JP2734480B2 - Nitrogen oxide decomposition catalyst - Google Patents

Description

The present invention relates to a catalyst for removing NOx from a gas containing nitrogen oxides (hereinafter abbreviated as NOx),
More specifically, the present invention provides a catalyst for directly decomposing NOx.

[Conventional technology]

NOx in combustion exhaust gas emitted from industrial plants, automobiles and the like is a substance that can cause the generation of photochemical smog, and the development of a method for removing NOx is a serious and urgent social issue from the standpoint of environmental protection. Among NOx, it is particularly difficult to remove NO, and various methods have been studied so far. For example, the catalytic reduction method has been proposed as one of the effective means and is under development, but requires a reducing agent such as ammonia, hydrogen or carbon monoxide, and a special method for recovering or decomposing the unreacted reducing agent. Requires equipment. On the other hand, the catalytic cracking method does not require a special additive such as a reducing agent, is a method of decomposing into nitrogen and oxygen only by passing through a catalyst layer, and is the most desirable method because the process is simple. According to previous research, Pt, CuO, Co ₃ O ₄
Although NO decomposition activity was observed in any of them, they could not be used as a practical catalyst due to the poisoning of oxygen, which is a decomposition product.References Toshio Uchijima, Surface, Vol.18 No.3, (198
0) p.132｝.

[Problems to be solved by the invention]

As a catalyst replacing the above catalyst, a method of ion-exchanging Cu with silica gel or zeolite has been proposed, but has the following problems.

(1) The catalyst supporting silica gel copper ions by the ion exchange method has a considerably high initial activity, but the activity rapidly decreases with time.

(2) A catalyst ion-exchanged with copper using Y-type zeolite or mordenite has a low decomposition activity in the presence of oxygen.

(3) A catalyst in which ZSM-5 type zeolite is used for ion exchange with copper has been proposed.
There is a problem that the selectivity for conversion to N ₂ (2NO → N ₂ + O ₂ ) is low. (JP-A-60-125250).

[Means for solving the problem]

An object of the present invention is to solve the above-described problems of the prior art, and a catalyst in which copper and a Group VIII element are contained using a crystalline silicate having a specific composition and a crystal structure is used as an NO catalyst. not only a very high activity as catalytic cracking catalyst has high selectivity to N ₂ NO, the and oxygen, S
They found that the activity was stable even in the presence of Ox, and completed the present invention.

That is, according to the present invention, in the dehydrated state, the molar ratio of oxides is (1.0 ± 0.4) R ₂ O · [aM ₂ O ₃ · bAl ₂ O ₃ ] · ySiO ₂ (wherein R: alkali metal ion And / or hydrogen ion, M: Group VIII element, rare earth element, titanium, vanadium,
An ion of one or more elements of the group consisting of chromium, niobium, and antimony, a crystalline silicate having a chemical composition of a + b = 1, a> 0, b> 0, y> 12) containing copper and group VIII elements A catalyst for decomposing nitrogen oxides.

[Action]

The catalyst of the present invention shows extremely high activity in the decomposition reaction of NO, or its activity is maintained for a long time. Regarding the action of the catalyst of the present invention, the mechanism for easily performing the oxidation-reduction cycle (Cu ² Cu ⁺ ) of ion-exchanged copper ions and releasing oxygen at a relatively low temperature, the specific crystal structure of the catalyst of the present invention, Structural stability and heat resistance are acting in a complex manner, and by further containing a Group VIII element, the decomposition reaction of NO is promoted and copper ions ion-exchanged by the Group VIII element are stabilized, and heat resistance is improved. It is thought that the property is improved.

 Hereinafter, the catalyst used in the present invention will be described in detail.

Zeolite is a crystalline aluminosilicate containing zeolite water as indicated by the Greek word "boiling stone", and its composition is generally represented by the following formula.

