Catalyst and method for exhaust gas purification

Abstract: A catalyst and method for exhaust gas purification comprising a crystalline silicate which comprises a laminar composite crystalline silicate having an X-ray diffraction pattern described in Table 1 in the specification, and said crystalline silicate being formed by growing a crystalline silicate made from Si and O over a mother crystal of a crystalline silicate synthesized in advance, said mother crystal containing an alkali metal ion and/or hydrogen ion and at least one element selected from the group consisting of the Group VIII elements of the periodic table, rare earth elements, titanium, vanadium, chromium, niobium, antimony and gallium, said mother crystal also having an X-ray diffraction pattern described in Table 1, the crystalline silicate and the laminar composite have the same structure, 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 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 that 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 ]·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 (WO 91/06508). While these recently introduced catalysts exhibit satisfactory initial activities, they have the disadvantage of limited durability. 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 this invention is to provide a catalyst for purifying exhaust gases which is highly resistant to heat and steam and which does not have the drawbacks of conventional catalysts. In order to develop a catalyst with great resistance to heat and steam, it is necessary to avoid the separation of aluminium or iron from the carrier, and it has been found that if aluminium 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. Furthermore, it was discovered that by employing a laminar composite structure of a crystalline silicate catalyst, the demetallization in high temperature steam atmosphere can be prevented, and hence a new type of catalysts was developed for the purification of exhaust gases containing nitrogen oxides, hydrocarbons, and carbon monoxide. A first aspect of this invention provides a catalyst for exhaust gas purification comprising a laminar composite crystalline silicate having an X-ray diffraction pattern described in the following Table 1, said laminar composite crystalline silicate being formed by growing a crystalline silicate made from Si and O over a mother crystal of a crystalline silicate synthesized in advance, said mother crystal being represented, in a dehydrated state and in terms of the molar ratio of oxides by the following chemical formula: (1±0.6)R 2 O·[aM 2 O 3 ·bAl 2 O 3 ]·ySiO 2 , wherein R is an alkali metal ion and/or hydrogen ion; M is an ion of at least one element selected from the group consisting of Group VIII elements, rare earth elements, titanium, vanadium, chromium, niobium, antimony, and gallium; a ≥ 0, b ≥ 0, a+b = 1, and y > 12; M is contained in the Si-O-Al structure of the mother crystal; said mother crystal also has the X-ray diffraction pattern described in the following Table 1; said crystalline silicate grown over said mother crystal having the same crystalline structure as said mother crystal, and said laminar composite crystalline silicate containing at least one metal chosen from Groups Ib and VIII of the periodic table, said metal being supported onto the silicate by ion exchange or by impregnation. Table 1 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; The mother crystal to be used in the first aspect of the present invention is synthesized under the following conditions: SiO 2 /(M 2 O 3 + Al 2 O 3 ) 12-3000 (preferably 20-200) OH - /SiO 2 0-1.0 (preferably 0.2-0.8) H 2 O/SiO 2 2-1000 (preferably 10-200) Organonitrogen compound/(M 2 O 3 + Al 2 O 3 ) 0-200 (preferably 0-50) M signifies one or more elements chosen from Group VIII elements, rare earth elements, titanium, vanadium, chromium, niobium, antimony, and gallium, and the organonitrogen compound when used may be tetrapropylammonium bromide or the like. The crystalline silicate used as the mother crystal can be synthesized by heating a mixture of the above materials at a temperature and for a time period sufficient for producing a crystalline silicate. The temperature for hydrothermal synthesis is in the range of 80 to 300°C, preferably 130 to 200°C, and the hydrothermal synthesis may be carried out for 0.5 to 14 days, preferably 1 to 10 days. The pressure is not