JP2004322077A - Exhaust gas clarification catalyst and exhaust gas clarification method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas clarification catalyst which has excellent durability, excellent practicality and excellent denitrification performance in that nitrogen oxides (NOx) can be removed even if exhaust gas contains sulfur oxides and that is not deteriorated even when the exhaust gas has comparatively low temperature of 300-400°C, and to provide an exhaust gas clarification method using the catalyst. <P>SOLUTION: This catalyst is β-zeolite on which at least one metal selected from iron, cobalt, silver, molybdenum and tungsten is deposited. The molar ratio of SiO<SB>2</SB>/Al<SB>2</SB>O<SB>3</SB>in the β-zeolite is preferably made to be 20-70. NOx in the exhaust gas is reduced/removed by bringing the oxygen-excessive exhaust gas into contact with this catalyst in the presence of methanol and/or dimethyl ether being reducing agents. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an exhaust gas purifying catalyst effective for removing various combustion exhaust gas from a boiler, a diesel engine generator, or a diesel engine vehicle, and nitrogen oxides contained in exhaust gas from industrial equipment, and an exhaust gas using the catalyst. It relates to a purification method.

  Various exhaust gases emitted from factories, power generation facilities, other industrial facilities, automobiles, and the like contain nitrogen oxides (NOx) such as nitric oxide and nitrogen dioxide. This NOx not only has an adverse effect on the human body, especially the respiratory system, but also is one of the causes of acid rain, which is regarded as a problem from the viewpoint of global environmental protection.

  Therefore, development of a technology for efficiently removing nitrogen oxides (NOx) from these various exhaust gases is desired. As one of such flue gas denitration methods, a three-way catalyst method used for exhaust gas treatment of automobiles (gasoline vehicles) is known. However, this three-way catalyst method cannot be applied to exhaust gas containing oxygen in excess of the theoretical amount required to completely oxidize unburned components such as hydrocarbons and carbon monoxide remaining in the exhaust gas.

On the other hand, as a method for reducing and removing NOx in an atmosphere in which oxygen is excessively present, an ammonia selective catalytic reduction method using a V ₂ O ₅ —TiO ₂ catalyst and using ammonia as a reducing agent is known. However, in this method, harmful ammonia having a strong odor is used, so that handling is not easy. In addition, since it requires a special device to prevent the emission of unreacted ammonia, the equipment becomes large, so it is not suitable for application to small-scale exhaust gas sources or mobile sources, It is not preferable in terms of economy.

  In recent years, it has been reported that the use of unburned hydrocarbons remaining in an oxygen-rich lean combustion exhaust gas as a reducing agent can promote the reduction reaction of NOx in the exhaust gas. Since this report, various catalysts have been developed to promote the NOx reduction reaction. For example, alumina and a catalyst supporting a transition metal on alumina are effective for the reduction removal reaction of NOx using hydrocarbons as a reducing agent. There have been many reports that

  Examples of catalysts for reducing and removing nitrogen oxides in combustion exhaust gas containing excess oxygen by using such hydrocarbons as a reducing agent include, in addition to alumina and a catalyst having a transition metal supported on alumina, 0.1 to 4% by weight. A reduction catalyst comprising alumina or silica-alumina containing Cu, Fe, Cr, Zn, Ni, or V has been reported (see JP-A-4-284848).

It is also reported that when a catalyst in which Pt or the like is carried on alumina is used, the reduction reaction of NOx proceeds even in a low temperature range of about 200 to 300 ° C. (JP-A-4-267946, JP-A-5-267946). 68855, JP-A-5-103949). However, in these noble metal-supported catalysts, the combustion reaction of hydrocarbons as a reducing agent is excessively promoted, and a large amount of N ₂ O, which is one of the substances causing global warming, is by-produced. be advanced reduction reaction to harmless N ₂ selectively had the disadvantage that it is difficult.

