JPH08112531A - Catalyst for purification of waste gas containing nitrogen oxide and method for purifying the same - Google Patents

Abstract

PURPOSE: To obtain a catalyst having remarkably superior catalytic characteristics even to waste gas contg. a very small amt. of NOx and having superior NOx removing ability even at a very high space velocity by using synthetic mordenite with iron incorporated into the structure by using iron-contg. stock at the time of synthesis. CONSTITUTION: When mordenite is produced by hydrothermal synthesis, iron- contg. stock such as iron nitrate is added to alumina stock such as sodium aluminate, silica stock such as amorphous silica and an alkali metallic compd. such as NaOH as starting materials for synthesizing mordenite and an aq. soln. of them is brought into a hydrothermal reaction in an autoclave to produce synthetic mordenite with iron incorporated into the structure at the time of synthesis. The objective catalyst is made of the synthetic mordenite. This catalyst has a superior NOx removing effect in the presence of ammonium acetate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst for purifying NOx-containing exhaust gas and a method for purifying NOx-containing exhaust gas using the same, and more specifically, by using a raw material containing iron during the synthesis of mordenite. The present invention relates to a NOx-containing exhaust gas purification catalyst that is a synthetic mordenite containing iron (Fe) in its structure and uses ammonium acetate as a reducing agent, and a NOx-containing exhaust gas purification method using this catalyst.

[0002]

2. Description of the Related Art When incinerating industrial waste, municipal waste, etc., it depends on the origin, type, composition, etc. of the waste.
NOx, CO, or SOx, hydrogen chloride, odor, etc. are generated. For this reason, various measures have been taken against exhaust gas containing these substances, and further research and development are under way. In this respect, it is a goal to use automobiles, thermal power generation, gas engines, etc.
The same applies to the exhaust gas discharged from the generation system and the like.

Of the harmful components in various exhaust gases, particularly for the treatment of NOx (nitrogen oxide), so-called flue gas denitration technology, for example, non-catalytic reduction method, catalytic cracking method, non-selective or selective catalytic reduction method, Various methods such as an adsorption method, an electron beam irradiation method, a molten salt absorption method, a reduction absorption method, etc. are known. Of these, the catalytic reduction method in which a catalyst is used for purification is usually used to remove NOx. To N
It is particularly noticeable because it changes to ₂ and is harmless.

The catalyst used in the catalytic reduction method is
Until now, precious metals such as Pt, Rh, and Pd, TiO ₂ , V ₂ O
There are various kinds of metal oxides such as ₅ , Cr ₂ O ₃ , Fe ₂ O _{3 and the} like, rare earth oxides, sulfides, etc. Zeolite type is known as one of them, and this zeolite is a catalyst. As a matter of course, it can be effectively used as a carrier for various catalysts.

Zeolite is composed of aluminosilicate in its composition, is crystalline and has a three-dimensional network structure, is easily reversibly dehydrated, binds with water weakly retained therein, ie, zeolitic water, and also undergoes ion exchange. It has characteristics such as containing a cation having a high property, but depending on the composition ratio of the alumina (Al ₂ O ₃ ) component and the silica (SiO ₂ ) component and the crystal structure of the component, There are many kinds, for example, typical examples thereof include analcime, chabazite, mordenite, faujasite, clinoptilolite and the like.

Not only natural zeolites but also synthetic zeolites produced by hydrothermal synthesis and the like are manufactured and commercially available. For example, a filler for paper, a hygroscopic agent, an ion exchanger, a molecular sieve, It has been used for a wide range of applications as catalysts for various reactions, catalyst carriers, etc., and its modification, reforming and other improvements have been made successively according to its applications, reactions, etc. As one system relating to purification catalysts, proposals have been made to improve purification efficiency and the like by supporting metals such as copper and iron on zeolite.

