Catalyst and method for exhaust gas purification

Abstract: A catalyst and method for exhaust gas purification comprising a crystalline silicate which comprises an oxide of an alkali metal and/or hydrogen ion, aluminum oxide, an oxide of an alkaline earth metal, Ca, Mg, Sr, or Ba, and which silicate has an X-ray diffraction pattern described in Table 1 in the specification, and said crystalline silicate containing at least one chosen from the group consisting of the metals in Groups Ib and VIII of the periodic table, rare earth elements, titanium, vanadium, chromium, antimony, zinc, and manganese. With the use of a crystalline silicate containing alkaline earth elements together with other metals or with a laminar composite structure of catalyst, the separation of aluminum or other metals from the crystal lattices (dealuminization or demetallization) can be controlled, and heat and steam resistance may be achieved.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst and method for purifying exhaust gases that contain nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbons. 2. Description of the Related Art For the purification treatment of exhaust gases from automobiles and other similar sources, catalysts (composition: Pt, Rh/Al 2 O 3 system) known as three-way catalysts which utilize hydrocarbons and CO in the automotive emissions are usually used. The three-way catalysts are normally prepared by applying a γ-alumina slurry to a refractory carrier of cordierite or the like, baking, and then causing the carrier to support a metal, e.g., Pt, Rh, or Pd. They cannot remove NOx, hydrocarbons, and CO, however, unless the combustion occurs within a very narrow range close to the theoretical air-fuel ratio (e.g., A/F = 14.6). With growing concern over the global environments in recent years, there has been strong demand for fuel-efficient cars. Accordingly, lean-burn engines for combustion above the theoretical air-fuel ratio have arrested attention as a key technology. It is well-known that the fuel efficiency is improved by burning in the engine a fuel mixture of an increased creased air-fuel ratio (A/F). However, this is accompanied by an increase in the oxygen concentration in the exhaust to such a level that while an ordinary three-way catalyst can remove hydrocarbons and CO, it does not remove NOx. To purify the exhaust gas in the "lean-burn" region where the oxygen concentration is high, it has already been proposed to use catalysts consisting of a transition metal, such as Cu, supported by a zeolite of the general formula, xM 2/n O·Al 2 O 3 ·ySiO 2 , wherein M is an n-valent metal (see, e.g., Japanese Patent Provisional Publications Nos. 125250/1985 (60-125250) and 130735/1989 (1-130735)). Another catalyst has also been introduced to meet the end which comprises a crystalline silicate to which Cu is added, the silicate itself having a chemical composition, (1±0.4)R 2 O·[aM 2 O 3 ·bAl 2 O 3 ]·ySiO 2 , in which R is an alkali metal ion and/or hydrogen ion, M is the ion of at least one element selected from the group consisting of Group VIII elements, rare earth elements, titanium, vanadium, chromium, niobium, and antimony, a + b = 1.0 , a ≥ 0, b > 0, and y > 12 (Japanese Patent Provisional Publication No. 303194/1989 (1-303194)). While these recently introduced catalysts exhibit satisfactory initial activities, they have the disadvantage of limited durability. A known catalyst described in EP-A-0 434 063 has a basic structure consisting of Si, Al, and O only. Various metal ions including alkaline earth metal, cobalt, and rare earth metal ions are carried by the basic zeolite structure at ion exchange sites without getting into the structural backbone. The strong acidic points in the crystalline silicate have been found to promote demetallization. If the alkaline earth metal is carried by the silicate at its ion exchange sites, the acidity of the strong acidic points would not be reduced sufficiently; and, if the amount of the alkaline earth metal ions is increased, less