  Further, it has been reported that a catalyst in which silver is supported on alumina or the like selectively promotes a NOx reduction reaction using a hydrocarbon as a reducing agent in an oxygen-excess atmosphere (see JP-A-4-281844). . After this report, many similar NOx reduction and removal methods using a catalyst containing silver have been developed and reported (see JP-A-4-354536).

  However, any of the above-mentioned methods for purifying exhaust gas using a denitration catalyst has the problem that the NOx removal performance is significantly reduced in an oxygen-excess exhaust gas containing sulfur oxides, and the practical durability is insufficient. was there. In addition, when the temperature of the exhaust gas is relatively low, such as about 300 ° C. to 400 ° C., there is a problem that the NOx removal performance is low.

  Further, a method of reducing and removing NOx in the presence of an organic compound using a hydrogenated zeolite catalyst or a catalyst in which V, Cr, Mn, Fe, Co, Ni, or the like is supported on the zeolite has been reported. Y-type zeolites, L-type zeolites, offretite-erionite mixed crystal zeolites, ferrierite-type zeolites, and ZSM-5-type zeolites are disclosed (see Japanese Patent No. 2139645). Furthermore, a method of reducing and removing NOx in the presence of ethanol using a proton-type zeolite has also been reported, and as the zeolites, Y-type zeolites, ZSM-5-type zeolites, and mordenite are disclosed (see Japanese Patent No. 2506598). .

  However, none of the above-described methods for reducing and removing NOx using a catalyst composed of a specific zeolite has provided practically sufficient NOx removal performance and has not yet been put to practical use.

JP-A-4-284848 JP-A-4-267946 JP-A-5-68855 JP-A-5-103949 JP-A-4-281844 JP-A-4-354536 Japanese Patent No. 2139645 Japanese Patent No. 2506598

  In view of such conventional circumstances, the present invention has excellent NOx removal performance and durability even in an oxygen-excess exhaust gas containing sulfur oxides, and denitration even at a relatively low exhaust gas temperature of 300 ° C to 400 ° C. An object of the present invention is to provide an exhaust gas purifying catalyst having high performance and excellent practicality, and an exhaust gas purifying method using the same.

  In order to achieve the above object, an exhaust gas purifying catalyst provided by the present invention is a catalyst for reducing and removing nitrogen oxides in an oxygen-excess exhaust gas in the presence of methanol and / or dimethyl ether. It is characterized by supporting at least one metal selected from cobalt, silver, molybdenum and tungsten.

  Further, the exhaust gas purification method provided by the present invention is characterized in that an excess oxygen exhaust gas is selected from iron, cobalt, silver, molybdenum, and tungsten in β zeolite in the presence of methanol and / or dimethyl ether as a reducing agent. And contacting the catalyst with at least one metal supported thereon to reduce and remove nitrogen oxides in the exhaust gas.

In the exhaust gas purifying catalyst and the exhaust gas purifying method of the present invention, the β zeolite preferably has a SiO ₂ / Al ₂ O ₃ molar ratio of 20 to 70.

  According to the present invention, it is possible to provide an exhaust gas purifying catalyst having both excellent NOx removal performance and durability, and it is possible to efficiently remove nitrogen oxides NOx from an oxygen-excess exhaust gas using this catalyst. . Moreover, the exhaust gas purifying catalyst of the present invention exhibits a high denitration rate even in a combustion exhaust gas containing a large amount of sulfur oxides, and the denitration performance does not decrease even at a relatively low exhaust gas temperature of about 300 ° C to 400 ° C. Extremely practical.

  In the present invention, as a catalyst for reducing and removing nitrogen oxides (NOx) in an oxygen-excess exhaust gas in the presence of methanol and / or dimethyl ether, β zeolite is used as a carrier, and iron, cobalt, silver, molybdenum, and One supporting at least one selected from tungsten is used. There are many types of zeolite such as Y type, L type, ZSM-5 (MFI) type, mordenite type in addition to β type. Among them, a catalyst using β zeolite as a carrier is particularly sulfur type. It was found to be extremely effective in purifying exhaust gas containing oxides.

