JP2007038182A - Nitrogen oxide catalytic reduction purification method - Google Patents

Abstract

Provided is a method for efficiently purifying nitrogen oxides with ammonia at low temperature using an inexpensive zeolite catalyst not supporting an active metal and ammonia.
A catalyst comprising a hydrogen-type and / or ammonium-type β zeolite is added to a processing gas containing nitrogen monoxide and nitrogen dioxide as a reducing component without supporting a catalytically active metal species, A method for catalytically reducing and purifying nitrogen oxides, which comprises contacting them. The molar ratio of silica (SiO ₂ ) to alumina (Al ₂ O ₃ ) in the β zeolite is 20 to 200, and the content of nitrogen dioxide in the processing gas is 0.5 to 1.5 in terms of molar ratio to nitrogen monoxide. The treatment temperature is preferably 150 ° C. or higher.
[Selection figure] None

Description

  The present invention provides a method for efficiently catalytically reducing and purifying nitrogen oxides in exhaust gas using an inexpensive catalyst.

Various methods have been proposed or put to practical use for purifying nitrogen oxides in exhaust gas. As a method for purifying nitrogen oxides emitted from fixed sources such as boilers, a selective catalytic reduction method in which a treatment gas obtained by adding ammonia to exhaust gas is brought into contact with a V ₂ O ₅ —TiO ₂ catalyst is industrially adopted. ing. As a method for purifying nitrogen oxides emitted from mobile sources such as automobiles, in addition to the selective catalytic reduction method using ammonia as in the case of stationary sources, hydrocarbons, carbon monoxide, and hydrogen are used as reducing components. Known catalytic reduction methods, direct decomposition methods, and storage reduction methods using noble metal catalysts containing nitrogen oxide storage components are known.

  The catalytic reduction method using ammonia as a reducing component has a high reaction selectivity with respect to the reduction of nitrogen oxides and is one of the excellent purification methods. It has been clarified that the reduction reaction of nitrogen oxides by ammonia proceeds mainly according to Formula 1, and it is considered that a pair of a solid acid point and active oxygen of the catalyst used acts as an active point.

一方、ゼオライトのみからなる触媒成分として、例えば特許文献１に於いて、水素型合成ゼオライトが提案されている。ここでは５００〜７００℃の高温域で十分な浄化性能が得られることを目的に、一酸化窒素を含む排ガスに一酸化窒素に対するモル比で１．０〜１．３の範囲内のアンモニアを混合した処理ガスを、水素型ゼオライトからなる触媒に接触させている。特許文献１で使用される合成ゼオライトはＸ型、Ｙ型、及びモルデナイトである。しかしながら、特許文献１に開示されている水素型ゼオライトを用いた方法では、５００℃未満の低温域での浄化は十分ではなかった。

On the other hand, as a catalyst component composed only of zeolite, for example, Patent Document 1 proposes a hydrogen-type synthetic zeolite. Here, for the purpose of obtaining sufficient purification performance in a high temperature range of 500 to 700 ° C., ammonia within a range of 1.0 to 1.3 is mixed with exhaust gas containing nitric oxide in a molar ratio with respect to nitric oxide. The treated gas is brought into contact with a catalyst made of hydrogen-type zeolite. The synthetic zeolite used in Patent Document 1 is X-type, Y-type, and mordenite. However, the method using hydrogen-type zeolite disclosed in Patent Document 1 has not been sufficiently purified in a low temperature region of less than 500 ° C.

  Therefore, for the purpose of efficient catalyst purification in a low temperature range, many zeolite catalysts supporting catalytically active metal species and purification methods using the catalysts have been proposed.