[0007] For example, in Japanese Unexamined Patent Publication (Kokai) No. 2-194819 / 1990, ammonia is mixed with a nitrogen oxide-containing exhaust gas, and the mixed gas is introduced onto an acid-resistant zeolite catalyst containing Cu and / or Fe. The method of reducing the nitrogen oxides present in the above with ammonia is introduced. As a method of containing the metals, "ion exchange or impregnation or precipitation and mixing of oxides or metal salts with zeolite, followed by It can be counted as calcination in an oxidizing or reducing atmosphere. "
e is included, and is not included in the synthesis process of zeolite itself.

Further, JP-A-2-4453 proposes a method for producing a denitration catalyst by catalytically reducing nitrogen oxides in flue gas in the presence of ammonia, using a zeolite carrying a transition metal. Here, the zeolite catalyst is made of Al, Si, a compound of an alkali metal or an alkaline earth metal and a salt of a transition metal as a raw material, and the molar ratio of the raw material components is set to a predetermined ratio, and the effective alkali concentration is adjusted to a gel solution. It is produced by hydrothermally treating, as the raw material transition metal, specifically Cu, Fe, Ni, Co and V, and as the zeolite obtained here, mordenite and pentasil type zeolite. It is shown.

By the way, NO in the NOx-containing exhaust gas
In the catalytic reduction denitration method for purifying x using a catalyst, a separate reducing agent is required in addition to the catalyst. As this reducing agent, ammonia, hydrogen, methane and other hydrocarbons, carbon monoxide, etc. can be used. However, among these, particularly ammonia, because it has excellent selective reactivity with NO,
It has been actually adopted as a practical denitration method, and the above-mentioned JP-A-2-194819 and JP-A-2-4453 both require the use of this ammonia as a prerequisite.

Further, in JP-A-6-71141, in the catalytic reduction denitration method using ammonia as the reducing agent, the ammonia itself is harmful and dangerous, and salts such as ammonium sulfate are formed on the catalyst surface at low temperature. Therefore, there is an inconvenience such as a decrease in the denitrification rate. The inconvenience and the problem are that X-type and Y-type containing Na as a denitrifying agent and at least one of K, Na and Ca are contained. It is said that denitration can be carried out without causing a decrease in catalytic activity by using a type A zeolite on which iron is supported by ion exchange and treating it in the presence of hydrocarbons such as heavy oil, light oil, and kerosene. It is a thing.

The role of the hydrocarbon in the above technique is to react it with SO ₂ gas so as to avoid the catalytic poisoning effect of SO ₂ , and there is no explicit description of the presence or absence of a reducing effect. JP-A-2-203923
According to the gazette, as in the case of the above-mentioned technique, the problem of using ammonia as its reducing agent is pointed out, and it is proposed to use an ammonium salt or an amine compound as an alternative thereto. Examples of the ammonium salt here include ammonium carbonate, ammonium hydrogencarbonate, ammonium formate, and ammonium acetate, and examples of the amino compound include urea. On the other hand, the catalyst is a carrier-supported type or a non-supported type. It is said that well-known materials such as Raney type can be used, and in particular, anatase type titania carrying a metal oxide such as V ₂ O ₅ shows excellent denitration performance. There is no description about the system catalyst, and specifically, this V ₂
Only the O ₅ catalyst is mentioned.

In this proposal, as an example,
For the test exhaust gas containing 100 ppm NO,
Using "anatase-type titania with V ₂ O ₅ supported (vanadium loading of 4% by weight)" as a catalyst, the reaction temperature was in the range of 200 to 450 ° C. The reaction rate when an aqueous solution is injected is shown. According to this, for example, when ammonium acetate is used as the reducing agent, an effect almost equal to that of the case of ammonia described as a comparative example is obtained, but the reaction rate is still higher than that of NO and ammonium. It has not been obtained beyond the equivalent reaction with the nitrogen atom in the salt.