  The method of supporting iron, cobalt, silver, molybdenum, or tungsten on the β zeolite is not particularly limited, and a conventionally known supporting method can be used. For example, iron, cobalt, silver, molybdenum, or β zeolite in an aqueous solution of a water-soluble salt of tungsten, and ion-exchange by stirring, followed by filtration and washing with an ion-exchange method, or iron in β zeolite, An impregnation method in which an aqueous solution of a water-soluble salt of cobalt, silver, molybdenum, or tungsten is impregnated may be used.

  A β zeolite supporting at least one metal selected from iron, cobalt, silver, molybdenum, and tungsten is then dried and calcined to prepare a catalyst. The drying temperature at the time of preparing the catalyst is not particularly limited, but it is usually dried at about 80 to 120 ° C. The firing temperature is preferably from 300 to 600 ° C. The atmosphere during firing is not particularly limited, but an atmosphere such as air, an inert gas, or oxygen may be appropriately selected according to the catalyst composition.

The silica (SiO ₂ ) / alumina (Al ₂ O ₃ ) ratio of the β zeolite used is preferably in the range of 20 to 70 in terms of molar ratio from the viewpoint of NOx removal performance (denitration performance). The denitration performance becomes lower as the molar ratio becomes larger than 40, and considering the stability against heat and water vapor, the SiO ₂ / Al ₂ O ₃ molar ratio is more preferably in the range of 20 to 40.

  The catalyst of the present invention can be formed into various shapes such as a spherical shape, a honeycomb shape, and a pellet shape by a conventionally known forming method. These shapes and sizes may be arbitrarily selected according to the conditions for using the catalyst. Further, at least one metal selected from iron, cobalt, silver, molybdenum, and tungsten is supported on the surface of a support base having a refractory integrated structure having a large number of through holes in the flow direction of the exhaust gas. The zeolite can be coated as a catalyst by a wash coat method or the like.

  In the method of purifying exhaust gas for reducing and removing nitrogen oxides, the exhaust gas may be brought into contact with the above-described catalyst of the present invention in the presence of methanol and / or dimethyl ether. The amount of methanol and / or dimethyl ether to be added to the exhaust gas as a reducing agent may be appropriately selected according to the denitration ratio and running cost required in operation, but usually, the molar ratio (carbon As (C) conversion, about 0.5 to 5 is preferable.

The exhaust gas containing nitrogen oxides (NOx) to be treated in the present invention includes various combustion facilities such as a boiler, an internal combustion engine such as a diesel engine automobile and a stationary diesel engine, and industrial facilities such as a nitric acid production facility. Exhaust gas can be mentioned. These exhaust gases generally contain reducing components such as CO, HC (hydrocarbon) and H ₂ and oxidizing components such as NOx and O ₂ , and the stoichiometry required for a complete redox reaction between them. It contains an excess amount of oxygen. NOx is reductively decomposed into N ₂ and H ₂ O by contacting such an oxygen-excess exhaust gas with the catalyst of the present invention in the presence of methanol and / or dimethyl ether.

  The gas space velocity (SV) in the exhaust gas purifying method using the catalyst of the present invention is not particularly limited, but is preferably 1,000 to 100,000 / h. Further, according to the catalyst of the present invention, excellent performance for removing nitrogen oxides can be obtained even when the temperature of the exhaust gas is relatively low at about 300 to 400 ° C. Furthermore, even an exhaust gas containing a sulfur oxide has excellent denitration performance, and its denitration performance does not decrease and is excellent in durability.

  When the exhaust gas is treated by the method of the present invention, unburned methanol, dimethyl ether, or incomplete combustion products may be discharged into the exhaust gas depending on the reaction conditions. In such a case, the exhaust gas after passing through the denitration catalyst of the present invention is brought into contact with an oxidation catalyst, for example, a noble metal-supported catalyst such as a Pt-based or Pd-based catalyst to remove unburned components and incomplete combustion products. be able to.

