JPH01245856A - Catalyst for use in catalytic reduction of nitrogen oxide - Google Patents

Abstract

PURPOSE:To obtain a catalyst having resistance against degradation due to catalyst poison in flue gases, by causing mordenite type zeolite having a specific ratio of SiO2/Al2O3 to carry metal such as copper, vanadium etc. CONSTITUTION:In case of a catalyst for removing nitrogen oxide in flue gases, mordenite type zeolite having a SiO2/Al2O3 ratio of 15-25 is caused to carry metal such as copper, vanadium, cobalt, iron. The catalyst so obtained has a resistance against SOx while maintaining a high resistance against arsenic oxide etc., preventing thereby the degradation of its activity. Further, the denitration of flue gases wherein arsenic, selenium, sulfur etc. coexist can be performed stably for a long period of time.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is directed to the treatment of nitrogen oxides (hereinafter referred to as NOx) in exhaust gas.
The present invention relates to catalysts for removing nitrogen oxides, and particularly to catalysts for removing nitrogen oxides that are not easily deteriorated by catalyst poisons in exhaust gas.

[Conventional technology]

Nitrogen oxide (NO,) emitted from power plants, sintering furnaces, various chemical factories, automobiles, and the like is considered to be a causative agent of photochemical smog, and effective treatment means are desired.

Among the flue gas denitrification methods that have been proposed in the past, the NOx catalytic reduction method using ammonia (NH, ) as a reducing agent reduces the
Since H3 selectively reacts with NOx, this method is considered advantageous in that it requires less reducing agent.

As catalysts used in this method, catalysts in which a transition metal compound is supported on a carrier such as activated alumina, silica gel, alumina, zeolite, or titanium oxide are known.

Among these, those currently in practical use are
The main component is titanium oxide as typified by No. 0-51966 and JP-A No. 5-2-122293, and vanadium (
V), molybdenum (MO), tungsten (W), and other oxides. These catalysts are excellent in that they are resistant to deterioration due to sulfur oxides (SoX) in exhaust gas. However, no consideration is given to deterioration caused by volatile metal oxides mainly generated from minerals in fuel, and oxides such as selenium, tellurium, thallium, and arsenic. For this reason, when coal or Chinese crude oil containing a large amount of mineral substances is used as fuel, there is a problem in that when the concentration of the volatile substances in the exhaust gas increases, the activity of the catalyst decreases significantly.

Such deterioration can be alleviated to some extent by using a zeolite-based catalyst that can introduce active ingredients into micropores where the volatile oxidizing substances are difficult to diffuse, and several patents have already been filed. (Unexamined Japanese Patent Publication No. 5
1-11063, JP-A-59-230642, etc.).

Silica-alumina ratio (hereinafter SiO
,/A/! .

Those with a low 03 ratio (abbreviated as 03 ratio) of 10 or less have a problem in that the catalytic activity decreases due to acidic substances (particularly sulfur oxides, So, etc.) contained in the coal. This is thought to be because the acidic substance reacts with the aluminum compound in the zeolite and destroys its structure.

[Problem to be solved by the invention] Even when a mordenite catalyst is used, S is generated during combustion.
When exposed to OX for a long time, the catalyst activity tends to gradually decrease. This is considered to be because So coats the surface of the catalyst or reacts with the catalytically active metal. Therefore, in order to prevent a decrease in activity due to SOX, it is necessary to search for mordenite that has active sites that are difficult to react with SOX. However, regarding mordenite, SiO□/Al t O
Although there are many reports that those with a high j ratio have excellent acid resistance, there are very few reports regarding acid resistance in gas phase and high temperature conditions such as in denitrification reactions. Among them, SiO□/A f ! when used as a denitrification reaction catalyst.
The relationship between the 03 ratio and acid resistance was not clear.

For this reason, 1. It was thought that it would be difficult to use a catalyst consisting only of active ingredients supported on mordenite for a long period of time as a denitrification catalyst.

