an organic solvent in advance, and subject the molded body to the treatment with silicon tetrachloride or to introduce an active metal. As the binder used when molding the zeolite, kaolin, attapulgite, montmorillonite, bentonite, allophane,
It is a clay mineral such as sepiolite. Alternatively, it may be a binderless zeolite molded product obtained by directly synthesizing a molded product without using a binder. Further, the exhaust gas-purifying catalyst used in the present invention can be wash-coated on a honeycomb-shaped substrate made of cordierite or metal.

The exhaust gas-purifying catalyst thus prepared is contacted with an oxygen-excess exhaust gas containing nitrogen oxides and hydrocarbons to remove nitrogen oxides. It is essential that the exhaust gas used in the present invention contains nitrogen oxides and hydrocarbons and is in excess of oxygen, but it is also effective when it contains carbon monoxide, hydrogen, ammonia and the like. Exhaust gas in excess of oxygen means that excess oxygen is contained in excess of the amount of oxygen required to completely oxidize carbon monoxide, hydrocarbons and hydrogen contained in the exhaust gas. For example, in the case of exhaust gas discharged from an internal combustion engine of an automobile or the like, the air-fuel ratio is large (lean region).

The space velocity, temperature, etc. when removing nitrogen oxides are not particularly limited, but the space velocity is 100 to 50,000.
It is preferable that the temperature is 0 / hr and the temperature is 200 to 800 ° C.

[0034]

The present invention will be described in more detail with reference to the following examples. However, the present invention is not limited to these examples.

Example 1 An aqueous sodium silicate solution (SiO ₂ ; 250 g) was placed in an overflow-type reaction tank having an actual volume of 2 liters in a stirring state.
/ L, Na ₂ O; 82 g / L, Al ₂ O ₃ ;
2.8 g / liter) and an aluminum sulfate aqueous solution (A
L ₂ O ₃ ; 8.8 g / liter, H ₂ SO ₄ ; 370 g / liter) and 3 liter / hr, 1 liter / hr, respectively.
It was continuously fed at a rate of hr. Reaction temperature is 30-32
C., the pH of the discharged slurry was 6.7 to 7.0.

After solid-liquid separation of the discharged slurry and sufficient washing with water, Na ₂ O; 0.75 wt%, Al ₂ O ₃ ; 0.77 w
t%, SiO ₂ ; 36.1 wt%, H ₂ O; 62.5 wt
% Of granular amorphous aluminosilicate homogeneous compound was obtained. 2860 g of the homogeneous compound and a 3.2 wt% NaOH aqueous solution 6
150g and charged into an autoclave, 72 at 160 ℃
Crystallized under stirring for hours.

The product is solid-liquid separated, washed with water and dried to obtain ZSM.
-5 type zeolite was obtained. As a result of chemical analysis, the composition had the following composition expressed as a molar ratio of oxides on an anhydrous basis.

1.03 Na ₂ O, Al ₂ O ₃ , 41SiO ₂ 10 g of this zeolite was added to 100 cc of an aqueous solution containing 2 g of NH ₄ Cl, stirred at 60 ° C. for 20 hours, washed, dried and ammonium. Ion exchange
An ammonium type zeolite was obtained.

This ammonium type zeolite: 10 g
Was charged in a reaction vessel and pretreated at 400 ° C. for 1 hour under exhaust to remove H ₂ O and the like adsorbed on zeolite. Next, under a nitrogen atmosphere, 40 cc of hexane containing 5% of silicon tetrachloride was dropped into the reaction vessel, stirred at room temperature for 20 hours, and then dried under reduced pressure at 40 ° C. The treated zeolite was washed with a 0.1N hydrochloric acid aqueous solution and then with pure water, and then calcined at 600 ° C. for 1 hour under air flow. The treated zeolite had a SiO ₂ / Al ₂ O ₃ molar ratio of 42.

The zeolite treated with silicon tetrachloride was added to 100 cc of an aqueous solution containing 0.7 g of copper acetate, the pH was adjusted to 10.5 with aqueous ammonia, and the mixture was stirred at room temperature for 20 hours, washed, and then washed with Cu ions. Exchange operation was performed. This operation was repeated twice and then dried to obtain a catalyst 1. As a result of chemical analysis, the composition had the following composition expressed as a molar ratio of oxides on an anhydrous basis.