(1) Preparation of catalyst (1-a) Catalysts 1-2 of the present invention: Fe-supported β zeolite 0.5 g of iron (II) chloride tetrahydrate was dissolved in 100 g of ion-exchanged water, and β zeolite was added to this solution. 10 g of powder (SiO ₂ / Al ₂ O ₃ molar ratio 27) was dispersed, and the mixture was stirred for 12 hours while maintaining the temperature at 60 ° C. Thereafter, the resultant was filtered and washed with water, dried at 110 ° C., and fired in air at 500 ° C. for 3 hours. This was press-molded, crushed and sized to a particle size of 350 to 500 μm to obtain Catalyst 1 of the present invention. In this catalyst 1, the amount of Fe supported was 1.0% by weight of the entire catalyst in terms of metal. Similarly, when preparing a catalyst, a catalyst 2 (Fe support amount: 0.8% by weight) was prepared by using a β zeolite powder having a SiO ₂ / Al ₂ O ₃ molar ratio of 37.

(1-b) Catalyst 3: Inventive Example 3: Co-Supported β Zeolite 1.3 g of cobalt acetate tetrahydrate was dissolved in 100 g of ion-exchanged water, and β zeolite powder ( ₃ moles of SiO ₂ / Al ₂ O) was added to this solution. Ratio 27) 10 g were dispersed, and the mixture was stirred for 12 hours while maintaining the temperature at 60 ° C. Thereafter, the resultant was filtered and washed with water, dried at 110 ° C., and fired in air at 500 ° C. for 3 hours. This was sized in the same manner as above to obtain Catalyst 3 of the present invention. The amount of Co supported on the catalyst 3 was 2.7% by weight of the entire catalyst in terms of metal.

(1-c) Catalyst 4 of the present invention: Ag-supported β zeolite 1.6 g of silver nitrate was dissolved in 100 g of ion-exchanged water, and 20 g of β zeolite powder (SiO ₂ / Al ₂ O ₃ molar ratio 27) was dissolved in this solution. It was dispersed and stirred for 12 hours while maintaining the temperature at 60 ° C. Thereafter, the resultant was filtered and washed with water, dried at 110 ° C., and fired in air at 500 ° C. for 3 hours. This was sized in the same manner as above to obtain Catalyst 4 of the present invention. The amount of Ag carried on the catalyst 4 was 2.5% by weight of the entire catalyst in terms of metal.

(1-d) Catalyst 5 of the present invention: Mo-supported β zeolite 0.38 g of hexaammonium hexamolybdate tetrahydrate was dissolved in 30 g of ion-exchanged water, and β zeolite powder (SiO ₂ / Al ₂ After immersing 10 g of O _{3 in a} molar ratio of 27), the mixture was heated with stirring to evaporate water, dried by ventilation at 110 ° C., and then fired in air at 500 ° C. for 3 hours. This was sized in the same manner as above to obtain Catalyst 5 of the present invention. The amount of Mo supported on the catalyst 5 was 2.0% by weight of the entire catalyst in terms of metal.

(1-e) Catalyst 6 of the present invention example: 0.29 g of ammonium tungstate parapentahydrate and 0.15 g of oxalic acid dihydrate were dissolved in 30 g of ion-exchanged water containing β zeolite carrying W, and the solution was added to this solution. After immersing 10 g of β zeolite powder (SiO ₂ / Al ₂ O ₃ molar ratio: 27), the mixture is heated with stirring to evaporate the water, and further dried by ventilation at 110 ° C., and then in air at 500 ° C. It was baked for 3 hours. This was sized in the same manner as above to obtain Catalyst 6 of the present invention. The amount of W carried on the catalyst 5 was 2.0% by weight of the entire catalyst in terms of metal.