An object of the present invention is to develop a mordenite-based catalyst that has strong resistance to SOX while maintaining high resistance to volatile oxidizing substances (particularly arsenic oxide) in order to prevent activity reduction due to SOX.

[Means to solve the problem]

The present invention aims to solve the above-mentioned problems, and provides a catalyst in which a catalytically active component is supported on zeolite and nitrogen oxides are catalytically reduced and removed using ammonia.
By a catalyst for catalytic reduction of nitrogen oxides, characterized in that it is formed by supporting one or more metals selected from copper, vanadium, cobalt, and iron on mordenite-type zeolite having an iOx/AlzOs ratio of 15 to 25. The above objectives are achieved.

〔Example〕

The mordenite used in the present invention can be a commercially available product or one synthesized by hydrothermal treatment. It is preferable to use mordenite in the H-type to support the active ingredient, but if it contains alkali or alkaline earth metal ions, use a 0.1 to 1 mol/l mineral acid aqueous solution or ammonium salt aqueous solution. Use after dealkalization.

If alkaline components remain, the active components Cu, VS
Co and Fe may become difficult to ion exchange, resulting in a decrease in activity.

When mordenite is synthesized by hydrothermal treatment using silica sol or water glass as the silicon source and aluminum salt of a mineral acid or sodium aluminate as the aluminum source, mordenite is stably produced and 5in2/A1.0 of the produced mordenite. ．． It is desirable to select conditions under which the ratio is around 20. S i 02 / A l
z Mordenite with an Os ratio of 15 or less is de-arsonized to give a predetermined SiO,/Az.

There is also a method of setting the ratio to 0゛, but this is not preferred because it destroys the original framework of mordenite and results in mordenite with low stability. Various methods have been reported for producing mordenite with a SiOz/A1203 ratio of around 15 to 25.
1) C, J, Whittemore synthesizes under hydrothermal conditions at 184°C using aluminosilicate gel containing sodium silicate and aluminum chloride as main raw materials.

Jr's method (American Mineralogy)
t, 57. 1146 (1972)), (2) No. 4
Using benzyltrimethylammonium ion, which is one of the class ammonium ions, for Sin, Al,
A method of obtaining mordenite with a maximum SiO2/Al2O3 ratio of 25.8 by carrying out the reaction at a relatively low temperature with an extremely small proportion of O (Japanese Patent Laid-Open No. 58-88
118), (3) S i Oz and /120. (7)
By making a mixed gel and then hydrothermally treating it in an aqueous alkali metal solution, the S i 02 /Ant Os ratio was reduced to 26.
．． Method for obtaining mordenite up to 5
No. 424), etc. can be used. It is desirable to use hydrothermally treated products after sufficiently removing the alkaline components present deep inside the pores. Treatment with organic acids such as acid, oxalic acid, maleic acid, chloroacetic acid to remove alkali or alkaline earth metal ions. However, if the active ingredient is sufficiently supported after several hours of dealkalization, the catalyst activity will not be affected significantly.

Sin obtained from commercial products or by hydrothermal synthesis.

/A2□0. Mordenite having a ratio of 15 to 25 is used to support an active ingredient, and as the active ingredient, Cu, Fe, VSCo, etc. are used as mentioned above. When using an ion exchange method as a supporting method, in the case of metal ions or metal oxy ions that are difficult to ion exchange, an unknown supporting method invented by the present inventors (hydrolysis of metal ions using a nitrogen-containing compound) is used. By using a metal support method that suppresses the metal and adjusts it to the optimal pH for ion exchange,
Active ingredients can be efficiently ion-exchanged. Further, the supporting method is not limited to the ion exchange method, but may also be a kneading method, a vapor deposition method, an impregnation method, or the like.

Mordenite S i O,z / A l z Ox
The SOX resistance is the highest when the ratio is about 20, and compared to this, the range where the durability is 70% is S i O2 / A l 20
+ ratio of 15 to 25, but if 50% durability is acceptable, S i O! /AlzO+ ratio in the range of 12 to 35 can be used.