of active metal ions can be carried by the silicate, affecting the catalytic activities. Furthermore, WO-A-9106508 = EP-A-0 451 291 discloses a catalyst comprising a crystalline silicate of (1±0.4)R 2 O·[a·M 2 O 3 ·bAl 2 O 3 ]·ySiO 2 and added M (Group VIII elements, titanium, chrome, niobium, or antimony. Exhaust gases from ordinary lean-burn engines are at high temperatures over a broad range of 300 to 700°C, and conventional catalysts used for such engines have been found to deteriorate due to sintering of the supported active metal. Also, the present inventors have found that the catalytic deterioration of such conventional catalysts is accelerated by demetallization, a phenomenon in which aluminum and iron in the crystal lattices, which form ion exchange sites in the catalyst carrier zeolite and crystalline silicate, are removed from the crystal lattices under the influence of the high temperature exhaust gas atmosphere, especially in the presence of steam. SUMMARY OF THE INVENTION In view of the state of the art summarized above, an object of the invention as defined in the appended claims is to provide catalysts for purifying exhaust gases which is highly resistant to heat and steam and which does not have the drawbacks of conventional catalysts, and methods for purifying exhaust gases. In order to develop a catalyst with great resistance to heat and steam, it is necessary to avoid the separation of aluminum or iron from the carrier, and it has been found that if aluminum remains stably in the crystal lattice, the degree of sintering of the active metal would be low under the exhaust gas conditions in the presence of high temperature steam. The present inventors have also found that a catalyst which uses an alkaline earth metal-containing crystalline silicate as a catalyst carrier and which supports Cu or other similar metals on the carrier inhibits the metal from separating from the crystal lattices at elevated temperatures and in the presence of steam; that is, the demetallization can be controlled. Thus the present inventors have developed a catalyst with excellent heat and steam resistance developed through the application of a crystalline silicate which incurs only a small possibility of dealuminization. Furthermore, the present inventors have also discovered that by employing a laminar composite structure of a crystalline silicate catalyst, the demetallization in high temperature steam atmosphere can be prevented, and developed new type of catalysts for the purification of exhaust gases containing nitrogen oxides, hydrocarbons, and carbon monoxide. The catalyst of claim 1 has specified in its chemical formula that a is 0 to 2 and a + b is more than one. This is because the components in these ranges permit the synthesis of the crystalline silicate as formulated. Also, the specified range of b being 0.03 to 40 is where the alkaline earth metal can function effectively. The further limitation of c being 11 to 3000 is based on the fact that if c is less than 11 the synthesis of the crystalline silicate is impossible and if c is more than 3000 the resulting catalyst according to the invention does not have adequate exhaust purification activity. Thus, this catalyst for exhaust gas purification is characterized by the use of a silicate containing an alkaline earth metal as a crystalline silicate which incurs only a limited possibility of removal of aluminum from the carrier. This silicate is produced through crystallization by hydrothermal synthesis, with the presence of an alkaline earth metal in the starting material mixture. For the synthesis of the crystalline silicate that contains an alkaline earth metal, the silica source to be used is water glass, silica sol, silica gel or the like. The alumina source to be chosen is aluminum nitrate, aluminum sulfate, sodium aluminate or the like. The alkali metal ion is, e.g., sodium oxide in water glass, sodium aluminate, sodium hydroxide. For alkaline earth metal ions, their acetates, nitrates, and chlorides may be used. As