specifically limited, but desirably the synthesis is carried out under the mixture's own pressure. The hydrothermal synthesis reaction is effected by heating the material mixture to a desired temperature and, with stirring where necessary, kept on until a crystalline silicate forms. Following the crystallization, the reaction mixture is cooled down to room temperature, filtered, rinsed with water, and separated. Usually, the product is further dried at 100°C or above for about 5 to 24 hours. The term "laminar composite crystalline silicate" as used herein means a composite crystalline silicate of a laminar structure formed by first synthesizing a crystalline silicate in the aforesaid manner as a mother crystal and then allowing a crystalline silicate (to be called "silicalite" hereinafter) which comprises Si and O and has the same structure as the mother crystal to grow thereon. The crystalline silicate serving as the mother crystal is desirably one which is represented, in a dehydrated state and in terms of the molar ratio of oxides, by the following chemical formula: (1±0.6)R 2 O·[aM 2 O 3 ·bAl 2 O 3 ]·ySiO 2, wherein R is an alkali metal ion and/or hydrogen ion, M is an ion of at least one element selected from the group consisting of Group VIII elements, rare earth elements, titanium, vanadium, chromium, niobium, antimony, and gallium, a ≥ 0, b ≥ 0, a + b = 1, and y > 12, said crystalline silicate having an X-ray diffraction pattern shown in Table 1 above. One method for the crystal growth of silicalite on the outer surface of the crystalline silicate serving as the mother crystal is hydrothermal synthesis. For the crystal growth of silicalite on the outer surface of the mother crystal by hydrothermal synthesis, water glass, silica sol or the like is employed as a silica source. The alkali metal ion to be used is, e.g., sodium oxide or sodium hydroxide in water glass, and the crystallizing mineralizer is, e.g., tetrapropylammonium bromide. The proportion of the silicalite to be grown as crystal with respect to the mother crystal crystalline silicate is desirably, in terms of the silicalite/mother crystal weight ratio, from 0.01 to 100 for the synthesis. Also, as a prerequisite for the crystal growth of silicalite on the mother crystal, the mixing ratios of the materials should be: OH - /SiO 2 = 0.01 to 10, H 2 O/SiO 2 = 1 to 1000, and organic matter (e.g., tetrapropylammonium compound)/SiO 2 = 0 to 10. The synthesis procedure consists of adding finely powdered mother crystals to the above mixture, stirring the whole mixture homogeneously, heating it with stirring at 80 to 200°C for 1 to 200 hours, removing excess ions by water rinsing, drying, and baking. In this way a laminar composite crystalline silicate according to the first aspect of the present invention is obtained. The catalyst is prepared by immersing the laminar composite crystalline silicate in an aqueous solution of a salt of a Group Ib or VIII metal, e.g., copper or cobalt, and allowing the silicate to support the metal ion by ion exchange or by impregnation with an aqueous solution of a metal salt, such as chloride or nitrate. In the first aspect of the invention, the use of a silicalite-covered laminar composite crystalline silicate improves the durability of the resulting catalyst for the following reason. In exhaust emissions present are such gases as NO, CO, hydrocarbons, H 2 O (steam), and O 2 . The exhaust is purified at the active reaction sites of the catalyst while, at the same time, the presence of high temperature H 2 O tends to cause metals to separate from the silicate (demetallization). However, the coating of silicalite, which is hydrophobic by nature, makes it difficult for H 2 O alone to permeate deep into the crystalline silicate. As a consequence, the H 2 O concentrations become low around the active reaction sites in the catalyst, and the demetallization is inhibited. A second aspect of this invention provides a catalyst for exhaust gas purification comprising a laminar composite crystalline silicate having an X-ray diffraction pattern described in the following Table 1, said laminar composite crystalline silicate being formed by growing a crystalline silicate made from Si and O over a mother crystal of a crystalline silicate synthesized in advance, said mother crystal being represented, in a dehydrated state and in terms of the molar ratio of oxides by the following