(1-f) Catalysts C1 to C3 of Comparative Example
A β-zeolite powder having a SiO ₂ / Al ₂ O ₃ molar ratio of 75 was used, and Fe was supported in the same manner as in the preparation of the catalyst 1 and Ag was supported in the same manner as in the preparation of the catalyst 4 in the comparative example. Catalyst C2 of Comparative Example and Catalyst C3 of Comparative Example carrying Mo in the same manner as in the preparation of Catalyst 5 were obtained. The amount of Fe supported in the catalyst C1 was 0.8% by weight of the entire catalyst in terms of metal, the amount of Ag supported in the catalyst C2 was 2.2% by weight in total of the catalyst in terms of metal, and the amount of Mo supported in the catalyst C3 was metal conversion. Was 2.0% by weight of the whole catalyst.

(1-g) Comparative catalysts C4 to C9
A catalyst C4 of a comparative example was prepared in which ZSM-5 (SiO ₂ / Al ₂ O ₃ molar ratio 27) was used as a carrier in place of the β zeolite and the amount of Fe supported was 1.2% by weight. Similarly, a catalyst C5 of a comparative example was prepared in which mordenite (SiO ₂ / Al ₂ O ₃ molar ratio: 20) was used as a carrier in place of the β zeolite and the amount of Ag supported was 2.7% by weight. Similarly, a catalyst C6 of a comparative example was prepared in which ZSM-5 (SiO ₂ / Al ₂ O ₃ molar ratio 27) was used as a carrier in place of the β zeolite and the amount of Co supported was 2.3% by weight. Similarly, a catalyst C7 of a comparative example was prepared in which ZSM-5 (SiO ₂ / Al ₂ O ₃ molar ratio 27) was used as a carrier instead of the β zeolite and the amount of Mo supported was 2.0% by weight. Similarly, a catalyst C8 of a comparative example was prepared in which mordenite (SiO ₂ / Al ₂ O ₃ molar ratio: 20) was used as a carrier in place of the above-mentioned β zeolite and the amount of W supported was 2.0% by weight. Further, a catalyst C9 of a comparative example was prepared in which a Y-type zeolite (SiO ₂ / Al ₂ O ₃ molar ratio of 30) was used as a carrier instead of the β zeolite, and the amount of Ag supported was 2.2% by weight.

(2) Evaluation of Catalysts The catalysts 1 to 6 of the present invention and the catalysts C1 to C9 of the comparative examples were each filled in a stainless steel reaction tube having an inner diameter of 15 mm to form a catalyst, which was then passed through a fixed pressure bed under normal pressure. It was attached to the reactor. In the reaction tube, as a model exhaust gas, a mixed gas consisting of NO: 1,000 ppm, O ₂ : 10%, methanol: 2,000 ppm, H ₂ O: 10%, SO ₂ : 100 ppm, and the balance: N _{2 was} charged at a space velocity. The catalyst was supplied under the condition of 30,000 / h, and the denitration performance of each of the above catalysts was examined.

The denitration rate as the NOx removal performance (denitration performance) of the catalyst was calculated according to the following formula 1. Incidentally, the reaction tube analysis of the outlet gas composition, the concentration of NOx was measured by chemiluminescence NOx meter, _{N 2} O concentration Porapack
The measurement was performed using a gas chromatograph / thermal conductivity detector equipped with a Q column.

When the denitration performance was evaluated as described above, the exhaust gas temperature was changed to 300 ° C., 350 ° C., and 400 ° C., respectively, and the obtained results are shown in Table 1 below. The methanol was added as a reducing agent, and the molar ratio of methanol to nitrogen oxide in the model exhaust gas (in terms of C) was 2. Also, in the case of any of the catalysts, almost no N ₂ O was found in the outlet gas of the reaction tube.

The denitration performance of each catalyst was evaluated using the above-described catalysts 1, 3 to 6 of the present invention and the catalysts C4 and C6 to 9 of the comparative examples while changing the composition (reducing agent) of the model exhaust gas. That is, the denitration performance was evaluated in the same manner as above except that dimethyl ether: 1000 ppm was added instead of 2,000 ppm of methanol added as a reducing agent to the model exhaust gas, and the results are shown in Table 2 below. The molar ratio of dimethyl ether to nitrogen oxides in the model exhaust gas (in terms of C) was 2. Also, in the case of any of the catalysts, almost no N ₂ O was found in the outlet gas of the reaction tube.