_{xM 2 / n O · Al 2} O 3 · ySiO 2 · zH 2 O ( where n is the valence of the cation M, x is a number in the range of 0.8 to 2, y is a number of 2 or more, z is 0 or more The basic structure is as follows: SiO ₄ tetrahedron in which four oxygens are coordinated at its apex with silicon as the center, and AlO ₄ tetrahedron in which aluminum is at the center instead of this silicon is O / ( It is one in which oxygen is shared with each other so that the atomic ratio of (Al + Si) becomes 2 and regularly and three-dimensionally bonded. As a result, a three-dimensional network structure having pores of different sizes and shapes is formed due to the difference in the bonding method between the tetrahedrons. The negative charge of the AlO ₄ tetrahedron is electrically neutralized by combining with a cation such as an alkali metal or an alkaline earth metal. Generally, the pores thus formed have a size of a few to several tens of squares, but the metal cations associated with the AlO ₄ tetrahedron are converted to other metal ions of different size or valence. The size of the pores can be changed by exchanging.

Zeolite is a gas or liquid industrial desiccant using these pores, or a molecular sieve that adsorbs and separates molecules in a mixture of two or more molecules, and a metal cation exchanged with hydrogen ions is a solid acid. It is widely used as an industrial catalyst utilizing this property.

There are many types of zeolites, and different zeolites are given different names depending on the crystal structure characterized by the X-ray diffraction diagram. Naturally occurring substances include chabazite, erionite, mordenite, clinoptilolite and the like, and synthetic zeolites A,
X, Y, large port mordenite, ZSM-5 and the like are well known.

Of these many zeolites, those usable in the present invention are limited to crystalline silicates having a specific compositional structure. The crystalline silicate of the present invention does not exist in nature, but is synthesized by the following method.

The crystalline silicate used in the present invention is a source of silica, a Group VIII element, a rare earth element, titanium, vanadium,
Create a reaction mixture containing a source of chromium, niobium, and antimony oxides, a source of alumina, a source of alkali, water and an organic nitrogen-containing compound, and heat the mixture for the time and temperature required to form a crystalline silicate. To be synthesized.

The source of silica may be any silica source as long as it is a compound of silica commonly used in zeolite synthesis, such as solid silica powder, colloidal silica, or silicates such as water glass. Used.

Compounds such as sulfates, nitrates, and chlorides are used as the source of the Group VIII element, rare earth element, titanium, vanadium, chromium, niobium, and antimony.

Examples of group VIII elements include iron, cobalt, ruthenium, rhodium, platinum, palladium and the like, and examples of rare earth elements include lanthanum, cerium, neodymium and the like.

The source of alumina is most preferably sodium aluminate, but compounds such as chlorides, nitrates, sulfates, oxides or hydroxides are used.

As a source of the alkali, a hydroxide of an alkali metal such as sodium or a compound with aluminate or silicic acid is used.

The following compounds can be used as the organic nitrogen-containing compound, which is one of the raw materials for hydrothermal synthesis of crystalline silicate.

(1) Organic amines; primary amines such as n-propylamine and monoethanolamine; secondary amines such as dipropylamine and diethanolamine; tertiary amines such as tripropylamine and triethanolamine; or ethylenediamine Or a mixture of the above compound with a halogenated hydrocarbon (such as propyl bromide); and other quaternary ammonium salts such as tetrapropylammonium salt. (2) Organic nitrogen compounds other than organic amines; These various organic compounds such as pyrazine and pyrazole are merely examples, and the present invention is not limited thereto.

In the crystalline silicate used in the present invention, a part of Al in the structure of the conventional zeolite is a group VIII element, a rare earth element, titanium, vanadium, chromium, niobium, antimony, and further SiO ₂ / (M ₂ O ₃ + Al ₂ O ₃ ) ratio is
It is characterized by being 12 or more, and is produced from a reaction mixture having the following molar composition.

SiO ₂ / (M ₂ O ₃ + Al ₂ O ₃ ) 12 to 3000 (preferably 20 to 20
0) OH ⁻ / SiO ₂ 0 to 1.0 (preferably 0.2 to 0.8) H ₂ O / SiO _{2 2 to} 1000 (preferably 10 to 200) Organic nitrogen-containing compound / (M ₂ O ₃ + Al ₂ O ₃ ) (Preferably,
5 to 50) The crystalline silicate used in the present invention is synthesized by heating the raw material mixture at a temperature and for a time sufficient to produce a crystalline silicate, and the hydrothermal synthesis temperature is 80 to 300.
° C, preferably in the range of 130 to 200 ° C, and the hydrothermal synthesis time is 0.5 to 14 days, preferably 1 to 10 days. The pressure is not particularly limited, but it is preferable to carry out the operation under its own pressure.