The zeolite carrying the active ingredient is dried and, if necessary, calcined, and then shaped. Firing temperature is 250~
700″C1, preferably selected from the range of 400 to 600″C. A temperature of 700°C or higher is not preferable because a decrease in activity is observed.

The shape of the molded product can be arbitrarily selected, such as granules, pellets, granules, and honeycomb shapes, and a molding machine suitable for each shape, such as an extrusion molding machine, tablet machine, transfer granulator, etc., can be selected. It is molded using.

The molded product molded as described above is then heat-treated. Although the strength of the molded product can be improved by simply drying the molded product by heating it to a temperature of about io'c or lower, heating at an even higher temperature is effective.

That is, the heat treatment is usually carried out under an inert gas or air atmosphere at temperatures below 800"C, preferably from 300"C to 70"C.
It is appropriately selected within the range of 0°C. Calcination time also affects the physical properties and strength of the catalyst, but is usually 1 hour to 10 hours.
This can be done in a timely manner.

In order to reduce NOx in exhaust gas using a catalyst manufactured by such a method, a mixed gas of exhaust gas and ammour gas may be passed through the catalyst at a temperature of preferably 300° C. or higher.

Using the catalyst of the present invention, No.
When it is reductively decomposed, not only a high NOX decomposition rate can be obtained, but also it is not easily deteriorated by SOX in the exhaust gas and can maintain high activity for a long time.

-m, acid resistance of mordenite and S l 02/A l
The relationship between the t03 ratio is SiO□/Al2O, and it is said that the higher the ratio, the better the acid resistance. However, when used as a denitrification catalyst, which is the purpose of the present invention, S i O,
,//M! , O, the So resistance shows maximum at a ratio of 20.

This is considered to be due to the following reasons.

The denitrification catalyst is capable of reacting to high temperatures of 250 to 500°C and
Since it is used while being exposed to O1X, heat resistance and acid resistance are required. It is believed that the higher the 5iOz/Ant 03 ratio, the less susceptible the aluminum in the zeolite lattice is to attack by SOX. However, under such harsh conditions, even if the SiO□/Al,O, ratio becomes high, it does not mean that it will not be attacked at all.
Inactivation due to de-A2 gradually progresses. Experimental results from a pilot plant show that S i Oz / A I! ,
When the z 03 ratio becomes extremely high, such as 30 or more, deterioration tends to progress. On the other hand, S i 02 / A lz O*
If the ratio is less than 10, the acid resistance and heat resistance will decrease, and since Al removal will occur rapidly, the activity will also decrease.

However, this intermediate SiO2/AlzO ratio is 20
In terms of degree, it has a large number of active sites, Al2, and has moderately strong acid resistance and heat resistance, so it is considered to have the best acid resistance under denitrification reaction conditions. Wait, S i O2/
When the A l z Oz ratio is around 20, the active points are relatively close to each other, so one AI! , desorbs and liberates the active ingredient, the nearby A! It is thought that the reason why it is difficult to deactivate is that it can quickly take over and become an active site.

Next, the present invention will be explained using specific examples.

The SOX resistance measurement test was conducted in the following manner.

In the Examples and Comparative Examples, accelerated tests were conducted using a simulated gas simulating combustion exhaust gas from coal with a high mineral content. S
O and I are sulfur trioxide (SO+) and sulfur dioxide (
SOz) was vaporized and added to the gas. The gas composition is: oxygen, 3% by volume, carbon dioxide, 12% by volume, moisture, 12% by volume, NOx: 200ppm, ammonia: 240ppm.
, S02500PPIII, SO3 100ppm - the balance is nitrogen. This test was performed by molding and firing, and
The experiment was carried out by passing the above mixed gas through 1 ml of the catalyst sieved to 0 mesh at a space velocity of 120° o o o h-'. The reaction temperature is 400°C.

The denitrification catalyst performance was continuously measured during the implementation of the above accelerated test. Note that the NOX content was measured with a chemiluminescent NOX meter, and the NOx decomposition rate (denitrification rate) was calculated using the following formula by measuring the NO8 concentration before and after the addition of ammonia.