chemical formula: (1±0.6)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 an ion of at least one element selected from the group consisting of Group VIII elements, rare earth elements, titanium, vanadium, chromium, niobium, antimony, and gallium; a ≥ 0, b ≥ 0, a+b = 1, y > 12, and y/c > 12; M is contained in the Si-O-Al structure of the mother crystal; said mother crystal also has the X-ray diffraction pattern described in the above Table 1; said crystalline silicate grown over said mother crystal having the same crystalline structure as said mother crystal, and said laminar composite crystalline silicate containing at least one metal chosen from Groups Ib and VIII of the periodic table, said metal having been incorporated into the silicate by ion exchange or by impregnation. The mother crystal to be used in the first aspect of the present invention is synthesized under the following conditions: SiO 2 /(M 2 O 3 + Al 2 O 3 ) 12-3000 (preferably 20-200) SiO 2 /MeO 12-∞ OH - /SiO 2 0-1.0 (preferably 0.2-0.8) H 2 O/SiO 2 2-1000 (preferably 10-200) Organonitrogen compound/(M 2 O 3 + Al 2 O 3 ) 0-200 (preferably 0-50) M is at least one element selected from the group consisting of Group VIII elements, rare earth elements, titanium, vanadium, chromium, niobium, antimony, and gallium, Me is an alkaline earth metal, and the organonitrogen compound when used may be tetrapropylammonium bromide or the like. The crystalline silicate used as the mother crystal is synthesized by heating a mixture of the above materials at a temperature and for a time period sufficient for producing a crystalline silicate. The temperature for hydrothermal synthesis is in the range of 80 to 300°C, preferably 130 to 200°C, and the hydrothermal synthesis is carried out for 0.5 to 14 days, preferably 1 to 10 days. The pressure is not specifically limited, but desirably the synthesis is carried out under the mixture's own pressure. The hydrothermal synthesis reaction is effected by heating the material mixture to a desired temperature and, with stirring where necessary, kept on until a crystalline silicate forms. Following the crystallization, the reaction mixture is cooled down to room temperature, filtered, rinsed with water, and separated. Usually, the product is further dried at 100°C or above for about 5 to 24 hours. The term "laminar composite crystalline silicate" as used herein means a composite crystalline silicate of a laminar structure formed by first synthesizing a crystalline silicate in the aforesaid manner as a mother crystal and then allowing a crystalline silicate (to be called "silicalite" hereinafter) which comprises Si and O and has the same structure as the mother crystal to grow thereon. The crystalline silicate serving as the mother crystal is desirably one which is represented, in a dehydrated state and in terms of the molar ratio of oxides, by the following chemical formula: (1±0.6)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 the ion of at least one element selected from the group consisting of Group VIII elements, rare earth elements, titanium, vanadium, chromium, niobium, antimony, and gallium, Me is an alkaline earth element, a ≥ 0, b ≥ 0, c ≥ 0, a + b = 1, y / c > 12, and y > 12, said crystalline silicate having an X-ray diffraction pattern shown in Table 1 above. One method for the crystal growth of silicalite on the outer surface of the crystalline silicate serving as the mother crystal is hydrothermal synthesis. For the crystal growth of silicalite on the outer surface of the mother crystal by hydrothermal synthesis, water glass, silica sol or the like is employed as a silica source. The alkali metal ion to be used is, e.g., sodium oxide or sodium hydroxide in water glass, and the crystallizing mineralizer is, e.g., tetrapropylammonium bromide. The proportion of the silicalite to be grown as crystal with respect to the crystalline silicate as the mother crystal is desirably, in terms of the silicalite/mother crystal weight ratio, from 0.01 to 100 for the synthesis. Also, as a prerequisite for the crystal growth of silicalite on the mother crystal, the mixing ratios of the materials should be: OH - /SiO 2 = 0.01 to 10, H 2 O/SiO 2 = 1 to 1000, and organic matter (e.g., tetrapropylammonium compound)/SiO 2 = 0 to 10. The synthesis procedure consists of adding finely powdered mother crystals to the above mixture, stirring the whole mixture