JPH06198134A - Method for removing nitrogen oxide - Google Patents

Abstract

PURPOSE:To provide a method for removing NOx with a catalyst hardly causing aggregation due to heat and excellent in durability. CONSTITUTION:Zeolite having at least >=15 molar ratio of SiO2 to Al2O3 is vacuum-fired after ion exchange with an aq. soln. contg. monovalent copper ions or monovalent copper complex ions to obtain a catalyst and NOx is removed from waste gas contg. excess oxygen as well as NOx and hydrocarbon with the catalyst.

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 oxygen-rich exhaust gas containing nitrogen oxides discharged from a boiler, an automobile engine, etc. by using a catalyst, and more specifically, to a method for treating activity and durability. The present invention relates to a method for removing nitrogen oxides using a very excellent catalyst.

[0002]

2. Description of the Related Art A lean-burn gasoline engine has been developed in order to reduce carbon dioxide emissions and reduce fuel consumption. However, since the exhaust gas of this lean burn gasoline engine or diesel engine is in an oxygen excess atmosphere, it is difficult to denitrate with a conventional three-way catalyst, and a method for removing nitrogen oxides has not been put to practical use.

In recent years, it has been reported that a zeolite catalyst in which copper is ion-exchanged can reduce and remove nitrogen oxides in exhaust gas having excess oxygen. In order to achieve a high purification rate, a catalyst with a high copper content and high dispersion is required. JP-A-1
No. 960111 discloses a catalyst to which ammonia is added at the time of ion exchange, and Japanese Patent Application Laid-Open No. 3-8621 discloses a catalyst to be subjected to ion exchange with monovalent copper complex ions and then calcined in air.

[0004]

However, the catalyst in the nitrogen oxide removing method proposed above has a large copper content, but has a problem in that it does not have high dispersion. In the former case, since it is exchanged with a divalent ammine complex, copper is also present on the outer surface of the zeolite in an unstable and non-uniformly loaded state. In the latter case, the copper complex ions are highly dispersed immediately after the exchange due to the exchange with the monovalent copper complex ions, but the monovalent copper complex ions that are unstable in the air become divalent since they are fired in the air thereafter. Easily oxidized. At this time, due to the charge balance, the copper complex ions are detached from the ion exchange site to be in a supported state, and the degree of dispersion is remarkably reduced. Since the dispersibility is low as described above, there is a problem that the NOx purification rate is low in spite of the high copper content, particularly the purification rate is low in a low temperature region of 400 ° C. or lower. Furthermore, the copper agglomerates due to heat, so the durability was low.

[0005]

Means for Solving the Problems As a result of intensive studies on the above problems, the present inventors have found that monovalent copper ions in zeolite are present in the ion-exchange site in a highly dispersed state, and the purification rate in the low temperature region is high. The inventors have found a method of purifying exhaust gas by using a catalyst having high durability and excellent durability, and completed the present invention.

That is, the present invention uses SiO ₂ / Al ₂ O ₃
Excess oxygen containing nitrogen oxides and hydrocarbons using a catalyst that has been subjected to vacuum calcination after ion exchange of zeolite having a molar ratio of at least 15 or more with an aqueous solution containing monovalent copper ions or monovalent copper complex ions. This is a method of removing nitrogen oxides from the exhaust gas.

By ion exchange with monovalent copper ions or monovalent copper complex ions and subsequent vacuum calcination, a catalyst having a high copper content and having copper present as monovalent ions at ion exchange sites and having high dispersion is prepared. it can. Therefore, the partially oxidized intermediate of hydrocarbon necessary for reducing and removing nitrogen oxides is generated at a low temperature, the NOx purification rate at a low temperature is higher than that of the conventional one, and the copper monovalent ion is retained at the exchange site. For,
Aggregation due to heat hardly occurs, and a catalyst having excellent durability can be obtained.

The present invention will be described in more detail below.