On the basis of these known facts, the inventors of the present invention are conducting various experiments and researches on the catalytic reduction denitration method, the combination of the catalyst and the reducing agent, the interaction between the two, etc. , (A) NOx and CO-containing exhaust gas purification method, wherein NOx and CO-containing exhaust gas purification method using ammonium acetate as a reducing agent in a zeolite-based catalyst (Japanese Patent Application No. 6-42000), and (B) N
As a catalyst for purifying exhaust gas containing Ox and CO, an "iron-treated mordenite" catalyst was previously developed and proposed (Japanese Patent Application No. 6-194799).

Of these, in (A), a zeolite-based catalyst is used as the catalyst, and ammonium acetate is also used as the reducing agent, so that NOx (NO) and CO
In addition to the effective removal of carbon monoxide, it is possible to obtain a removal effect more than the original equivalent amount to the NOx amount due to its unique characteristic of ammonium acetate. It is essential to use ammonium acetate reducing agent as
(1) Mordenite carrying iron ion exchange, (2)
When applied and used as any form of mordenite that has been subjected to a metamorphic treatment with iron, NOx (NO) in exhaust gas can be extremely effectively removed without accompanying slip ammonia.

The present inventor has further advanced these results in the case of using ammonium acetate as a reducing agent, and while conducting experiments and studies, regarding the mordenite, alumina at the time of synthesizing mordenite itself,
In addition to the essential raw materials such as silica components, a raw material containing iron is present as the synthetic raw material itself, and thus iron is contained in the structure itself of the synthetic mordenite, which is excellent for NOx-containing exhaust gas. It has excellent catalytic properties and is excellent for exhaust gas containing a very small amount of NOx, and also excellent under high space velocity conditions.
The present invention has been found to exhibit a purifying ability, and has led to the present invention.

[0016]

That is, the present invention consists of synthetic mordenite containing iron in its structure, which is obtained by allowing a raw material containing iron to exist during its synthesis, and produces a very small amount of NOx. An object is to provide a catalyst for purifying NOx-containing exhaust gas, which has significantly excellent catalytic properties for exhaust gas containing it, and has excellent NOx purifying ability even under extremely high space velocity conditions, and ammonium acetate reduction NOx using this catalyst in the presence of agents
It is an object of the present invention to provide a method for purifying contained exhaust gas.

[0017]

The present invention relates to a NOx-containing exhaust gas purifying catalyst using ammonium acetate as a reducing agent, and by using a raw material containing iron at the time of its synthesis, iron is incorporated into the structure. Provided is a catalyst for purifying NOx-containing exhaust gas, which is characterized by comprising synthetic mordenite contained therein, and the present invention provides a NOx-containing exhaust gas in the presence of an ammonium acetate reducing agent as a raw material containing iron at the time of its synthesis. A method for purifying a NOx-containing exhaust gas, characterized by treating with a synthetic mordenite catalyst containing iron in its structure, is provided.

The synthetic mordenite catalyst containing iron in its structure according to the present invention is used for producing mordenite by hydrothermal synthesis, and alumina raw materials such as sodium aluminate as a raw material for the synthesis, amorphous silica, etc. In addition to the silica raw material and alkali metal compound or alkaline earth metal compound such as caustic soda, a raw material containing iron such as iron nitrate as the raw material component itself is present, and the raw material aqueous solution is autoclaved. It can be produced by carrying out a hydrothermal reaction in.

Here, as the alumina raw material, in addition to sodium aluminate, alumina sol, alumina gel, aluminum sulfate, aluminum nitrate, aluminum hydroxide or the like can be used, and as the silica raw material, amorphous silica or water glass is used. , Silica sol, silica gel, diatomaceous earth, clay mineral and the like can be used. In the present invention, in addition to these raw materials, a raw material containing iron as the raw material component itself is present, and as the raw material containing iron, iron chloride, iron bromide, iron oxide, sulfuric acid in addition to the above iron nitrate. Iron, organic iron compounds and the like can be used. Moreover, as the quantitative ratio of each raw material, an amount corresponding to a composition capable of obtaining a product having a mordenite structure is used.