homogeneously, heating it with stirring at 80 to 200°C for 1 to 200 hours, removing excess ions by water rinsing, drying, and baking. In this way a laminar composite crystalline silicate according to the present invention is obtained. The catalyst is prepared by immersing the laminar composite crystalline silicate in an aqueous solution of a salt of a Group Ib or VIII metal, e.g., copper or cobalt, and allowing the silicate to support the metal ion by ion exchange or by impregnation with an aqueous solution of a metal salt, such as chloride or nitrate. The use of a silicalite-covered laminar composite crystalline silicate improves the durability of the resulting catalyst for the following reason. In exhaust emissions are present such gases as NO, CO, hydrocarbons, H 2 O (steam), and O 2 . The exhaust is purified at the active reaction sites of the catalyst while, at the same time, the presence of high temperature H 2 O tends to cause metals to separate from the silicate (demetallization). However, the coating of silicalite, which is hydrophobic by nature, makes it difficult for H 2 O alone to permeate deep into the crystalline silicate. As a consequence, the H 2 O concentrations around the active reaction sites in the catalyst are low and the demetallization is inhibited. Moreover, in the catalyst according to the second aspect of the invention, the alkaline earth element contained in the mother crystal weakens the strongly acidic sites in the silicate which would otherwise promote the removal of metal from the resulting catalyst and thereby inhibits the demetallization in the presence of high-temperature steam. As discussed in connection with the first to second aspects 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 the two embodiments for this invention and comparative examples for conventional catalyst. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The first aspect of the present invention will now be described in detail by way of Examples 1 to 4 and Comparative Example 1. The composition of laminar composite crystalline silicate is listed in Table 2. Example 1 Synthesis of mother crystal 3A A solution of 5616 g of water glass #1 (contg. 30% SiO 2 ) in 5429 g of water is prepared and referred to as solution 3A. Meanwhile 718.9 g of aluminum sulfate, 110 g of ferric chloride, 262 g of sodium chloride, and 2020 g of concentrated hydrochloric acid are dissolved in 4175 g of water, and the solution is designated as solution 3B. Solutions 3A and 3B are fed at a predetermined ratio, and the mixture is caused to precipitate. Thorough agitation yields a slurry of pH 8.0. This slurry is charged into a 20-liter autoclave and, with the addition of 500 g tetrapropylammonium bromide, the mixture is subjected to hydrothermal synthesis at 160°C for 72 hours. The synthesis is followed by water rinsing, drying, and baking at 500°C for 3 hours to obtain crystalline silicate 3A. This crystalline silicate is represented, in terms of the molar ratio of the oxides (excluding crystal water), by the compositional formula: 0.5Na 2 O·0.5H 2 O·[0.8Al 2 O 3 ·0.2Fe 2 O 3 ]·25SiO 2 and has a crystal structure according to an X-ray diffraction analysis as shown in Table 1 above. Synthesis of laminar composite crystalline silicate A One thousand grams of mother crystal A (crystalline silicate A) in a finely powdered state is added to 2160 g of water. With the further addition of 4590 g of colloidal silica (contg. 20% SiO 2 ), the mixture is thoroughly stirred, and the resulting solution is designated solution 3a. In the meantime 105.8 g of sodium hydroxide is dissolved in 2008 g of water to prepare solution 3b. While solution 3a is kept under agitation, solution 3b is slowly added drop-wise into solution 3a to form a precipitate and obtain a slurry. This slurry is placed in an autoclave and a solution of 568 g tetrapropylammonium bromide in 2106 g of water is added. Inside the autoclave hydrothermal synthesis is carried out at 160°C for 72 hours (with stirring at 200 rpm). After the stirring, the synthesis product is rinsed, dried, and baked at 500°C for 3 hours to yield laminar composite crystalline silicate 3A. Preparation of catalysts Laminar composite crystalline silicate 3A was subjected to copper ion exchange by immersion in a 0.04M aqueous copper acetate solution at 30°C with stirring. After 24-hour stirring, it was rinsed, and this cycle was repeated to conduct the copper ion exchange with the aqueous solution three times. Final rinsing and drying gave powder catalyst 3a. To 100 parts of powder catalyst 3a added as binders were 3 parts of alumina sol, 55 parts of silica sol (contg. 