Although many natural and synthetic zeolites are known, it is essential that the raw material zeolite used in the present invention has a SiO ₂ / Al ₂ O ₃ molar ratio of 15 or more. _Also, SiO 2 / _Al 2 _{O 3} molar ratio is not the upper limit thereof is limited. SiO ₂ / A
If the l ₂ O ₃ molar ratio is less than 15, sufficient durability cannot be obtained. Further, the type of zeolite is not particularly limited, but for example, mordenite, ferrierite, 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.

In the present invention, the monovalent copper or monovalent copper complex ion used for ion exchange is not particularly limited as long as it is an ion that can exist monovalently in the aqueous solution, but since a high ion concentration can be obtained, A monovalent copper ammine complex ion obtained by reducing a divalent copper ammine complex ion obtained by dissolving cupric halide or cupric oxide in an aqueous ammonia solution with hydrazine or hydrazine hydrate is preferable.

The concentration of monovalent copper or monovalent copper complex ions, the exchange temperature, the time, etc., during the ion exchange are not particularly limited,
A commonly used method may be used. The monovalent copper or the monovalent copper complex ion concentration is 0. 0 to obtain a high copper content.
01 to 1 mol / liter is preferable. The ion exchange temperature is preferably 20 to 80 ° C. at which monovalent copper or monovalent copper complex ions can stably exist. The ion exchange time is preferably 5 minutes to 3 days, when the exchange is sufficiently performed. The concentration of ion-exchanged zeolite slurry is preferably 1 to 50% so that the zeolite powder and the aqueous solution are in uniform contact with each other.

Ion exchange and filtration after ion exchange are carried out in an atmosphere in which oxygen is sufficiently replaced with nitrogen or the like so that the above-mentioned monovalent copper ion or monovalent copper ammine complex ion is not oxidized. At this time, you may wash with distilled water or diluted ammonia water.

The vacuum of the vacuum firing in the present invention may be a vacuum degree of 10 mmHg or less at which monovalent copper ions or monovalent copper ammine complex ions are not oxidized, but preferably 0.1 mmHg or less. good. The temperature of the vacuum calcination is such that ammonia as the ligand of the ammine complex ion is scattered and the monovalent copper ion is stabilized at the ion exchange site. 3
The structure of zeolite is not destroyed at temperatures above 00 ° C 800
℃ or less is good. It is preferably 400 to 700 ° C. The time for vacuum firing may be 5 minutes to 1 day because monovalent copper ions are stabilized at the exchange site. In addition, vacuum drying may be performed to remove water before vacuum baking.

The above ion exchange and vacuum firing can be repeated to increase the copper content.

The copper content is preferably high in order to obtain a high purification rate, but if it is excessive, it tends to aggregate, so CuO / Al
_{The 2} O ₃ molar ratio may be 1.0 to 3.0, but 1.2 to 2.
5 is preferable, and 1.7 to 2.2 is more preferable.

The exhaust gas purifying catalyst used in the present invention can be prepared as described above. The exhaust gas-purifying catalyst used in the present invention can be mixed with a binder such as a clay mineral and molded before use. As the binder used when forming the zeolite, clay minerals such as kaolin, attapulgite, montmorillonite, bentonite, allophane and sepiolite, and inorganic oxides such as silica, titania and zirconia can be used. 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 oxygen-rich 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 emitted from an internal combustion engine of an automobile or the like, the air fuel consumption is high (lean region).

The space velocity, temperature, etc. for 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 ⁻¹ and the temperature is 200 to 800 ° C.