an alkaline earth metal calcium (Ca), magnesium (Mg), strontium (Sr), or barium (Ba) can be used. Further, as a crystallizing mineralizer tetrapropylammonium bromide or the like is used. The reaction mixture for the hydrothermal synthesis of the alkaline earth metal-containing crystalline silicate is made up with the following mixing ratios: SiO 2 /Al 2 O 3 = 11 to 3000 (molar ratio); OH - /SiO 2 = 0.01 to 10; H 2 O/SiO 2 = 1 to 1000; tetrapropylammonium compound/SiO 2 = 0 to 4; alkaline earth metal/Al (atomic ratio) = 0.03 to 40. The crystalline silicate can be synthesized by heating the above mixture at 80 to 200°C for about 1 to 200 hours with stirring, removing excess ions by water rinsing, drying, and then baking. The catalyst of claim 1 is prepared by allowing the alkaline earth metal-containing crystalline silicate to support at least one metal in Groups Ib and VIII, e.g., copper or cobalt, either by way of ion exchange with the immersion of the silicate in an aqueous solution of such a metal or by impregnating the silicate with an aqueous solution of a salt of the metal, such as chloride or nitrate. Although the catalyst made in accordance with the invention is desired to have a honeycomb shape that permits the reduction of pressure loss, it may be pelletized instead. Such binder as silica sol or alumina sol in use for the molding of the catalyst has already been confirmed to have little adverse effect upon the catalyst performance. This catalyst is adequately durable in automotive exhaust purification and other similar services with only little deterioration of activity in the presence of steam at 600°C and even higher temperatures. The outstanding durability of this catalyst of claim 1 is attributable to the use of an alkaline earth-containing silicate as a carrier. The alkaline earth contained in the silicate functions to decrease the number of strongly acidic points in the silicate which would otherwise accelerate the dealuminization. On the other hand, it causes little change in the catalytic activities for NO adsorption and the activation of hydrocarbons. Since the dealuminization is thus avoided, the Group Ib or VIII metal or metals that are active can be stably supported by the silicate carrier. Another catalyst of this invention is defined in claim 3. The alkaline earth metal-containing crystalline silicate is prepared by mixing sources of silica, alkaline earth metal, alkali metal, transition metal, and aluminum with water and a quaternary alkylammonium salt such as tetrapropylammonium bromide, or an alcohol amine, alkylamine or the like, and then holding this reaction mixture for a period and at a temperature adequate for forming a crystalline silicate. The silica source to be used may be water glass, silica sol, silica gel or the like. The alkaline earth metal source may be an acetate, nitrate, chloride, or the like of Ca, Mg, Sr, or Ba. The alkali metal is, e.g., sodium in water glass, sodium hydroxide, or potassium hydroxide. The aluminum source may be sodium aluminate, aluminum nitrate, aluminum sulfate, or the like. Transition metal sources include the Group VIII elements, such as Fe, Ni, Co, Rh, Ru, and Pd, rare earth elements, such as La and Ce, as well as Ti, V, Cr, Nb, and Sb. Such a source of transition metals may be used in the form of, e.g., a sulfate, nitrate, or chloride as a starting material. The catalyst is prepared by adding the chloride or nitrate of Cu, Co, Ni, Zn, Fe, Cr, or Mn to the alkaline earth metal-containing crystalline silicate either through impregnation or ion exchange with an aqueous solution of their salt. The catalyst according to claim 3 is characterized in that it is prepared by allowing an alkaline earth metal to be present in the reaction mixture at the time of crystalline silicate synthesis. Presumably, the alkaline earth metal incorporated and stabilized in the lattice of the silicate crystal in this way helps improve the catalyst durability. As discussed above, the catalyst for exhaust gas purification in accordance with the invention is highly durable and stable. It is useful as a catalyst for purifying exhaust gases from lean-burn gasoline engines and diesel engines of motor vehicles. In the following the present invention will be described in further detail with reference to a number of embodiments for this invention and comparative examples for conventional catalyst. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention of claim 1 is described in detail by way of Examples 1 to 3, Comparative Example 1 and Experiments 1 and 2. Example 1 Catalysts were prepared in the following way. Synthesis of alkaline earth metal-containing silicate 1A A solution prepared by dissolving 395.5 g of aluminum nitrate and 94.4 g of calcium acetate in 6552 g of water was designated solution 1A. Solution 1B was prepared by dissolving 4212 g of water glass sold as "Cataloid SI-30" (a product of Catalysts & Chemicals Industries Co., Japan, contg. 30.5% SiO 2 and 0.42% Na 2 O) in 2808 g of water. Solution 1B was added to solution 1A with vigorous stirring. Next, a solution of 234 g sodium hydroxide in 1404 g water and then a solution of 568.6 g tetrapropylammonium bromide (TPABr) in 2106 g water were added. Stirring was continued for about 10 minutes, and an aqueous gel mixture resulted. The molar ratios of the components were: SiO 2 /Al 2 O 3 = 40 and SiO 2 /CaO = 40. This aqueous gel mixture was charged into an autoclave having a capacity of 20 liters and hydrothermally treated at the charge's own pressure and at 160°C for 72 hours with stirring (at 200 rpm). Following the treatment, the reaction product was centrifuged for solid-liquid separation. The solid matter was thoroughly washed with water and dried at 120°C for 5 hours in the air. It was then baked at 500°C for 5 hours. The Ca-containing crystalline silicate thus obtained is herein referred to as silicate 1A. Synthesis of alkaline earth metal-containing silicates 1B-1N The procedure described above for the synthesis of the alkaline earth metal-containing silicate 1A was repeated except for changes in the alkaline earth salts used as an ingredient and in compositional formulation to obtain various alkaline earth metal-containing silicates 1B to 1N. The proportions of ingredients are listed in Table 1 below. Preparation of catalysts Alkaline earth metal-containing silicates 1A to 1N were subjected to copper ion exchange by immersion in a 0.04M aqueous copper acetate with stirring, and after 24-hour stirring at room temperature they were rinsed. This cycle of copper ion exchange was repeated three times each with a fresh supply of the solution. Final rinsing and drying gave powder catalysts 1a to 1n. To 100 parts of each of the powder catalysts added as binders were 3 parts of alumina sol, 55 parts of silica sol, and 230 parts of water. The mixtures upon thorough stirring afforded slurries for wash coating. Divided portions of cordierite type monolithic substrates (with a 400-cell lattice pattern) were separately dipped in the slurries and taken out, excess slurries were blown away, and then dried at 200°C. The coating weighed 200 g per liter of the substrate. The coated substrates are designated as honeycomb catalysts 1A to 1N. Example 2 Divided portions of silicate 1A obtained in Example 1 were immersed with agitation in 0.04M aqueous solutions of cupric chloride, cobalt chloride, nickel chloride, ferric chloride, and silver nitrate, and in the same way as described in Example 1, powder catalysts 1O to 1S were prepared. These powder catalysts were supported by monolithic substrates in the manner described in Example 1, and honeycomb catalysts 1O to 1S were obtained. Example 3 Silicate 1A prepared in Example 1, with the addition of a binder, was applied to a cordierite type monolithic substrate to form a coating. Divided portions of the coated monolithic substrate were separately immersed for impregnation over a one-hour period in a solution of cuprous chloride in hydrochloric acid (26.8 g in 200 cm 3 of HCl), aqueous cupric chloride solution (46.1 g in 200 cm 3 of water), aqueous cobalt chloride (64.4 g in 200 cm 3 of water), aqueous nickel chloride (64.4 g in 200 cm 3 of water), aqueous solution mixture of cupric chloride and cobalt chloride (23.1 g of CuCl 2 ·2H 2 O and 32.2 g of CoCl 2 ·6H 2 O in 200 cm 3 of water), and aqueous solution mixture of nickel chloride and cobalt chloride (32.2 g of NiCl 2 ·2H 2 O and 32.2g of CoCl 2 ·6H 2 O in 200 cm 3 of water), respectively. The solutions left on the walls of the substrates were wiped off, and the coated substrates were dried at 200°C. A 12-hour purging in a nitrogen atmosphere at 500°C yielded honeycomb catalysts 1T to 1Y. Comparative Example 1 Two types of crystalline silicates were synthesized by the same method as described in Example 1 with the exception that no alkaline earth metal was added. The material proportions of these silicates, designated as 1O and 1P, are also given in Table 1. These silicates 1O and 1P were subjected to Cu ion exchange in the same manner as for the catalyst preparation in Example 1 to obtain powder catalysts 1t and 1u. Likewise monolithic substrates were coated with them to give honeycomb catalysts 1Z and 1ZA. The details of thus obtained honeycomb catalysts 1A to 1ZA are summarized in Table 2. Experiment 1 Honeycomb catalysts 1A to 1ZA prepared in Examples 1 to 3 and in Comparative Example 1 were tested for the evaluation of their activities. The conditions for activity evaluation were as follows. Gas composition NO = 400 ppm; CO = 1000 ppm; C 2 H 4 = 1000 ppm; C 3 H 6 = 340 ppm; O 2 = 8%; CO 2 = 10%; H 2 O = 10%; the remainder = N 2 ; GHSV = 30000 hr -1 ; catalyst shape = 15 mm x 15 mm x 60 mm (144 cells); reaction temperature = 350°C. The denitration rates of the catalysts in the initial state at 450°C are given in Table 3. Experiment 2 As a durability test the catalysts were forced to deteriorate with the supply of a gaseous mixture consisting of 10% H 2 O and the balance N 2 gas at a GHSV of 30000 hr -1 for 24 hours. The forcibly deteriorated honeycomb catalysts 1A to 1ZA were evaluated for their activities under the same conditions as used in Experiment 1. The results are also shown in Table 3. As Table 4 indicates, honeycomb catalysts 1A to 1Y prepared using alkaline earth metal-containing silicates had high denitration activities after the forced deterioration as well as in the initial state, proving that they were durable. Table 4 also indicates that the ordinary silicate catalysts (honeycomb catalysts 1Z and 1ZA) showed substantial reductions in activity after the forced deterioration, revealing that they were less durable. Honeycomb catalyst No. Powder catalyst No. Qty of active substance supported (mmol per gram of silicate) Type of silicate No. SiO 2 /Al 2 O 3 SiO 2 /MO 1A 1a Cu (0.55) 1A 40 M:Ca 40 1B 1b Cu (0.5) 1B 40 M:Mg 40 1C 1c Cu (0.55) 1C 40 M:Sr 40 1D 1d Cu (0.5) 1D 40 M:Ba 40 1E 1e Cu (0.5) 1E 40 M:Ca 80 1F 1f Cu (0.45) 1F 40 M:Ca 20 1G 1g Cu (0.4) 1G 40 M:Ca 40 1H 1h Cu (0.45) 1H 40 M:Ba 40 1I 1i Cu (0.5) 1I 40 M:Ca 40 1J 1j Cu (0.4) 1J 80 M:Ca 40 1K 1k Cu (0.35) 1K 80 M:Ca 40 1L 1l Cu (0.45) 1L 80 M:Sr 40 1M 1m Cu (0.4) 1M 80 M:Ba 40 1N 1n Cu (0.35) 1N 80 M:Mg 40 1O 1o Cu (0.55) 1A 40 M:Ca 40 1P 1p Co (0.5) 1A 40 M:Ca 40 1Q 1q Ni (0.55) 1A 40 M:Ca 40 1R 1r Fe (0.35) 1A 40 M:Ca 40 1S 1s Ag (0.8) 1A 40 M:Ca 40 1T - Cu (0.8) 1A 40 M:Ca 40 1U - Cu (0.8) 1A 40 M:Ca 40 1V - Co (0.8) 1A 40 M:Ca 40 1W - Ni (0.8) 1A 40 M:Ca 40 1X - Cu(0.4)Co(0.4) 1A 40 M:Ca 40 1Y - Ni(0.4)Co(0.4) 1A 40 M:Ca 40 1Z 1t Cu (0.6) 1O 40 - 1ZA 1u Cu (0.4) 1P 40 - Honeycomb Catalyst No. Initial activity (ηNOx %) After forced deterioration (ηNOx %) 350°C 450°C 350°C 450°C 1A 42 39 37 39 1B 41 40 35 39 1C 38 42 33 40 1D 41 39 36 38 1E 42 39 38 39 1F 44 41 38 42 1G 45 41 40 41 1H 42 43 38 41 1I 45 41 39 40 1J 46 43 38 41 1K 43 41 39 40 1L 42 39 38 39 1M 40 40 36 37 1N 41 42 35 38 1O 42 40 34 39 1P 41 39 38 40 1Q 43 41 36 39 1R 28 29 28 29 1S 31 31 30 28 1T 45 40 35 40 1U 44 39 39 40 1V 46 41 36 39 1W 45 40 35 39 1X 43 39 36 38 1Y 41 38 