20% SiO 2 ), and 200 parts of water. The mixture on thorough stirring afforded a slurry for wash coating. A monolithic substrate (with a 400-cell lattice pattern) for cordierite was dipped in the slurry and taken out, excess slurry was blown away, and then dried at 200°C. The coating so supported weighed 200 g per liter of the substrate, and this coated matter is designated as honeycomb catalyst 3A. Example 2 Mother crystals 3B to 3L were prepared by repeating the procedure for the synthesis of the mother crystal 3A in Example 1 with the exception that the ferric chloride was replaced by the chlorides of cobalt, ruthenium, rhodium, lanthanum, cerium, titanium, vanadium, chromium, antimony, gallium, or niobium, respectively, each in the same molar amount as Fe 2 O 3 in terms of equivalency as oxide. The compositions of these mother crystals are represented, as the molar ratio of the oxides (in the dehydrated state), by the formula: 0.5Na 2 O·0.5H 2 O·(0.2M 2 O 3 ·0.8Al 2 O 3 )·25SiO 2, wherein M is Co, Ru, Rh, La, Ce, Ti, V, Cr, Sb, Ga, or Nb, constituting mother crystals 3B to 3L, respectively. In the same manner as with mother crystal 3A but without the addition of the ferric chloride, mother crystal 3M was obtained. These mother crystals 3B to 3M were finely powdered and used in place of mother crystal 3A in the synthesis of the laminar crystalline silicate in Example 1. Repeated runs of hydrothermal synthesis using the autoclave gave laminar composite crystalline silicates 3B to 3M. Using these laminar crystalline silicates 3B to 3M, the procedure of Example 1 for the preparation of a catalyst was repeated, and powder catalysts 3b to 3m were obtained. These powder catalysts were used to coat monolithic substrates of cordierite, in the same way as in the preparation of catalyst in Example 1, and honeycomb catalysts 3B to 3M were obtained. Example 3 Divided portions of laminar composite crystalline silicate 3A 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 at 60°C for ion exchange with the respective metals. Then, in the same way as described in Example 1, powder catalysts 3n to 3r were prepared. These powder catalysts were supported by monolithic substrates in the manner described in Example 1, and honeycomb catalysts 3N to 3R were obtained. Example 4 Laminar composite crystalline silicate A prepared in Example 1 was immersed in a 4N aqueous NH 4 Cl solution at 80°C and was stirred for 24 hours for NH 4 ion exchange. Following the ion exchange, the product was rinsed, dried at 100°C for 24 hours, and baked at 500°C for 3 hours to give H type laminar crystalline silicate 3A. With the addition of a binder, this silicate 3A was applied to a cordierite type monolithic substrate as a coating. Divided portions of the coated monolithic substrate were separately immersed for impregnation over a one-hour period in a cuprous chloride solution 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 solution (64.4 g in 200 cm 3 of water), aqueous nickel chloride solution (64.4 g in 200 cm 3 of water), aqueous solution mixture of cupric chloride and cobalt chloride (23.1 g CuCl 2 ·2H 2 O and 32.2 g CoCl 2 ·6H 2 O in 200 cm 3 of water), and aqueous solution mixture of nickel chloride and cobalt chloride (32.2 g NiCl 2 ·2H 2 O and 32.2g 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 dried at 200°C. A 12-hour purging in a nitrogen atmosphere at 500°C gave honeycomb catalysts 3S to 3X. Comparative Example 1 Mother crystal 3M obtained in Example 2 was directly subjected to Cu ion exchange in the same manner as with the preparation of catalyst in Example 1 to obtain powder catalyst 3m'. Likewise a monolithic substrate was coated with it to yield honeycomb catalyst 3Y. Experiment 1 Honeycomb catalysts 3A to 3Y prepared in Examples 1 to 4 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 ; and catalyst shape = 15 mm x 15 mm x 60 mm (144 cells). The denitration rates of the catalysts in the initial state at reaction temperatures of 350°C and 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 3A to 3Y were evaluated for their activities under the same conditions as used in Experiment 1. The results are also shown in Table 3. As Table 3 