1.35 CuO, Al ₂ O ₃ , 42 SiO ₂ Example 2 ZSM-5 type zeolite synthesized in Example 1; 10 g
Was added to 100 cc of an aqueous solution containing 14 g of CsCl, and the mixture was stirred at 60 ° C. for 20 hours, washed and dried to obtain Cs.
Ion exchange was performed. This operation was repeated 3 times and then dried to obtain Cs-type zeolite.

Using 10 g of this Cs type zeolite,
Silicon tetrachloride was treated in the same manner as in Example 1 except that 40 cc of hexane containing 14% of silicon tetrachloride was used. The treated zeolite was added to 100 cc of an aqueous solution containing 2 g of NH ₄ Cl, stirred at 60 ° C. for 20 hours, washed and dried to perform ammonium ion exchange,
It was an ammonium type.

Thereafter, the Cu ion exchange operation was performed in the same manner as in Example 1. The resulting catalyst 2 had the following composition, expressed as the molar ratio of oxide on an anhydrous basis.

2.04 CuO, Al ₂ O ₃ , 42 SiO ₂ Example 3 The Cs-type zeolite obtained in Example 2 was pretreated in the same manner as in Example 1 and then treated with silicon tetrachloride in a nitrogen atmosphere. Hexane containing 5%; 40 cc was dropped into the reaction vessel, and the mixture was stirred at room temperature for 22 hours and then filtered, and hexane; 2
It was washed with 00 cc.

Catalyst 3 obtained by subjecting the treated zeolite to ammonium ion exchange and Cu ion exchange in the same manner as in Example 2 had the following composition expressed in terms of molar ratio of oxide on an anhydrous basis.

1.01 CuO, Al ₂ O ₃ , 41SiO ₂ Example 4 Durability was evaluated using the catalysts 1 to 3 obtained in Examples 1 to 3.

Each catalyst is press-molded and then pulverized to 12 to
The size was adjusted to 20 mesh. 2 cc of the gas was filled in a fixed pressure fixed bed flow type reaction tube, and a gas simulating the exhaust gas of the lean burn engine (Table 1) was flowed at a space velocity of 120,000 / hr. After pretreatment at 550 ° C. for 30 minutes, the steady-state purification activity at each temperature was measured. The steady-state purification activity was the NO purification rate after holding at each temperature for 1 hour.

Each catalyst was subjected to a durability treatment at 800 ° C. for 5 hours while flowing a gas having the composition shown in Table 1 at a space velocity of 120,000 / hr. Then, the steady-state purification activity was measured by the same method as described above, and the durability test was conducted.

The results obtained are shown in Table 2.

[0050]

[Table 1]

[0051]

[Table 2]

Comparative Example 1 Cu type ZSM-5 (Comparative Catalyst 1) was obtained in the same manner as in Example 1 except that the silicon tetrachloride treatment was not carried out. As a result of chemical analysis, the composition had the following composition expressed as a molar ratio of oxides on an anhydrous basis.

1.03 CuO, Al ₂ O ₃ , 41 SiO ₂ Comparative Example 2 In the same manner as in Example 1 except that H ₂ O was used as the solvent, Cu type ZSM-5 (comparative catalyst 2) was used. ) Got. As a result of chemical analysis, the composition had the following composition expressed as a molar ratio of oxides on an anhydrous basis.

1.05 CuO, Al ₂ O ₃ , 45SiO ₂ Comparative Example 3 Using Comparative Catalysts 1 and 2 obtained in Comparative Examples 1 and 2,
The catalyst durability was evaluated in the same manner as in Example 4. The results obtained are shown in Table 3.

[0055]

[Table 3]

[0056]

As is clear from Tables 2 and 3, according to the method of the present invention, nitrogen oxides can be efficiently removed, and the nitrogen oxides can be efficiently oxidized even after the catalyst is used at a high temperature. Things can be removed.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location B01D 53/36 102 H

Claims (1)

[Claims] 1. S, which has been treated with silicon tetrachloride in an organic solvent.
Removal of nitrogen oxides from oxygen-rich exhaust gas containing nitrogen oxides and hydrocarbons using a catalyst composed of zeolite having an iO ₂ / Al ₂ O ₃ molar ratio of at least 15 and one or more active metals how to.