Further, using the catalysts 1, 3 to 6 of the present invention described above and the catalysts C4, C6 to 9 of the comparative examples, the amount of methanol added as a reducing agent was changed from the above 2,000 ppm to 1,500 ppm. The results of evaluating the denitration performance of each catalyst in the same manner as above except for the above are shown in Table 3 below. The molar ratio of methanol to nitrogen oxides in the model exhaust gas (in terms of C) was 1.5. Also, in the case of any of the catalysts, almost no N ₂ O was found in the outlet gas of the reaction tube.

As can be seen from these results, the catalysts 1 to 6 of the present invention were prepared by using methanol or dimethyl ether as a reducing agent. It can be seen that even at very low temperatures, the NOx removal performance is remarkably superior to the catalysts C1 to C9 of the comparative example. Note that, even when β zeolite is used as the carrier, the catalysts C1 to C3 of the comparative examples whose SiO ₂ / Al ₂ O ₃ molar ratio exceeds 70 significantly decrease the denitration rate.

  Further, as shown in Table 3, when the molar ratio of the methanol of the reducing agent to the nitrogen oxides in the exhaust gas (in terms of C) was reduced, the catalysts of the present invention, Catalyst 3 (Co-supported β zeolite) The NOx removal performance of 6 (W-supported β zeolite) is larger than that of Catalyst 1 (Fe-supported β zeolite), Catalyst 4 (Ag-supported β zeolite) and Catalyst 5 (Mo-supported β zeolite), It has sufficiently high performance as compared with the catalysts of Comparative Examples using other than β zeolite.

Next, the durability of the catalyst against sulfur oxides was evaluated by the following durability test using the above-described catalysts 1, 3 to 6 of the present invention and the catalysts C4 to C8 of the comparative examples. That is, in each of the reaction tubes constructed in the same manner as above, as model exhaust gas having a high SO ₂ concentration, NO: 1,000 ppm, O ₂ : 10%, methanol: 2,000 ppm or dimethyl ether: 1,000 ppm, H ₂ O: A mixed gas consisting of 10%, SO ₂ : 1,000 ppm, and balance: N ₂ was supplied for 20 hours under the conditions of an exhaust gas temperature of 350 ° C. and a space velocity of 30,000 / h.

After the endurance test using the model exhaust gas with the increased SO ₂ concentration, a model exhaust gas having the same composition as that described above except that the SO ₂ concentration was changed to 100 ppm was similarly placed in each reaction tube at an exhaust gas temperature of 350 ° C. and a space velocity of 30,000 / h, and the denitration rate was determined in the same manner as described above. The results obtained are shown in Table 4 below.

As can be seen from the results, even after the endurance test for 20 hours using the exhaust gas containing high concentration of SO ₂ , the catalyst composed of the metal-supported β zeolite of the present invention shows a high catalytic activity in the presence of methanol or dimethyl ether as the reducing agent. Denitration performance is maintained and durability is excellent.

Claims (4)

A catalyst for reducing and removing nitrogen oxides in an oxygen-excess exhaust gas in the presence of methanol and / or dimethyl ether, wherein β zeolite is at least one selected from iron, cobalt, silver, molybdenum, and tungsten An exhaust gas purifying catalyst characterized by carrying the above metals. _2. The exhaust gas purifying catalyst according to claim 1, wherein the β zeolite has a SiO ₂ / Al ₂ O ₃ molar ratio of 20 to 70. ₃ . The exhaust gas containing excess oxygen is converted to a catalyst in which β zeolite supports at least one metal selected from iron, cobalt, silver, molybdenum, and tungsten in the presence of methanol and / or dimethyl ether as a reducing agent. A method for purifying exhaust gas, comprising contacting and reducing nitrogen oxides in exhaust gas. Wherein the _SiO 2 / _Al 2 _{O 3} molar ratio of said β-zeolite is from 20 to 70, the exhaust gas purifying method according to claim 3.