The hydrothermal synthesis reaction is continued by heating the raw material mixture to the desired temperature and, if necessary, stirring to form a crystalline silicate. After the crystals have thus formed, the reaction mixture is cooled to room temperature, filtered, washed with water and the crystals are separated. Further, drying is usually performed at 100 ° C. or higher for about 5 to 24 hours.

The crystalline silicate produced by the above-described method can be used as is, by a well-known technique, as it is, or by mixing it with a binder or the like conventionally used for catalyst molding to form an appropriate size to form a catalyst. Can be used.

The crystalline silicate used in the present invention is a regular porous crystalline material having a certain crystal structure, and generally shows an X-ray diffraction pattern shown in Table 1.

The crystalline silicate obtained by the above hydrothermal synthesis is Na ⁺
Contains alkali metal ions such as (C ₃ H ₇ ) ₄ N ⁺ and organic nitrogen-containing compound ions. In order to replace some or all of these ions with hydrogen ions, the organic compounds are removed by baking in air at a temperature in the range of 400 to 700 ° C. for 2 to 48 hours, and then the mixture is treated with a strong acid such as hydrochloric acid. There is a method of immersing directly in an H-type, or a method of immersing in an aqueous solution of an ammonium compound to obtain an NH ₄ type, followed by baking to obtain an H-type.

Ion exchange sites such as alkali metal ions and hydrogen ions in the crystalline silicate obtained by the above method are exchanged for copper ions and further ions of group VIII elements by the following method. The catalyst of the present invention is obtained by supporting a metal or an oxide.

Copper ion exchange is performed by a usual method such as immersing a crystalline silicate in an aqueous solution in which a mineral acid salt such as copper sulfate or copper nitrate or an organic acid salt such as copper acetate is dissolved. The concentration of copper ions in the aqueous solution can be arbitrarily selected according to the desired copper ion exchange rate, and the copper ions are Cu ⁺ , Cu ²⁺ , C
Any form of uOH ⁺ is exchanged with the cation in the crystalline silicate. After the completion of ion exchange, the substrate is sufficiently washed with water and dried. The copper ion exchange rate of the catalyst used in the present invention is essential to be at least 10% or more of all exchangeable cations contained in the crystalline silicate as a catalyst base, and the exchange rate is high. The higher the NO decomposition activity,
Preferably it is in the range of 40 to 180%. When the exchange rate is less than 10%, no effective NO decomposition activity is exhibited.

The ion exchange rate of copper indicates the ratio of copper ions in the ion exchange site, and is calculated assuming that Na ⁺² atoms and Cu 2 ⁺ 1 atoms exchange ions.

Therefore, an exchange rate of 200% indicates a state in which all ion exchange sites are ion-exchanged with copper (ion exchange is performed with one Cu). In the case of ion-exchanging copper ions and group VIII element ions, a mixed aqueous solution of a copper compound and a group VIII element compound is used in the above step.

As a method of supporting the metal or oxide of the Group VIII element on the crystalline silicate after the copper ion exchange, the crystalline silicate is immersed in an aqueous solution of a compound of the Group VIII element, dried,
A firing method and a reduction method are used.

When CuO and a group VIII element are supported on the crystalline silicate, a method is used in which the crystalline silicate is immersed in an aqueous solution containing a compound of the Cu and the group VIII element, dried, and fired.

Here, as Group VIII elements, Fe, Co, Ni, Pt, Pd, Rh, Os, I
Point to r. The content of the Group VIII metal is preferably in the range of 0.1 to 20% by weight (based on the total weight of the catalyst).

The NO decomposition catalyst of the present invention has a wider use temperature range as compared with the conventional catalyst, in the range of 300 to 1000 ° C, preferably 400 to 70 ° C.
Used in the range of 0 ° C.