[0020]

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 <Preparation of catalyst 1> A sodium silicate aqueous solution (SiO ₂ ; 250 g) was placed in an overflow type reaction tank having an actual volume of 2 liters in a stirring state.
/ Liter, Na ₂ O; 82 g / liter, Al
₂ O ₃ ; 2.8 g / liter) and an aqueous solution of aluminum sulfate (Al ₂ O ₃ ; 8.8 g / liter, H ₂ SO ₄ ;
370 g / liter) and 3 liters / hr,
It was continuously fed at a rate of 1 liter / hr. The reaction temperature is 30 to 32 ° C., and the pH of the discharged slurry is 6.7 to
It was 7.0. After solid-liquid separation of the discharged slurry and sufficient washing, Na ₂ O; 0.75 wt%, Al ₂ O ₃ ; 0.7
7 wt%, SiO ₂ ; 36.1 wt%, H ₂ O; 62.
A 5 wt% granular amorphous aluminosilicate homogeneous compound was obtained. 2860 g of the homogeneous compound and 3.2 wt% NaOH
Charge the autoclave with 6150 g of the aqueous solution, and add 160
Crystallized at 72 ° C under stirring for 72 hours. Solid-liquid separation of products,
It was washed with water and dried to obtain Na-type ZSM-5. As a result of chemical analysis, the composition had the following composition represented by the molar ratio of oxide on an anhydrous basis. The _{1.0Na 2 O · Al 2 O 3} · 40SiO 2 ammonium type ZSM-5 This was prepared by ion exchange in an aqueous solution of ammonium chloride was prepared. This ammonium type ZSM-5; 72 mmo so that the number of moles of copper is twice the number of moles of alumina contained in 20 g.
200 ml of a 1 / liter cupric chloride aqueous solution was prepared, and ammonia was added in a molar ratio of 20 times that of copper to prepare a divalent copper ammine complex aqueous solution. After moving to a nitrogen box in which the aqueous solution was sufficiently replaced, hydrazine in a mole number of 0.25 times that of copper was added as a reducing agent to this aqueous solution to prepare a colorless and transparent monovalent copper ammine complex aqueous solution. The above ammonium type ZSM-
5; 20 g was added, and the mixture was stirred at 20 ° C. for 20 hours to perform copper ion exchange. After the replacement, solid-liquid separation was carried out in a nitrogen box, followed by washing with water, followed by vacuum drying at 110 ° C. for 20 hours. Then, 20 g of this copper-containing zeolite was filled in a quartz boat, the degree of vacuum was 0.1 mmHg or less, and the temperature rising rate was 1
The temperature was raised to 500 ° C. at 00 ° C./hr, and the temperature was maintained for 1 hour for vacuum firing. Then, it cooled to room temperature under vacuum and obtained the catalyst 1. As a result of investigating the copper content of this copper monovalent exchange zeolite by a chemical analysis, it had the following composition in terms of oxide molar ratio in anhydrous base.

1.30 CuO.Al ₂ O ₃ .40 SiO ₂ Example 2 <Preparation of catalyst 2> In Example 1, ammonium type ZSM-5; 3 times as many copper moles as the alumina mole number contained in 20 g. A catalyst 2 was obtained in the same manner as in Example 1 except that 200 ml of a 108 mmol / liter cupric chloride aqueous solution was prepared so that the number became equal to the number. As a result of investigating the copper content of this copper monovalent exchange zeolite by a chemical analysis, it had the following composition in terms of oxide molar ratio in anhydrous base.

1.66 CuO.Al ₂ O ₃ .40SiO ₂ Example 3 <Preparation of catalyst 3> 20 g of ammonium type ZSM-5 obtained in Example 1 was subjected to copper ion exchange and vacuum drying in the same manner as in Example 1. After performing the vacuum calcination, copper ion exchange, vacuum drying and vacuum calcination were repeated to obtain a catalyst 3. As a result of investigating the copper content of this copper monovalent exchange zeolite by a chemical analysis, it had the following composition in terms of oxide molar ratio in anhydrous base.

1.90 CuO.Al ₂ O ₃ .40SiO ₂ Example 4 <Preparation of Catalyst 4> Catalyst 4 was prepared in the same manner as in Example 3 except that copper ion exchange, vacuum drying and vacuum firing were repeated twice. Got As a result of investigating the copper content of this copper monovalent exchange zeolite by a chemical analysis, it had the following composition in terms of oxide molar ratio in anhydrous base.