In addition, the denitrification reaction rate constant ratio (ratio of the rate constant after deterioration to the initial rate k) is defined as η for the denitrification rate after deterioration and η for the initial denitrification rate. In this case, it was defined as below and used in the comparative calculation of the deterioration test.

k/ko=1. (100/(100-77))/j2
゜(100/(100-η.))(2) BET specific surface area measurement: The specific surface area and pore distribution were determined by taking approximately 0.3 g of the catalyst, which had been calcined and molded and sieved to 10 to 20 mesh, and using QIJAN.
TACHROMf! The measurement was carried out using a gas adsorption test device of the AUTOSORB-1 type.

The specific surface area is relative pressure CP/Po) from 0.025 to 0.
．． B, E, and T in 3 were calculated by the multi-point method.

Ratio measurement of S-i 0□//120; Measured using a fluorescent X-ray analyzer manufactured by Rigaku Denki Co., Ltd.

Example 1 Sio2/A1□0. produced by the present inventors.
Copper acetate was kneaded and supported on H-type mordenite with a ratio of 17.1 to a ratio of 3% of the mordenite, and after drying, it was formed into a cylindrical shape of 10φ x 3L with a press molding machine, and then C. for 2 hours under an air atmosphere. This was crushed and sifted into 10 to 20 meshes to be used as a catalyst.

As a result of an accelerated SO8 resistance test using this catalyst, k/
The time required for the ko ratio to deteriorate to 0.6 was 380
It was time.

Examples 2 to 7 In place of the mordenite in Example 1, S from mordenite produced by the present inventors or commercially available mordenite was used.
i O2/Al120ff ratio 15.2.18°2.19
, 5.20.1.23.2 and 24.8 type 1 mordenites were selected to prepare catalysts, and accelerated SOX resistance tests were conducted in the same manner as in Example 'l. As a result, the time required for the ratio to deteriorate to 0.6 for k/ is 2
6B, 322.375.342.276 and 266 hours.

Examples 8 to 10 Catalysts were produced by supporting ■ (using vanadium sulfate), Co (using cobalt acetate), and Fe (using ferrous sulfate) in place of copper in Example 1 as active ingredients. Then, an accelerated SO8 resistance test was conducted in the same manner as in Example 1. As a result, the time required for the ratio to deteriorate to 0°6 was 281.295 and 356 hours, respectively.

Comparative Examples 1 to 6 In place of the mordenite in Example I, silicon was used from mordenite manufactured by the present inventors or commercially available mordenite.
O□//1203 ratio 10.5.12゜1.27.0.3
4.8. H-type mordenites of 36.8 and 40.6 were selected to produce catalysts, and S-resistant was tested in the same manner as in Example 1.
An OX acceleration test was conducted. As a result, the time required for the k/kO ratio to deteriorate to 0.6 was 56.1, respectively.
90.198.191.184 and 14.1 hours.

The above results are summarized in Table 1.

Below is Table 1 in the margin [Effects of the Invention] According to the present invention, a mordenite-based denitrification catalyst that is less likely to be degraded by 5O1I, especially SOI, in exhaust gas can be obtained. Mordenite-based denitrification catalysts exhibit extremely high toxicity resistance to volatile catalyst poisons such as arsenic and selenium compounds, where conventional titanium-based catalysts supporting transition metal elements inevitably deteriorate in catalytic activity after short-term use. By using the catalyst of the present invention, it becomes possible to denitrate exhaust gas containing large amounts of arsenic, selenium, and sulfur while maintaining stable catalytic activity over a long period of time.

Agent: Patent Attorney Kawakita Takecho

Claims (1)

[Claims] (1) In a catalyst that carries a catalytically active component on zeolite and catalytically reduces nitrogen oxides using ammonia,
A catalyst for the catalytic reduction of nitrogen oxides, comprising a mordenite-type zeolite having an O_2/Al_2O_3 ratio of 15 to 25 and supporting one or more metals selected from copper, vanadium, cobalt, and iron.