For example, in Patent Document 2, a crystalline aluminosilicate having a specific lattice spacing by X-ray powder diffraction, a SiO ₂ / Al ₂ O ₃ molar ratio of 15 to 60, and a catalyst containing copper ions And a method for selectively reducing nitrogen oxides in exhaust gas with ammonia using the catalyst. In US Pat. No. 6,057,049, a zeolite having a three-dimensional structure in which pores having a silica to alumina ratio of at least about 10 and an average dynamic pore size of at least about 7 angstroms are connected in a three-dimensional structure. A method for reducing nitrogen oxides by ammonia using a catalyst on which at least one or more is supported is disclosed. Preferred zeolite species for enhancing catalyst performance are selected from the group consisting of ultra-stabilized Y, β zeolite and ZSM-20. Further, Patent Document 4 discloses a nitrogen oxide reduction catalyst in which an active metal component is supported on a faujasite type zeolite, and a method for reducing and purifying nitrogen oxide. Patent Document 5 discloses a method for removing nitrogen oxides by ammonia using hydrogen and / or iron-substituted mordenite having a Si / Al molar ratio of 10 or more.

  In these conventional techniques, metal species such as iron and copper supported on zeolite are active ingredients for catalytically promoting selective reduction of nitrogen oxides by ammonia, and these active ingredients are not supported. Indicates that the reaction of Formula 1 is difficult to proceed, and the reduction and purification performance of nitrogen oxides is extremely small.

  However, the loading of the catalytically active metal component has been a factor of cost increase in terms of the catalyst manufacturing process and variable costs. Furthermore, some metal components have a risk of secondary pollution due to metal scattering during use. From these viewpoints, it is required to efficiently purify nitrogen oxides over a wide temperature range using a catalyst that does not support catalytically active metal species. In the case of receiving the catalyst, a catalyst having sufficient durability has been demanded.

Japanese Unexamined Patent Publication No. 57-140628 JP 59-230642 A JP-A-2-2933021 JP-A-5-138030 JP-A-2-68120

  As described above, in the prior art, as a method for efficiently purifying nitrogen oxides from a low temperature, it is necessary to use a catalyst having a metal species that can be an active point of a catalytic reaction as a constituent element. In contrast, the present invention provides a method for efficiently purifying nitrogen oxides even at a low temperature by using an inexpensive catalyst that does not carry an active metal.

  As a result of intensive studies on the denitration performance of the zeolite catalyst and the influence of the reaction conditions on the denitration performance, the present inventors have found that the hydrogen-type and / or ammonium-type β-zeolite catalyst itself has the catalytic activity as previously proposed. The present inventors have found that nitrogen oxides in exhaust gas can be reduced and purified with ammonia without supporting any metal species, and the present invention has been completed.

  That is, the present invention provides a catalyst composed of hydrogen-type and / or ammonium-type β zeolite without supporting a catalytically active metal species, exhaust gas containing nitrogen monoxide and nitrogen dioxide, and ammonia added as a reducing component. This is a method for catalytically reducing and purifying nitrogen oxides in exhaust gas, wherein the mixed processing gas is brought into contact at 150 ° C. or higher.

  Hereinafter, the method for reducing and purifying nitrogen oxides of the present invention will be described in detail.

  The catalyst used in the present invention is hydrogen-type and / or ammonium-type β zeolite.

β zeolite is a 12-membered ring pore having a pore size of 0.55 × 0.55 nm in the c-axis direction, and a 12-membered ring having a pore size of 0.76 × 0.64 nm in the a-axis and b-axis directions. It is composed of pores, and these pores intersect to form a three-dimensional pore. Beta zeolite has the X-ray diffraction pattern shown in Table 1. β-zeolite can be produced by a known technique, and the production method of β-zeolite used in the present invention is not limited at all. For example, a hydrothermal synthesis method in which a raw material mixture slurry such as silica, alumina, alkali metal, organic matter, water, etc. is crystallized in an enclosed pressure vessel at an arbitrary temperature of 100 to 180 ° C., or a gel obtained by drying the raw material mixture slurry A method of crystallization in a sealed pressure vessel using a solid solid can be employed. Further, the molar ratio of SiO ₂ to Al ₂ O ₃ in β zeolite (hereinafter referred to as SiO ₂ / Al ₂ O ₃ molar ratio) is not particularly limited, but if it is a SiO ₂ / Al ₂ O ₃ molar ratio of 20 or more, A sufficiently heat-resistant β zeolite is obtained, and on the other hand, a SiO ₂ / Al ₂ O ₃ molar ratio of 200 or less is preferred because the purification performance of the target nitric oxide and nitrogen dioxide is high.