FIGS. 1 (a) and 1 (b) show the relationship between the amounts of Fe contained in the mordenite product obtained by varying the amount of the Fe raw material in the raw materials during the synthesis. Fig. 1 (a) shows "Fe / (A
"F +" in the formed mordenite to "F +" molar ratio
"e / (Al + Fe)" molar ratio, and FIG. 1 (b) shows the Fe content (wt%) in the produced mordenite with respect to the "Fe / (Al + Fe)" molar ratio in the raw material.

Fe added to the raw material as shown in FIG.
The amount is "Fe / (Al + Fe)" molar ratio up to a little less than 0.4,
Almost as it is contained in the produced mordenite structure, and thereafter, the ratio of incorporation into the produced mordenite structure decreases as the amount of Fe is gradually increased. However, as shown in FIG. I understand. Na-type mordenite is Na / Al
(Atom number ratio) = 1, but even if Fe is present in the mordenite structure, Na is Na / (Al + Fe)
The amount corresponding to = 1 was contained, and no change was observed in this content.

Further, FIG. 2 shows the change in the NOx removal rate depending on the content of Fe in the synthetic mordenite structure according to the present invention, which is experimentally investigated [Si / Al].
Molar ratio: 5 to 10, processing temperature (= reaction temperature): 300
The conditions such as C, gas to be treated, space velocity, etc. are the same as those in the examples described later], and as shown in the figure, the Fe content is 0.3 to 4.7% by weight and the NOx removal rate is 50% or more. It is understood that the NOx removal rate of 60% or more can be obtained when the content is 0.4 to 4.2% by weight. Here, the NOx removal rate is calculated by the following equation (1), where the NOx concentration before the treatment is X and the NOx concentration after the treatment is Y, and the NOx removal rate in the embodiment is the same. is there.

Mordenite is a kind of aluminosilicate, and is orthorhombic and has a tunnel structure therein. In the present invention, however, the mordenite has a composition of Si / Al molar ratio of 5 to 50 (= SiO _2). / Al ₂ O ₃ molar ratio is 10-
Within the range of 100), it can be effectively applied as a NOx-containing exhaust gas catalyst. Therefore, the quantitative ratio of the silica raw material and the alumina raw material can be selected according to these final compositions, but the quantitative ratio of the Fe raw material to be contained is necessary to obtain the characteristics shown in FIG. The amount of Fe is added and used according to the tendency or fact as shown in FIGS. 1 (a) and 1 (b).

The synthesis in the present invention is carried out by a hydrothermal reaction, and the heating condition is a temperature of about 100 to
It is carried out at 200 ° C, preferably about 150 to 190 ° C,
The pressure during the reaction can be carried out under autogenous pressure in an autoclave or under a pressure higher than that. Although this reaction can be carried out in a stationary state, iron is surely and strongly present in the structure of the produced mordenite, and in terms of its yield, reaction rate, etc., under stirring conditions, especially under mild stirring. It is desirable to carry out. Even in the operation of mixing the raw materials, an aqueous solution of the raw material containing iron is gradually added dropwise to the aqueous solution containing the alumina raw material, the silica raw material, and the alkali metal compound or the alkaline earth metal compound under stirring conditions to add a uniform mixture. By devising such as forming a mixed solution, iron can be surely present in the structure itself of the produced mordenite.

The synthetic mordenite containing iron in its structure at the time of its synthesis, which is obtained through the above manufacturing process, is
NO when using ammonium acetate as the reducing agent
Although it exhibits excellent characteristics as an x-containing exhaust gas purification catalyst, this catalyst has an excellent NOx purification ability even when the NOx concentration in the exhaust gas is extremely low, and, for example, the space velocity (S
V) Exhibits an extremely excellent catalytic property of NOx removal rate of 60 to 78% (at a relatively low processing temperature of 300 to 325 ° C.) even under an instantaneous or severe processing condition of up to 220,000 h ^−1. It is a thing.

