JP3511638B2 - Exhaust gas purification method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst for removing nitrogen oxides and hydrocarbons in exhaust gas discharged from an internal combustion engine of an automobile or the like, and particularly to an oxygen purifying catalyst. The present invention relates to a method for purifying excess combustion exhaust gas. 2. Description of the Related Art Nitrogen oxides, carbon monoxide and hydrocarbons, which are harmful substances in exhaust gas discharged from an internal combustion engine, are, for example, ternary materials in which Pt, Rh, Pd and the like are supported on a carrier. Removed by catalyst. However, since the exhaust gas of a diesel engine has an oxygen-excess atmosphere, there is no effective catalyst, and the exhaust gas is not purified by the catalyst. In recent gasoline engines, it has become necessary to perform lean combustion for the purpose of reducing fuel consumption and reducing carbon dioxide emissions. However, since the exhaust gas of this lean-burn gasoline engine has an oxygen-excess atmosphere, the above-described conventional three-way catalyst cannot be used, and a method for removing harmful components has not been put to practical use. As a method for removing nitrogen oxides in exhaust gas containing excess oxygen, there is a method of adding a reducing agent such as ammonia and reducing it to nitrogen using a catalyst, or a method of absorbing nitrogen oxides in an alkali. There are known removal methods,
These methods are not effective methods for purifying exhaust gas from automobiles, which are mobile sources, and their application is limited to fixed sources. Under such circumstances, as a catalyst for purifying exhaust gas used in an exhaust system of a lean burn engine or a diesel engine, a transition metal / zeolite catalyst in which a transition metal such as Cu is ion-exchanged into zeolite has been proposed ( JP-A-63-100919), among which, a catalyst obtained by ion-exchanging Cu with zeolite having a high SiO ₂ / Al ₂ O ₃ molar ratio such as ZSM-5 is the most excellent nitrogen oxide in an oxygen-excess atmosphere. It is known to exhibit a purifying ability, that is, a selective nitrogen oxide reducing ability by hydrocarbons. However, these zeolites having a high molar ratio of SiO ₂ / Al ₂ O ₃ have small pore diameters, and hydrocarbons having a large molecular diameter contained in exhaust gas cannot enter the pores. Is considered low. On the other hand, a zeolite having a large pore size cannot provide sufficient performance. In order to solve this problem, Japanese Unexamined Patent Publication (Kokai) No. 2-139040 discloses a method in which a zeolite having a large supercage diameter among zeolites exchanged with a transition metal is arranged upstream in the exhaust gas flow direction. A catalyst in which a small-diameter zeolite is arranged on the downstream side has been proposed. However, with this catalyst, although the activity is slightly improved, sufficient performance is not obtained. SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art, and without using a reducing agent such as ammonia.
It is an object of the present invention to provide an exhaust gas purifying catalyst suitable for efficiently purifying an oxygen-excess exhaust gas discharged from an internal combustion engine, particularly, the exhaust gas of the above-mentioned automobiles and the like. Means for Solving the Problems The inventors of the present invention have conducted intensive studies on the above-mentioned problems, and as a result, by using a catalyst obtained by mixing two types of zeolite-based catalysts, regardless of the type of hydrocarbon present. It has been found that a high nitrogen oxide purification rate can be achieved, and the present invention has been completed. That is,
The present invention relates to a 7 Å ion exchanged with a transition metal .
Large-pore zeolite having the above pores and transition metal
Disclosed is a method for removing nitrogen oxides from an oxygen-excess exhaust gas containing hydrocarbons and nitrogen oxides by using a catalyst mixed with an on-exchanged zeolite having pores of less than 7 angstroms. It is. It is unknown why the catalyst used in the present invention exhibits high activity in purifying exhaust gas. However, by mixing a large pore zeolite and a small pore zeolite, hydrocarbons having a large molecular diameter contained in the exhaust gas can be reduced. It is considered that partial oxidation and cracking occur in the pores of the large-pore zeolite, and as a result, hydrocarbons having a reduced molecular diameter can reduce nitrogen oxides in the pores of the adjacent small-pore zeolite. On the other hand, if a large-pore zeolite is placed on the upstream side and a small-pore zeolite is placed on the downstream side, hydrocarbons are completely oxidized to water and carbon dioxide by excess oxygen on the upstream side, and nitrogen oxides are reduced by the small-pore zeolite placed on the downstream side. Does not progress, and the effect of the present invention has not been obtained. Hereinafter, the present invention will be described in more detail. The zeolite used in the present invention is a crystalline aluminosilicate, or Al, Si is converted to Fe, G
a skeleton-substituted zeolite substituted with a, P, or the like; or aluminum phosphate molecular sieves such as SAPO, AlPO. The large pore zeolite used in the present invention has a pore size of 7 angstroms or more, and includes zeolite β, Y, L, and Mazzite. Further, these zeolites may be dealuminated. Preferably, Y and zeolite β are used.