更に、本発明に係る触媒は既述の様に水素型及び／又はアンモニウム型のβゼオライトであり、加えて活性金属を担持させたものではない。一般的にβゼオライトは、構造指向作用を有する有機物及びアルカリ金属を含む原料混合物から結晶化して製造されるものであり、結晶化後は有機物及びアルカリ金属を含有しているが、これを水素型及び／又はアンモニウム型に変化するためには、余分な有機物、アルカリ金属を洗浄にて除去する方法、プロトン或いはアンモニウムイオンを含む水溶液で処理する方法、及び十分な条件での熱処理で有機物を分解／燃焼させて除去する方法等が例示できる。これらの方法を適宜組合せることも可能である。このようにして製造された水素型及び／又はアンモニウム型のβゼオライトは、活性金属を担持させることなく窒素酸化物の浄化触媒として用いることができる。

Furthermore, the catalyst according to the present invention is a hydrogen-type and / or ammonium-type β-zeolite as described above, and does not additionally support an active metal. In general, β zeolite is produced by crystallization from a raw material mixture containing an organic substance and an alkali metal having a structure directing action, and after crystallization contains an organic substance and an alkali metal. In order to change to the ammonium type, the organic matter is decomposed / decomposed by a method of removing excess organic matter, alkali metal by washing, a method of treating with an aqueous solution containing protons or ammonium ions, and a heat treatment under sufficient conditions. The method of removing by burning can be exemplified. These methods can be appropriately combined. The hydrogen-type and / or ammonium-type β-zeolite thus produced can be used as a nitrogen oxide purification catalyst without supporting an active metal.

  Here, it is preferable that the alkali metal used in the crystallization of the zeolite is reduced to less than 2% in a molar ratio with respect to the aluminum of the zeolite framework. In addition, when the hydrogen-type and / or ammonium-type β-type zeolite is industrially produced, impurities may be slightly mixed from the material of the production equipment. For example, when a stainless steel manufacturing facility is used, a trace amount of metal such as iron is contained, but the content is generally 0.1% by weight or less.

  The catalyst used in the present invention can be used by mixing and molding hydrogen-type and / or ammonium-type β zeolite with binders such as silica, alumina and clay mineral. Examples of the clay mineral used for molding include kaolin, attapulgite, montmorillonite, bentonite, allophane, and sepiolite. Further, a cordierite or metal honeycomb base material may be used by washing with a hydrogen-type or ammonium-type β zeolite.

  In the nitrogen oxide reduction and purification method of the present invention, it is essential that nitrogen monoxide and nitrogen dioxide are contained in the processing gas as nitrogen oxides to be purified. When only nitrogen monoxide is used, nitrogen oxides cannot be sufficiently purified at a low temperature as disclosed in JP-A-2-2933021, and when only nitrogen dioxide is used, the rate of the reduction reaction of nitrogen dioxide by ammonia. As a result, nitrogen oxides cannot be sufficiently purified.

  The abundance ratio of nitrogen monoxide and nitrogen dioxide contained in the processing gas is not particularly limited, but in order to increase the reduction and purification rate of nitrogen oxides, nitrogen dioxide is in a molar ratio of 0.5 to 1.5 with respect to nitrogen monoxide. Is preferable, and the range of 0.9 to 1.1 is more preferable. When purifying exhaust gas that consists of only nitrogen monoxide, it is possible to use a treatment gas containing nitrogen monoxide and nitrogen dioxide in combination with a system that converts a portion of nitrogen monoxide to nitrogen dioxide in the previous stage of the catalyst. is there. For example, in the case of diesel automobile exhaust gas, most of the nitrogen oxides in the exhaust gas are nitrogen monoxide. Therefore, by placing an oxidation catalyst etc. in front of the nitrogen oxide reduction purification catalyst, a part of the nitrogen monoxide is oxidized. It can be converted to nitrogen and processed. Here, the nitrogen oxide concentration of the processing gas that can be processed by the present invention is not limited.