35 38 1Z 41 38 8 18 1ZA 42 39 7 17 As for the purification activity against CO and hydrocarbons, honeycomb catalysts 1A to 1Y according to the first aspect of the invention showed little changes in activity from the initial stage till after the forced deterioration. They all achieved 100% removal of those pollutants at both 350°C and 450°C. Comparative honeycomb catalysts 1Z and 1ZA also exhibited initial activities of 100% CO and hydrocarbon removal at the two temperature levels. After the forced deterioration, however, honeycomb catalyst 1Z showed much lower CO removal rates at 350°C and 450°C of 15% and 60%, respectively. Its hydrocarbon removal rates at the two temperature levels dropped to 45% and 80%. At 350°C and 450°C, honeycomb catalyst 1ZA too showed reduced CO removal rates of 10% and 50% and hydrocarbon removal rates of 35% and 70%, respectively. The invention of claim 3 is described in more detail by way of Example 4, Comparative Example 2 and Experiment 3. Example 4 Synthesis of alkaline earth metal-containing silicate 2A A solution prepared by dissolving 5616 g of water glass #1 (contg. 30 wt% SiO 2 ) in 5429 g of water is referred to as solution 2A. Into 4175 g of water were dissolved 453.6 g of aluminum sulfate (Al 2 (SO 4 ) 3 ·17H 2 O), 94.6 g of ferric chloride (FeCl 3 ·6H 2 O), 153.3 g of calcium chloride (CaCl 2 ·6H 2 O), 262 g of sodium chloride, and 2020 g of concentrated hydrochloric acid. The resulting solution is designated as solution 2B. Solutions 2A and 2B were gradually mixed and sufficiently stirred to form a precipitate, thus preparing a slurry with pH 8.0. This slurry was charged into an autoclave and, with the addition of 500 g tetrapropylammonium bromide, the mixture was kept at 160°C for 72 hours for crystallization. The crystalline product was filtered, rinsed, dried, and baked in air at 500°C for 3 hours, and crystalline silicate 2A resulted. Synthesis of alkaline earth metal-containing crystalline silicates 2B, 2C, and 2D The procedure for the synthesis of silicate 2A was repeated with the exception that calcium chloride (as 6H 2 O salt) was replaced by 142.3 g of magnesium chloride (as 6H 2 O salt), 186.6 g of strontium chloride (as 6H 2 O salt), or 170.9 g of barium chloride (as 2H 2 O salt) to obtain crystalline silicates 2B, 2C, and 2D, respectively. Preparation of catalysts With the addition of a binder, powders of crystalline silicates 2A to 2D were applied to cordierite monolithic substrates (meshed with 400 cells per square inch). The coated substrates were immersed in a 4M aqueous NH 4 Cl solution at 80°C for 24 hours to effect NH 4 ion exchange. The ion exchange was followed by rinsing, drying, and baking at 500°C for 3 hours to convert the coated crystalline silicates to proton type silicates. The monolithic catalysts so obtained were then separately dipped separately in an aqueous hydrochloric acid solution of CuCl and aqueous solutions of CoCl 2 , NiCl 2 , ZnCl 2 , FeCl 3 , and MnCl 2 . Excess liquid was wiped off from the substrate walls, and the catalysts were dried at 200°C. Purging in an N 2 atmosphere at 500°C for 12 hours gave catalysts C1 to C28 as listed in Table 4 below. The quantity of each metal supported by the crystalline silicate was 0.8 mmol/g. Comparative Example 2 As a comparative example, crystalline silicate 2E was obtained by the same procedure for the synthesis of crystalline silicate 2A but without the addition of calcium chloride. Then, catalysts R1 to R7 were prepared for comparison purposes are prepared using this crystalline silicate 2E in the same way as with the preparation of catalysts C1 to C28 with the supported metal listed in Table 4 below. Also, divided portions of the monolithic substrate coated with silicate 2A were converted to the proton type in the manner described above and then were subjected to ion exchange separately with aqueous solutions of the chlorides of Ca, Mg, Sr, and Ba. After water rinsing, drying, and dipping in aqueous hydrochloric acid solution of CuCl in the same way as