Further, the present invention is usually carried out by passing exhaust gas through a catalyst layer arranged in a reaction tube or the like. As a method of adding ammonium acetate as a reducing agent,
Various aspects such as injecting by spraying or other method into the exhaust gas stream in the catalyst layer or upstream thereof, preliminarily impregnating and supporting the zeolite-based catalyst as a catalyst, using these and the like, but for a long period of time can be adopted. For stable operation, it is advantageous to adopt one of these modes.

[0027]

EXAMPLES Examples of the present invention will be described below, but it goes without saying that the present invention is not limited to these examples. In this example, first, a production example of a synthetic mordenite catalyst containing iron in its structure in the production process is shown, and then an example of a NOx-containing exhaust gas purification test conducted by using this catalyst is described.

<Production Example> First, commercially available sodium aluminate, sodium hydroxide and Nipsil VN3 (manufactured by Nippon Silica Industry Co., Ltd., trade name, amorphous silica) are used as production raw materials.
And iron nitrate nonahydrate [Fe
_{(NO 3) 3 · 9H 2} O ] was prepared. Of these, sodium aluminate and sodium hydroxide were used as a beaker (capacity:
(1000 ml) in distilled water with stirring and dissolution, and amorphous silica is added to this to make solution A, while iron nitrate nonahydrate is distilled water in a beaker (volume: 500 ml). Was dissolved in the solution to obtain a solution B. The quantitative ratios of the respective raw materials and the distilled water in the respective liquids A and B are as shown in Table 1.

Next, the total amount of each liquid A was gradually added dropwise to the total amount of each liquid A, and the total amount was added, followed by stirring to form a uniform mixed solution, which was equipped with a rotary stirrer inside a Teflon container. Stainless steel oak equipped with (internal volume: 1000 ml)
After transfer to a toclave and encapsulation, hydrothermal synthesis was carried out.
As for this condition, while maintaining the temperature at 180 ° C., the treatment is carried out under stirring (300 rpm), this treatment state is continued for 3 days and nights (72 hours), then the heating is stopped and allowed to stand, and it is naturally cooled to room temperature. Let

[0030]

[Table 1]

Subsequently, the product is filtered and slurried.
After washing with water until the pH became 10 or less, it was dried overnight at a temperature of 80 ° C. to obtain a powdery product. When this product was subjected to X-ray diffraction analysis (XRD), each sample showed a clear mordenite structure (Na type). Further, as a result of fluorescent X-ray analysis, it was found that the samples containing iron nitrate nonahydrate added to the synthetic raw material contained a predetermined amount of Fe in its structure. Table 1 shows the above-mentioned various conditions and results for each sample. In Table 1,
The case of manufacturing without adding the liquid B is shown as a comparative example.

Next, 0.5 m of the Na-type Fe-containing mordenite (the products of Examples 1 to 8 in Table 1) was added.
ol / l (81 g / 1000 ml) of ammonium nitrate aqueous solution was added (about 5 times equivalent of Na = 500 ml / Na-type Fe mordenite 20 g), and the mixture was heated at 60 ° C.
Then, the mixture was stirred for 1 hour, filtered, washed with water, repeated this treatment, and then dried at a temperature of 110 ° C. for about 5 hours. Further, it was baked at 500 ° C. for 2 hours, and H
Type Fe-containing mordenites are obtained, which are
It was used as a test catalyst in a purification test of contained exhaust gas. In addition, the H-type was similarly used for the comparative example. When each sample obtained by converting the Na-type Fe-containing mordenite into the H-type was subjected to the fluorescent X-ray analysis in the same manner as described above, no change was observed in the Fe content.