When the catalyst of the present invention is used industrially, it is desirable to mold it into an appropriate shape before use. For example, it is formed into a spherical, columnar, or honeycomb shape by using an inorganic oxide such as silica or alumina or a clay as a binder and optionally using a molding aid such as an organic material. The crystalline silicate before exchange with the copper ions may be molded in advance, and the size of the molded body is not particularly limited.

Example 1 A crystalline silicate was synthesized as follows.

Water glass, ferric sulfate, aluminum sulfate, water 36Na
_{2 O · (0.1Fe 2 O 3} · 0.9Al 2 O 3) · 80SiO 2 · 1600H 2 O were blended so that the molar ratio of which was added an appropriate amount of sulfuric acid, pH of the mixture is 9 back and forth After that, propylamine and propyl bromide as organic compounds were added 20 times the total number of moles of Fe ₂ O ₃ and Al ₂ O ₃ , mixed well, and inserted into a 500 cc stainless steel autoclave.

The mixture was reacted at 160 ° C. for 3 days while stirring at about 500 rpm. After cooling, the solid content was filtered, washed thoroughly with water until the pH of the washing water was about 8, dried at 110 ° C for 12 hours,
Calcination was performed at 3 ° C. for 3 hours.

The crystal grain size of this product is about 1 μm, and the composition expressed by the molar ratio of oxide is expressed in the form of dehydration (H, N
a) was _{2 O · (0.1Fe 2 O 3} 0.9Al 2 O 3) · 80SiO 2. This is called crystalline silicate 1.

When synthesizing this crystalline silicate 1, hydrochloric acid or the like may be used instead of sulfuric acid among the raw materials, ferric chloride may be used instead of ferric sulfate, and silica sol may be used instead of water glass. A similar silicate was obtained by using.

The same silicate was obtained by reacting at 170 ° C. or 180 ° C. for 2 days instead of reacting at 160 ° C. for 3 days as hydrothermal synthesis conditions.

Except that the addition amount of ferric sulfate and aluminum sulfate at the time of mixing the raw materials of the crystalline silicate 1 was changed as follows in terms of the molar ratio of Fe ₂ O ₃ and Al ₂ O ₃ , Crystal silicates 2 to 4 were prepared by repeating the same operation as in the case.

In preparing the crystalline silicate 1, hydrochloric acid was used instead of sulfuric acid, and instead of ferric sulfate, cobalt chloride, ruthenium chloride, rhodium chloride, lanthanum chloride, cerium chloride, titanium chloride, vanadium chloride, chromium chloride, Crystalline silicates 5 to 13 were prepared by repeating the same operation as for crystalline silicate 1 except that antimony chloride was added in the same mole number as Fe ₂ O _{3 in} terms of oxide. The composition excluding the organic compounds of these crystalline silicates is expressed in terms of the molar ratio of oxides (form of dehydration), and is expressed as (H, Na) ₂ O ·
(0.1M ₂ O ₃ · 0.9Al ₂ O ₃ ) · 80SiO ₂ . Where M is C
o, Ru, Rh, La, Ce, Ti, V, Cr, and Sb (in numerical order of crystalline silicates 5 to 13).

In the crystalline silicate 1, SiO ₂ /(0.1
Fe ₂ O ₃ +0.9 Al ₂ O ₃ ) Except that the ratio was 20,200, the same operation as that of the crystalline silicate 1 was repeated, and each crystalline silicate was
14,15 were prepared.

It was confirmed that the powder X-ray diffraction patterns of the above crystalline silicates 1 to 15 showed the patterns shown in Table 1.

An ion exchange operation was performed in which 10 g of each of the above crystalline silicates 1 to 15 was placed in an aqueous solution in which 1 g of copper acetate was dissolved in 500 cc of water, and stirred at room temperature for 12 hours. After repeating this ion exchange operation three times, washing with water, further immersing in a predetermined concentration of platinum chloride aqueous solution, drying at 100 ° C. for 12 hours, hydrogen reduction at 300 ° C. for 3 hours, platinum 0.5% by weight
(Based on the total amount of catalyst) Supported catalysts 1 to 15 (corresponding to the number of crystalline silicate) were prepared.

Catalysts 1 to 15 were sized to 16 to 32 mesh, and 0.5 g of the catalyst was charged into a fixed-bed flow reactor under normal pressure, and an activity evaluation test was performed under the following reaction conditions. Table 2 shows the results.