Zeolite having a small pore size has pores of less than 7 Å, and includes ferrierite, ZSM-5,
ZSM-8, ZSM-11, ZSM-22, ZSM-3
5, ZSM-57 and the like. Preferably ZSM-
5 As these zeolites, synthetic products or calcined products thereof are used, but ions such as Na in the zeolites may be treated with an ammonium salt or a mineral acid to be used as H-type or ammonium-type. The large pore zeolite and the small pore zeolite used in the present invention are required to ion-exchange transition metals. The transition metal may be any metal that is usually used for purifying exhaust gas, such as Ib such as Cu, Ag, or Au.
Group, VIII group such as Fe, Co, Ru, Rh, Pt, C
VIa group such as r, Mo, etc .; IIIa such as Sc, Y, La, etc.
Group or VIIa group such as Mn is used. Preferably, it is Cu or a rare earth metal. The amount of the transition metal introduced into the catalyst may be 0.02 to 3 in atomic ratio with respect to Al in the zeolite. The method for ion-exchanging the transition metal is not particularly limited, but can be carried out by ordinary ion exchange. As the transition metal source to be used, salts such as chlorides, nitrates, sulfates, and acetates of transition metals are suitably used. Further, transition metal ammine complex salts and the like can also be suitably used. The amount, concentration, exchange temperature, time, and the like of the transition metal during ion exchange are not particularly limited, and a generally used method may be used. The addition amount of the transition metal is preferably 0.2 to 2 times the atomic ratio of Al in the zeolite. The slurry concentration for ion exchange is preferably 5 to 50%, which is usually used. Further, the ion exchange temperature and time are preferably a temperature of room temperature to 100 ° C. and a time of 5 minutes to 3 days. Further, if necessary, the ion exchange operation can be repeated. In the catalyst used in the present invention, it is essential to mix a large-pore zeolite and a small-pore zeolite. Each zeolite is mixed in powder form, or after uniformly mixed in a slurry state, and then solid-liquid separated. Then, it can be prepared by a method such as drying. Alternatively, each zeolite may be mixed after being formed into a pellet. Preferably, it is desirable to mix in a powder form or a slurry form. The mixing of the large pore zeolite and the small pore zeolite is carried out after ion exchange of the transition metal. After mixing the raw material zeolite, the transition metal can be subjected to ion exchange. The exhaust gas purifying catalyst used in the present invention comprises:
It can also be used by mixing with a binder such as a clay mineral and molding. Alternatively, the raw material zeolite may be formed in advance, and the active metal may be introduced using the formed body. As a binder used when molding zeolite,
Clay minerals such as kaolin, attapulgite, montmorillonite, bentonite, allophane and sepiolite. Alternatively, a binderless zeolite molded body obtained by directly synthesizing a molded body without using a binder may be used. Alternatively, the catalyst for purifying exhaust gas used in the present invention may be wash-coated on a cordierite or metal honeycomb substrate and used. The exhaust gas purifying catalyst thus prepared is brought into contact with an oxygen-excess exhaust gas containing nitrogen oxides and hydrocarbons to remove nitrogen oxides. It is essential that the exhaust gas used in the present invention contains nitrogen oxides and hydrocarbons and is in excess of oxygen, but it is also effective when it contains carbon monoxide, hydrogen, ammonia and the like. Although the hydrocarbon species is not particularly limited, it is particularly effective when a hydrocarbon containing C ₆ or more is contained. The target exhaust gas may be one to which fuel oil such as gasoline or light oil is added as required. The concentration of each component in the exhaust gas is not particularly limited, but NO = 0.02 to 1 mol%, hydrocarbon: THC = 0.02 to 3 mol%, O ₂ = _{2 to} 15 m
ol%, CO ₂ = 0 to 15 mol%, H ₂ O = 0 to 15 m
ol% is preferred. Particularly preferred hydrocarbon: THC =
0.1 to 1 mol%. Here, THC is the concentration when converted to methane. Exhaust gas with excess oxygen means that the exhaust gas contains oxygen in excess of the amount of oxygen required to completely oxidize carbon monoxide, hydrocarbons, and hydrogen. For example, in the case of exhaust gas discharged from an internal combustion engine of an automobile or the like, exhaust gas discharged from a combustion state (lean region) having a large air-fuel ratio can be mentioned. The space velocity, temperature and the like for removing nitrogen oxides are not particularly limited, but the space velocity is 100 to 50,000.