  In the nitrogen oxide reduction and purification method of the present invention, ammonia which is a nitrogen oxide reducing component is added. The method for adding ammonia is not particularly limited, and examples thereof include a method in which ammonia gas is directly added, a method in which ammonia water is sprayed and vaporized, and a method in which urea water is sprayed and thermally decomposed. The amount of ammonia added is not particularly limited, but a molar ratio of 0.8 to 1.05 is preferable with respect to the total of nitrogen monoxide and nitrogen dioxide in order to sufficiently purify nitrogen oxides. Further, the addition of excess ammonia is advantageous for the purification of nitrogen oxides, but it is not preferable because measures against the discharge of unreacted ammonia are required.

  In the method of the present invention, when purifying nitrogen oxides by bringing a mixed processing gas of ammonia and ammonia into contact with a catalyst composed of hydrogen-type and / or ammonium-type β zeolite, the processing temperature is set to 150 ° C. or higher. It is preferable. When the treatment temperature is lower than 150 ° C., sufficient reduction and purification of nitrogen oxides cannot be achieved.

  In the method of the present invention, other components contained in the processing gas are not particularly limited, and examples include carbon monoxide, carbon dioxide, hydrogen, oxygen, nitrogen, hydrocarbons, sulfur oxides, and water. It is also effective in some cases. Specifically, in the method of the present invention, nitrogen oxides can be purified from a wide variety of exhaust gases such as diesel automobiles, boilers, gas turbines and the like.

Moreover, the space velocity at the time of making process gas contact a beta zeolite catalyst is not specifically limited. Preferred space velocities 500～500000Hr ^-1 on a volume basis, more preferably from 2000~300000hr ^-1.

  In the method of the present invention, the reduction and purification of nitrogen oxides can be achieved with a β zeolite catalyst that does not carry active metals, so there is no problem of deterioration of active metals, and the purification efficiency of nitrogen oxides is sufficiently high even after durability at high temperatures. can get. Furthermore, the purification method of the present invention can not only reduce the catalyst cost based on the loading of the active metal, but also avoid the risk of secondary pollution caused by the scattering of the active metal species to be loaded.

EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to these Examples at all.
<Synthesis of β zeolite>
Β zeolite was synthesized with reference to the method disclosed in JP-A-2-2933021. A sodium silicate aqueous solution (SiO ₂ ; 130 g / l, Na ₂ O; 41.8 g / l, Al ₂ O ₃ ; 0.05 g / l) in an overflow type reaction tank (actual volume 4.8 liters) in a stirring state And an aqueous solution of aluminum sulfate (Al ₂ O ₃ ; 21.3 g / l, SO ₄ ; 240 g / l) were simultaneously supplied at a flow rate of 18.2 liter / Hr and 4.5 liter / Hr, respectively, and reacted with stirring. A slurry product was obtained. At this time, the average residence time of the slurry was 12.5 minutes. Moreover, the supply amount of the sodium silicate aqueous solution was adjusted so that the pH of the reaction tank during the reaction was 6-8. The slurry product overflowed from the reaction tank was dehydrated with Nutsche and washed with water to obtain a granular amorphous aluminosilicate. The composition of the obtained granular amorphous aluminosilicate has a SiO ₂ / Al ₂ O ₃ molar ratio of 41.5, a Na ₂ O / Al ₂ O ₃ molar ratio of 1.15, and a water content of 69.7 wt. %Met.

The granular amorphous aluminosilicate; 189 g, solid sodium hydroxide; 1.4 g, solid potassium hydroxide; 3.5 g and a 20 wt% tetraethylammonium aqueous solution; 480 g were stirred and mixed for 30 minutes to obtain a raw material for β zeolite. The raw slurry was transferred to a 1 liter closed pressure vessel and crystallized at 150 ° C. for 96 hours while stirring at a peripheral speed of 0.8 m / s. The reaction product was separated into solid and liquid, washed with pure water at 70 ° C., and dried at 110 ° C. overnight. The obtained reaction product had the same X-ray diffraction pattern as Table 1 and was β zeolite. Further, the resulting product was analyzed by ICP emission analysis, was the composition of _{0.02Na 2 O · 0.03K 2 O ·} Al 2 O 3 · 36.3SiO 2 in anhydrous basis.