<< Purification test >> Each of the test catalysts obtained in the above (Table 1
The products of Comparative Examples and Examples 1-8 were converted to H as described above.
0.5 ml of each of the cast catalysts) was used for NOx
A purification test of the contained exhaust gas was performed. As the exhaust gas to be treated, NO = 48 ppm, CO = 910 ppm, CO ₂ = 6.
8%, O ₂ = 9.1%, steam (steam) = 9.1
%, N ₂ = gas containing a balance amount is used, the processing temperature is 250 to 400 ° C., and the space velocity (SV) is 22000.
It was carried out as ⁰ hr ⁻¹ .

In this purification test, a tubular fixed bed flow type reactor was used as the reactor. Ammonium acetate as the reducing agent was sprayed as an aqueous solution thereof onto the reaction tube immediately before the catalyst layer to obtain the reducing agent. 36 in the exhaust gas to be treated
It was injected so as to have a ppm (converted to N). The results are shown in Table 2. In addition, in Table 2, the comparative example is
e The mordenite that does not contain Fe in its structure was manufactured without adding the raw material, and the NOx removal rate was measured at each reaction temperature of 250, 300, 325, 350, 375 and 400 ° C. is there.

[0035]

[Table 2]

As shown in Table 2, Example No. 1 to No. 8
That is, the mordenite catalyst containing Fe in its structure at the time of its synthesis has a very low NO content of 48 ppm in the exhaust gas in the presence of ammonium acetate as a reducing agent. NO
x is extremely effectively removed and purified, and 300 to 3
It is clear that it exhibits excellent characteristics at a relatively low temperature of about 25 ° C.

For example, the NOx removal rates at the reaction temperatures of 300 ° C. and 325 ° C. are only 7.4% and 9.3%, respectively, in the comparative example (H-type mordenite containing no Fe), respectively. Example No. 3 is 64.8
% And 59.3%, Example No. 5 in 7
It shows NOx removal rates as high as 7.4% and 60.0%. Considering this point, in particular, the instantaneous value of space velocity 220,000 h ⁻¹ under severe conditions, it is clear that this is an extremely excellent NOx removal effect.

[0038]

INDUSTRIAL APPLICABILITY As described above, N composed of synthetic mordenite containing iron in its structure during its synthesis according to the present invention.
The Ox-containing exhaust gas purifying catalyst has an excellent NOx removal effect in the presence of ammonium acetate, and can very effectively remove NOx in the NOx-containing exhaust gas, for example, reaching a space velocity (SV) of 220,000 h ^-1. Treatment temperature of 300 to 32, even under instantaneous and severe purification treatment conditions
NOx removal rate of 60-78% at a relatively low temperature of 5 ° C
That is, an extremely excellent purification effect can be obtained.

The catalyst according to the present invention, which is composed of synthetic mordenite with iron in the structure during its synthesis, is excellent even when the NOx concentration in the exhaust gas is extremely low.
Since it has a purifying ability, it is possible to effectively reduce and purify even a trace amount of NOx in the lean combustion exhaust gas from a gas engine or the like, and further, ammonium acetate is used as a reducing agent. Also, excellent effects such as no slip ammonia as in the case of using ammonia as a reducing agent can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the amount of Fe in a manufacturing raw material and the amount of Fe contained in generated mordenite.

FIG. 2 is a graph showing changes in the NOx removal rate depending on the Fe content contained during the synthesis of mordenite.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location B01J 29/88 ZAB A

Claims (2)

[Claims] 1. A NOx-containing exhaust gas purifying catalyst using ammonium acetate as a reducing agent, which is composed of a synthetic mordenite containing iron in its structure by using a raw material containing iron during its synthesis. A catalyst for purifying exhaust gas containing NOx. 2. A NOx-containing exhaust gas-purifying catalyst comprising synthetic mordenite having iron in its structure by using a raw material containing iron during the synthesis of the NOx-containing exhaust gas in the presence of an ammonium acetate reducing agent. A method for purifying NOx-containing exhaust gas, which comprises treating the exhaust gas.