The temperature may be 0 hr ^-1 and the temperature is 200 to 800 ° C. EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples. Production Example 1 Ammonium-exchanged Z having a SiO ₂ / Al ₂ O ₃ molar ratio of 40
SM-5; 50 g was weighed at a concentration of 0.1 mol, which was twice as large as the number of moles of Al ₂ O ₃ contained therein.
Per liter of an aqueous solution of copper (II) acetate hydrate,
Immediately, add 2.5% aqueous ammonia to adjust the pH of the aqueous solution to 1
The mixture was adjusted to 0.5 and stirred at room temperature for 16 hours. After the solid-liquid separation, the product was sufficiently washed with water and subjected to copper ion exchange. After performing this ion exchange operation twice, it was dried at 110 ° C. for 10 hours. The copper ion exchanged zeolite thus obtained was used as catalyst A. When the copper content of this catalyst was examined by chemical analysis, the copper content was expressed as a Cu / Al atomic ratio with respect to Al in the zeolite of 0.1.
53 were included. Production Example 2 Proton-exchanged Y-type zeolite having a SiO ₂ / Al ₂ O ₃ molar ratio of 6; 50 g was weighed so as to be 1 times the number of moles of Al ₂ O ₃ contained therein. Concentration 0.1mol /
Liter of an aqueous solution of copper (II) acetate hydrate
Stirring was performed at 0 ° C. for 20 hours. After the solid-liquid separation, the product was sufficiently washed with water and subjected to copper ion exchange. After performing this ion exchange operation five times, the resultant was dried at 110 ° C. for 10 hours. The copper ion exchanged zeolite thus obtained was used as catalyst B. When the copper content of this catalyst was examined by chemical analysis, it was found that the Cu / Al atomic ratio was 0.31 with respect to Al in the zeolite. Production Example 3 Proton-exchanged β-zeolite having a SiO ₂ / Al ₂ O ₃ molar ratio of 35; 50 g was weighed so as to be 4 times the number of moles of Al ₂ O ₃ contained therein. Concentration 0.1m
ol / liter of an aqueous solution of copper (II) acetate hydrate and stirred at 40 ° C. for 20 hours. After the solid-liquid separation, the product was sufficiently washed with water and subjected to copper ion exchange. After performing this ion exchange operation twice, it was dried at 110 ° C. for 10 hours. The copper ion exchanged zeolite thus obtained was used as catalyst C. When the copper content of this catalyst was examined by chemical analysis, it was found that the content of Cu in the zeolite was 0.72 based on the atomic ratio of Cu / Al. Production Example 4 Proton-exchanged Y-type zeolite having a SiO ₂ / Al ₂ O ₃ molar ratio of 6; 50 g was weighed so as to be 1 times the mole number of Al ₂ O ₃ contained therein. Concentration 1.0mol /
Liter of an aqueous solution of lanthanum (III) chloride,
Stirring was performed at 80 ° C. for 20 hours. After the solid-liquid separation, the mixture was sufficiently washed with water and ion exchanged. After performing this ion exchange operation once, it was dried at 110 ° C. for 10 hours. The lanthanum exchanged zeolite thus obtained was used as catalyst D. When the lanthanum content of this catalyst was examined by chemical analysis, it was found that the lanthanum content was 0.06 with respect to Al of the zeolite in La / Al atomic ratio. Example 1 2.0 g of the catalyst A obtained in Production Example 1 and 1.5 g of the catalyst B obtained in Production Example 2 were uniformly mixed in an agate mortar. After press molding, the mixture was crushed and sized to 12 to 20 mesh to obtain Catalyst 1. Example 2 The catalyst A obtained in Production Example 1 and the catalyst B obtained in Production Example 2 were each press-molded, sized to 12 to 20 mesh, and 1 cc of each was mixed. Prepared. Example 3 <Evaluation of catalyst activity 1> The catalysts 1 and 2 prepared in Examples 1 and 2 were each replaced with 2 cc.
Filled into a fixed-bed reaction tube at normal pressure, and isooctane was added to a gas simulating exhaust gas from a diesel engine with THC = 1700p
pm added gas (Table 1) at space velocity 30000 / hr
Flowed away. After pretreatment at 550 ° C for 30 minutes, N
The O purification rate was measured. Table 2 shows the obtained results. NO
The purification rate is expressed by the following equation. NO purification rate (%) = (NOin−NOou)
t) / NOin × 100 NOin: NO concentration at fixed bed reaction tube inlet NOout: NO concentration at fixed bed reaction tube outlet NO concentration [Table 2] Comparative Example 1 The catalyst A obtained in Production Example 1 was press-molded and then crushed.