Examples 1-3
<Preparation of catalyst 1>
The β zeolite obtained by the above-described method was calcined at 600 ° C. for 2 hours in a dry air stream to remove organic substances contained in the β zeolite. Thereafter, 10 g of the calcined β-zeolite was fractionated, and ion exchange treatment with stirring at 60 ° C. for 20 hours was repeated twice using 0.5 N aqueous ammonium chloride solution; 200 g. Subsequently, the solid-liquid separated β zeolite was washed with pure water and dried at 110 ° C. to obtain an ammonium type β zeolite. When the ammonium-type β zeolite was analyzed by ICP emission analysis, the composition ratio of Na and K to aluminum was less than 1%. The obtained ammonium-type β zeolite was subjected to a catalytic reaction test as a catalyst used in Example 1.
<Preparation of catalyst 2>
The ammonium type β zeolite was calcined at 500 ° C. for 1 hour in a dry air stream to convert the ammonium type into the hydrogen type. This hydrogen-type β zeolite was subjected to a catalytic reaction test as a catalyst used in Example 2.
<Preparation of catalyst 3>
The hydrogen type β zeolite was referred to the hydrochloric acid treatment disclosed in JP-A-58-208131, and the SiO ₂ / Al ₂ O ₃ molar ratio of the hydrogen type β zeolite was increased. 20 g of the hydrogen type β zeolite described above was added to 100 g of a 0.2 N aqueous hydrochloric acid solution and stirred at 80 ° C. for 2 hours. Thereafter, solid-liquid separation, washing with a sufficient amount of pure water, and drying at 110 ° C. overnight were performed. When the obtained hydrogen-type β zeolite was analyzed by ICP emission analysis, it had a composition of Al ₂ O ₃ · 108.5SiO _{2 in} terms of anhydrous. This hydrogen-type β zeolite was subjected to a catalytic reaction test as a catalyst used in Example 3.

Comparative Example 1
Fe support was carried out using hydrogen-type β zeolite having a SiO ₂ / Al ₂ O ₃ molar ratio of 36.3 prepared in the examples. Hydrogen-type β zeolite; 10 g was added to an iron nitrate aqueous solution corresponding to 4 wt% in terms of Fe metal based on the zeolite solid content, and stirred at 60 ° C. for 20 hours. Then, it separated into solid and liquid, washed with a sufficient amount of pure water, and dried at 110 ° C. overnight. When the obtained powder was analyzed by ICP emission analysis, the supported amount of Fe was 2.8% by weight in terms of metal. This powder was subjected to a catalytic reaction test as a catalyst used in Comparative Example 1.

Comparative Example 2
A hydrogen type mordenite structure zeolite (trade name; HSZ-660HOA, SiO ₂ / Al ₂ O ₃ molar ratio = 26) manufactured by Tosoh was used as a catalyst used in Comparative Example 2 and subjected to a catalytic reaction test.

Comparative Example 3
Tosoh's ammonium-type Y structure zeolite (trade name: HSZ-371NHA, SiO ₂ / Al ₂ O ₃ molar ratio = 27) was calcined at 500 ° C. for 1 hour in a dry air stream to form a hydrogen-type Y zeolite. Converted. The hydrogen-type Y zeolite was subjected to a catalytic reaction test as a catalyst used in Comparative Example 3.

  The catalysts produced in Examples 1 to 3 and Comparative Examples 1 to 3 were press-molded, pulverized, and sized to 12 to 20 mesh. Evaluation was performed by filling 1.5 cc of each sized catalyst into a normal pressure fixed flow type reaction tube. While flowing the gas having the composition shown in Table 2 at 1500 cc / min, steady nitrogen oxide purification activity was evaluated at an arbitrary temperature of 100 to 500 ° C.