2.40CuO.Al ₂ O ₃ .40SiO ₂ Example 5 <Preparation of catalyst 5> 20 g of Na-type ZSM-5 obtained in Example 1 was subjected to copper ion exchange and vacuum drying in the same manner as in Example 1. It was fired in vacuum to obtain a catalyst 5. As a result of investigating the copper content of this copper monovalent exchange zeolite by a chemical analysis, it had the following composition in terms of oxide molar ratio in anhydrous base.

1.29 CuO.Al ₂ O ₃ .40SiO ₂ Comparative Example 1 <Preparation of Comparative Catalyst 1> In Example 1, 400 ml / mi was used instead of vacuum firing.
Comparative catalyst 1 was obtained in the same manner except that the calcination was carried out under the flow of n air. As a result of investigating the copper content of this copper-containing zeolite by chemical analysis, it had the following composition in terms of oxide molar ratio on an anhydrous basis.

1.90 CuO.Al ₂ O ₃ .40SiO ₂ Comparative Example 2 <Preparation of Comparative Catalyst 2> Ammonium type ZSM-5 obtained in the same manner as in Example 1;
20 g was added to a 0.1 mol / liter cupric acetate aqueous solution so that the number of moles of copper was twice the number of moles of alumina contained in the zeolite. After that, 2.8% ammonia was added to adjust the pH to 7.5, and then the mixture was stirred at 20 ° C. for 20 hours to perform ion exchange treatment. After solid-liquid separation, the ion exchange operation was repeated. Vacuum drying and vacuum calcination were carried out in the same manner as in Example 1 to obtain comparative catalyst 2. As a result of investigating the copper content of this copper-containing zeolite by chemical analysis, it had the following composition in terms of oxide molar ratio on an anhydrous basis.

1.92 CuO.Al ₂ O ₃ .40SiO ₂ Example 2 <Catalytic activity evaluation test> Catalysts 1 to 4 and comparative catalysts 1 and 2 were press-molded, respectively, and then crushed and sized to 12 to 20 mesh. . Each sized catalyst 2
cc was filled in a fixed pressure fixed bed flow reaction tube and used for the reaction.
As a reaction pretreatment, a gas having the composition shown below (hereinafter, reaction gas) is flown at 4000 ml / min at 550 ° C.
The temperature was raised to and held for 30 minutes. Then 250-550
The purification rate of NOx and hydrocarbons was measured at an arbitrary temperature between 0 ° C (reaction 1). Space velocity at this time (volume basis)
Was 120,000 hr ⁻¹ . Table 2 shows the NOx purification rate at each temperature. The NOx and hydrocarbon purification rates are expressed by the following equations.

Further, a durability treatment was carried out at 800 ° C. for 5 hours while circulating the reaction gas through the catalyst 2 cc of which particle size was adjusted to 12 to 20 mesh. Then, the same pretreatment as in Reaction 1 was performed, and the NOx purification rate was measured under the same reaction conditions (Reaction 2). The results are shown in Table 3.

XNOX = {([NOx] in- [NOx] ou
t) / [NOx] in} × 100 XNOX: NOx purification rate [NOx] in: NOx concentration of inlet gas [NOx] out: NOx concentration of outlet gas

Table 1 Reaction gas composition CO 0.12vol% C ₃ H ₆ 0.08vol% NO 0.12vol% O ₂ 4vol% CO ₂ 12vol% H ₂ O 10vol% N ₂ balance

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

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

Claims (1)

[Claims] 1. A catalyst having a SiO ₂ / Al ₂ O ₃ molar ratio of at least 15 or more ion-exchanged with an aqueous solution containing a monovalent copper ion or a monovalent copper complex ion, followed by vacuum calcination using a catalyst. A method for removing nitrogen oxides from an oxygen-excess exhaust gas containing nitrogen oxides and hydrocarbons.