Gas _{composition: NO: 0.5%, O 2} : 1%, He: balance gas flow rate: 1 Nl / h, a reaction temperature of 500 ° C. Example 2 The crystalline silicate 1 of Example 1 was subjected to Cu ion exchange (Cu ion exchange rate: 120%) in the same manner as in Example 1, and was immersed in the following aqueous solution, dried, and then dried. Firing,
Hydrogen reduction treatment was performed to prepare Catalysts 16 to 23.

○ Catalysts 16 to 18: Catalysts 16 to 18 were prepared by immersing in respective aqueous solutions of iron nitrate, cobalt nitrate and nickel nitrate, drying and calcining at 500 ° C for 3 hours.

○ Catalysts 19 to 23: After immersion in aqueous solutions of palladium nitrate, rhodium chloride and ruthenium chloride, mixed aqueous solutions of platinum chloride and palladium chloride, and mixed aqueous solutions of platinum chloride and palladium chloride and rhodium chloride, respectively, then reduced at 300 ° C with hydrogen To prepare catalysts 19 to 23.

The activity of these catalysts was evaluated in the same manner as in Example 1. Table 3 shows the results.

Example 3 0.5 g of the catalyst 1 of Example 1 was packed in a fixed-bed flow reactor under normal pressure, and an activity evaluation test was conducted while changing the reaction conditions. Table 4 shows the results.

All the reacted NO was converted to N ₂ and O ₂ .

As described above, it was found that the catalyst of the present invention had high activity even when a gas containing SO ₂ was used, and that the change in activity with time was small.

Example 4 A mixture of ferric chloride and chromium chloride was used instead of ferric sulfate at the time of preparing the raw material of the crystalline silicate 1 of Example 1, and a mixture of 36Na ₂ O · (0.09Fe ₂ O ₃ · 0.01Cr ₂ O ₃ · 0.9 Al ₂ O ₃ ) · 80
A crystalline silicate is prepared in the same manner as the crystalline silicate 1 except that the molar ratio of SiO ₂ and 1600H ₂ O is adjusted.
16 was obtained and subjected to Cu ion exchange by the same method (however, the ion exchange operation was repeated twice), and then a catalyst 24 (Cu ion exchange rate 110%) carrying 0.5 weight of platinum was obtained by the same method.

As a result of performing the same activity evaluation as in Example 1, the NO conversion rate was 81.
%Met.

(Comparative example)

Ferric sulfate was not added at the time of preparing the raw material of the crystalline silicate 1 of Example 1, but the molar ratio of 36Na ₂ O · Al ₂ O ₃ · 80SiO ₂ · 1600H ₂ O was prepared. Then, the same operation as in Example 1 was repeated except that the mixture was adjusted to have a pH of about 9, and then tetrapropylammonium bromide was added as an organic compound 20 times that of Al ₂ O ₃ .

In the same manner as in Example 1, ion exchange was performed with copper (Cu ion exchange rate).
105%), 0.5% by weight of platinum was loaded, and the same condition activity evaluation was performed. As a result, the NO conversion was high at 76%, but reacted.
The conversion of NO to N ₂ was as low as 73%, indicating that there were many by-products such as NO ₂ .

〔The invention's effect〕

As described above, as shown in the examples, the catalyst containing copper in the crystalline silicate of the present invention can be used as a catalyst for decomposing NOx in exhaust gas into N ₂ and O ₂ .

Continued on the front page (51) Int.Cl. ⁶ Identification code Reference number in the agency FI Technical display location B01D 53/36 102H

Claims (1)

(57) [Claims] In the dehydrated state, the molar ratio of oxides is (1.0 ± 0.4) R ₂ O · [aM ₂ O ₃ · bAl ₂ O ₃ ] · ySiO ₂ (where R is an alkali metal ion And / or hydrogen ion, M: Group VIII element, rare earth element, titanium, vanadium,
An ion of one or more elements of the group consisting of chromium, niobium, and antimony, a crystalline silicate having a chemical composition of a + b = 1, a> 0, b> 0, y> 12) containing copper and group VIII elements A catalyst for decomposing nitrogen oxides.