窒素酸化物の浄化活性は数式２で表される。

Nitrogen oxide purification activity is expressed by Equation 2.

ここで、Ｘ_ＮＯｘは窒素酸化物の浄化率（％）、［ＮＯｘ］_ｉｎは入りガスの窒素酸化物濃度（一酸化窒素と二酸化窒素の和）、［ＮＯｘ］_ｏｕｔは出ガスの窒素酸化物濃度を示す。

Where X _NOx is the nitrogen oxide purification rate (%), [NOx] _in is the nitrogen oxide concentration of the incoming gas (sum of nitrogen monoxide and nitrogen dioxide), and [NOx] _out is the nitrogen oxide of the outgoing gas Indicates the concentration.

  Table 3 shows the nitrogen oxide purification rate (%) of each catalyst at an arbitrary temperature.

実施例４
更に、触媒２；３ｃｃを常圧固定床流通式反応管に充填し、７００℃で２０時間、Ｈ_２Ｏ＝１０ｖｏｌ％を含む空気を３００ｃｃ／ｍｉｎで流通させて処理した（耐久処理）。この耐久処理後の実施例２の触媒に関し、上述の触媒反応試験と同様な条件で窒素酸化物の浄化活性を評価した。評価結果を表４に示す。

Example 4
Further, Catalyst 2; 3 cc was charged into a normal pressure fixed bed flow type reaction tube, and treated by flowing air containing H ₂ O = 10 vol% at 700 cc for 20 hours at 300 cc / min (endurance treatment). With respect to the catalyst of Example 2 after the durability treatment, the purifying activity of nitrogen oxides was evaluated under the same conditions as in the catalytic reaction test described above. The evaluation results are shown in Table 4.

Comparative Example 4 and Comparative Example 5
The catalysts used in Comparative Example 1 and Comparative Example 3 were subjected to the same durability treatment as in Example 4, and the nitrogen oxide purification activity was evaluated under the same conditions as in the catalytic reaction test described above. The evaluation results are shown in Table 4.

本発明の方法では、高温耐久後も高い処理効果が得られた。

In the method of the present invention, a high treatment effect was obtained even after high temperature durability.

Comparative Example 6 and Reference Example 1
Using the catalyst 2 used in Example 2, the purifying activity of nitrogen oxides was evaluated with the gas composition not containing NO ₂ shown in Table 5. As Reference Example 1, the comparative catalyst 1 used in Comparative Example 1 was used for evaluation.

評価結果を表６に示す。

The evaluation results are shown in Table 6.

二酸化窒素を含まない処理ガスでは、処理効果が低下した。

With the treatment gas not containing nitrogen dioxide, the treatment effect was reduced.

Claims (7)

Contacting and purifying nitrogen oxides in a processing gas, characterized by contacting with a processing gas containing nitrogen monoxide and nitrogen dioxide to which ammonia is added as a reducing component to hydrogen-type and / or ammonium-type β zeolite Method. The method for catalytic reduction and purification of nitrogen oxide according to claim 1, wherein the contact temperature is 150 ° C or higher. The nitrogen oxide catalytic reduction and purification method according to claim 1 or 2, wherein the molar ratio of silica (SiO ₂ ) to alumina (Al ₂ O ₃ ) in the β zeolite is 20 to 200. The method for catalytic reduction and purification of nitrogen oxides according to claim 1, wherein the content of nitrogen dioxide in the processing gas is in the range of 0.5 to 1.5 in terms of a molar ratio to nitrogen monoxide. The method for catalytic reduction and purification of nitrogen oxides according to claims 1 to 4, wherein the ammonia is added in a molar ratio of 0.8 to 1.05 with respect to the total of nitrogen monoxide and nitrogen dioxide. Catalyst for catalytic reduction and purification of nitrogen oxides consisting of β zeolite. Alumina _(Al 2 _{O 3)} to silica catalytic reduction purification catalyst of nitrogen oxides according to claim 6 molar ratio is 20 to 200 of (SiO _2).