The particles were sized to 2 to 20 mesh to prepare Comparative Catalyst 1. Comparative Example 2 Catalyst B obtained in Production Example 2 was prepared in the same manner as in Comparative Example 1,
Comparative catalyst 2 was used. Comparative Example 3 The catalyst A obtained in Production Example 1 and the catalyst B obtained in Production Example 2 were each press-molded, crushed, and sized to 12 to 20 mesh. 1 cc of catalyst B; 1 cc of catalyst A; Comparative Example 4 <Evaluation 1 of Activity of Comparative Catalyst> Using the catalysts 1 to 3 prepared in Comparative Examples 1 to 3, the NO purification rate was measured in the same manner as in Example 3. Table 3 shows the obtained results. [Table 3] Example 4 A catalyst 3 was prepared in the same manner as in Example 1 by using 5.0 g of the catalyst A obtained in Production Example 1 and 4.2 g of the catalyst C obtained in Production Example 3. Example 5 <Evaluation 1 of catalyst activity> Using the catalyst 3 prepared in Example 4, the NO purification rate was measured in the same manner as in Example 3. Table 4 shows the obtained results. [Table 4] Comparative Example 5 The catalyst C obtained in Production Example 3 was prepared in the same manner as in Comparative Example 1,
Comparative catalyst 4 was used. Comparative Example 6 Catalyst A obtained in Production Example 1 and Catalyst C obtained in Production Example 3 were each press-molded, crushed and sized to 12 to 20 mesh. 1 cc of the catalyst C was disposed on the upstream side with respect to the exhaust gas flow direction, and 1 cc of the catalyst A was disposed on the downstream side with respect to the exhaust gas flow direction. Comparative Example 7 <Evaluation 1 of Activity of Comparative Catalyst> Using the catalysts 4 to 5 prepared in Comparative Examples 5 and 6, the NO purification rate was measured in the same manner as in Example 3. Table 5 shows the obtained results.
Shown in [Table 5] Example 6 A catalyst 4 was prepared in the same manner as in Example 1 using 4.0 g of the catalyst A obtained in Production Example 1 and 3.1 g of the catalyst D obtained in Production Example 4. Example 7 <Evaluation 1 of Catalyst Activity> Using the catalyst 4 prepared in Example 6, the NO purification rate was measured in the same manner as in Example 3. Table 6 shows the obtained results. [Table 6] Comparative Example 8 Catalyst D obtained in Production Example 4 was prepared in the same manner as in Comparative Example 1,
This was designated as Comparative Catalyst 6. Comparative Example 9 Catalyst A obtained in Production Example 1 and Catalyst D obtained in Production Example 4 were each press-molded, crushed and sized to 12 to 20 mesh. 1 cc of the catalyst D was disposed on the upstream side with respect to the exhaust gas flow direction, and 1 cc of the catalyst A was disposed on the downstream side with respect to the exhaust gas flow direction. Comparative Example 10 <Evaluation 1 of Activity of Comparative Catalyst> Using the catalysts 6 and 7 prepared in Comparative Examples 8 and 9, the NO purification rate was measured in the same manner as in Example 3. Table 7 shows the obtained results.
Shown in [Table 7] Example 8 <Evaluation 2 of activity of catalyst> The catalyst 3 prepared in Example 4 was packed in a 2 cc normal pressure fixed bed reaction tube, and 8000 ppm of light oil was added to gas simulating exhaust gas of a diesel engine by THC. Gas (Table 8)
Was flowed at a space velocity of 30000 / hr, and the activity was evaluated.
Table 9 shows the obtained results. [Table 8] [Table 9] Comparative Example 11 <Evaluation 2 of Comparative Catalyst Activity> Comparative Catalysts 1, 4 and Comparative Example 6 prepared in Comparative Examples 1 and 5
Table 10 shows the results of measurement of the NO purification rate in the same manner as in Example 8 using the comparative catalyst 5a prepared in the same manner as in Example 8. [Table 10] As is clear from Tables 2 to 7, 9, and 10, hydrocarbons and nitrogen oxides can be produced by uniformly mixing the large pore zeolite and the small pore zeolite ion-exchanged with the transition metal. Nitrogen oxides can be efficiently removed from the contained oxygen-excess exhaust gas.

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) B01D 53 / 86,53 / 94 B01J 21/00-38/74 JSPlus (JOIS) CAplus (STN)

Claims (1)

(57) [Claim 1] 7 angstros ion-exchanged with a transition metal
- large pore zeolite and a transition metal having pores greater than arm
Has pores of less than 7 angstroms ion-exchanged with
Small and pore zeolites using a mixed catalyst, a method for removing nitrogen oxides from an oxygen excess exhaust gas containing hydrocarbons and nitrogen oxides that.