above, catalysts R8 to R11 were obtained as comparative examples. Experiment 3 The monolithic catalysts prepared as above were tested for their catalytic performance before and after use in the treatment of a gas consisting of 10% H 2 O and the balance N 2 at 700°C and at a GHSV of 30000 Hr -1 for 24 hours. Their performance was determined under the following conditions. Testing conditions NO 400 ppm CO 1000 ppm C 2 H 4 1000 ppm C 3 H 6 340 ppm O 2 8% CO 2 10% H 2 O 10% N 2 balance GHSV 30000 hr -1 The denitration rates at 350°C and 450°C are given in Table 5. Catalyst No. Denitration rate (%) Catalyst No. Denitration rate (%) Before treatment After treatment Before treatment After treatment 350°C 400°C 350°C 400°C 350°C 400°C 350°C 400°C C1 41 40 37 40 C23 47 41 42 41 C2 47 41 42 41 C24 46 40 41 40 C3 47 39 42 39 C25 32 40 30 41 C4 32 40 29 40 C26 29 27 26 28 C5 30 29 29 29 C27 28 24 26 24 C6 28 25 25 25 C28 24 31 23 30 C7 25 33 21 33 R1 41 39 10 20 C8 40 39 36 40 R2 46 40 8 20 C9 46 40 42 39 R3 46 41 9 21 C10 45 40 41 41 R4 33 39 11 19 C11 33 41 30 41 R5 30 29 9 15 C12 30 28 28 28 R6 27 24 8 12 C13 29 24 26 24 R7 23 31 10 18 C14 24 34 21 34 R8 38 41 12 23 C15 40 42 37 42 R9 39 40 10 22 C16 46 40 41 40 R10 40 41 7 19 C17 45 41 40 40 R11 38 39 10 20 C18 33 41 30 41 C19 31 29 27 28 C20 29 25 25 25 C21 23 32 21 31 C22 39 40 35 40 Catalysts C1 to C28 according to the second aspect of the present invention were found to be clearly superior to comparative catalysts R1 to R11 in denitration after the treatment of a gaseous mixture with 10% H 2 O at 700°C, and it is now obvious that the catalysts of the second aspect of the invention is excellently durable and useful as a catalyst for the treatment of exhaust gases.

Claims: A catalyst for exhaust gas purification comprising a crystalline silicate which in a dehydrated state has a chemical formula, in terms of the molar ratio of oxides: aR 2 O·bMO·0Al 2 O 3 ·cSiO 2 , wherein R is an alkali metal ion and/or hydrogen ion, M is at least one alkaline earth metal selected from the group consisting of Ca, Mg, Sr, and Ba, a = 0-2, b = 0.03-40, wherein M is incorporated into the crystalline silicate through crystallization by hydrothermal synthesis, with the proviso that a + b > 1 and c = 11-3000, and which silicate has an X-ray diffraction pattern according to the following Table Lattice spacing (d value) Relative intensity 11.2 ± 0.3 very strong 10.0 ± 0.3 very strong 6.7 ± 0.2 weak 6.4 ± 0.2 moderate 6.0 ± 0.2 moderate 5.7 ± 0.2 weak 5.6 ± 0.2 moderate 4.6 + 0.1 weak 4.25 ± 0.1 moderate 3.85 ± 0.1 very strong 3.75 ± 0.1 strong 3.65 ± 0.1 strong 3.3 ± 0.1 moderate 3.05 ± 0.1 weak 3.0 ± 0.1 moderate and said crystalline silicate containing at least one metal chosen from Groups Ib and VIII of the periodic table. A method for purifying exhaust gases containing nitrogen oxides, hydrocarbons, and carbon monoxide, comprising the use of the catalyst as claimed in claim 1. A catalyst for purifying exhaust gases containing nitrogen oxides, hydrocarbons, and carbon monoxide, comprising a crystalline silicate which in a dehydrated state has a chemical formula: (1±0.4)R 2 O·[aM 2 O 3 ·bAl 2 O 3 ]·cMeO·ySiO 2 , wherein R is an alkali metal ion and/or hydrogen ion, M is at least one element selected from the group consisting of Group VIII elements, rare earth elements, titanium, vanadium, chromium, niobium, and antimony, Me is an alkaline earth element, a+b = 1.0 , a > 0, b > 0, y/c > 12 , and y > 12, and said crystalline silicate containing at least one metal selected from Zn, Mn, Cr, and the elements in Groups Ib and VIII of the periodic table. A method for purifying exhaust gases containing nitrogen oxides, hydrocarbons, and carbon monoxide, comprising the use of the catalyst as claimed in claim 3.

Inventors: IIDA KOZO, KOBAYASHI NORIHISA, NOJIMA SHIGERU, SERIZAWA SATURO, IIDA, KOZO, KOBAYASHI, NORIHISA, NOJIMA, SHIGERU, SERIZAWA, SATURO
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1992
Main classification: B01D53
IPC codes: B01J29, B01D53