indicates, honeycomb catalysts 3A to 3X prepared using laminar composite crystalline silicates had high denitration activities after the forced deterioration as well as in the initial state, proving that they were durable. The table also shows that the ordinary silicate catalyst (honeycomb catalyst 3Y) lost much of its activity after the forced deterioration, revealing that it was less durable. With regard to the catalytic activities for the removal of CO and hydrocarbons, honeycomb catalysts 3A to 3X according to the first aspect of the invention underwent little changes in activity from the initial stage till after the forced deterioration. They showed 100% removal rates at both 350°C and 450°C. Comparative honeycomb catalyst 3Y also had a CO and hydrocarbon purification activity of 100% initially at both 350°C and 450°C. After the forced deterioration, however, the CO removal rates of comparative honeycomb catalyst 3Y at 350°C and 450°C declined to 35% and 75%, respectively, and the hydrocarbon removal rates at those temperatures also dropped to 45% and 90%. The second aspect of the present invention will now be described in detail by way of Examples 5 to 8 and Comparative Example 2. Example 5 Synthesis of mother crystal 4A A solution of 5616 g water glass #1 (contg. 30% SiO 2 ) in 5429 g of water is prepared and referred to as solution 4A. Meanwhile 718.9 g of aluminum sulfate, 110 g of ferric chloride, 47.2 g of calcium acetate, 262 g of sodium chloride, and 2020 g of concentrated hydrochloric acid are dissolved in 4175 g of water, and the solution is designated solution 4B. Solutions 4A and 4B are fed at a predetermined ratio, and the mixture is caused to precipitate. Thorough agitation yields a slurry of pH 8.0. This slurry is charged into a 20-liter autoclave and, with the addition of 500 g of tetrapropylammonium bromide, the mixture is subjected to hydrothermal synthesis at 160°C for 72 hours. The synthesis is followed by water rinsing, drying, and baking at 500°C for 3 hours to obtain crystalline silicate 4A. This crystalline silicate is represented, in terms of the molar ratio of the oxides (excluding crystal water), by the compositional formula: 0.5Na 2 O·0.5H 2 O·[0.8Al 2 O 3 ·0.2Fe 2 O 3 ·0.25CaO]·25SiO 2 and has a crystal structure according to an X-ray diffraction analysis as shown in Table 1 above. Synthesis of laminar composite crystalline silicate 4A One thousand grams of the mother crystal 4A (crystalline silicate 4A) in a finely powdered state is added to 2160 g of water. With the further addition of 4590 g of colloidal silica (contg. 20% SiO 2 ), the mixture is thoroughly stirred, and the resulting solution is designated as solution 4a. In the meantime 105.8 g of sodium hydroxide is dissolved in 2008 g of water to prepare solution 4b. While solution 4a is kept under agitation, solution 4b is slowly added drop-wise into solution 4a to form a precipitate and obtain a slurry. This slurry is placed in an autoclave and a solution of 568 g of tetrapropylammonium bromide in 2106 g of water is added. Inside the autoclave hydrothermal synthesis is carried out at 160°C for 72 hours (with stirring at 200 rpm). After the stirring, the synthesis product is rinsed, dried, and baked at 500°C for 3 hours to yield laminar composite crystalline silicate 4A. Preparation of catalysts Laminar composite crystalline silicate 4A was subjected to copper ion exchange by immersion in a 0.04M aqueous copper acetate solution at 30°C with stirring. After 24-hour stirring, it was rinsed, and this cycle was repeated to conduct the copper ion exchange with the aqueous solution Three times. Final rinsing and drying gave powder catalyst 4a. To 100 parts of powder catalyst 4a added as binders were 3 parts of alumina sol, 55 parts silica sol (contg. 20% SiO 2 ), and 200 parts water. The mixture upon thorough stirring afforded a slurry for wash coating. A monolithic substrate (with a 400-cell lattice pattern) for cordierite was dipped in the slurry and taken out, excess slurry was blown away, and then dried at 200°C. The coating so supported weighed 200 g per liter of the substrate, and this coated matter is designated as honeycomb catalyst 4A. Example 6 Mother crystals 4B to 4L were prepared by repeating the procedure for the synthesis of mother crystal 4A in Example 5 with the exception that the ferric chloride was replaced by the chlorides of cobalt, ruthenium, rhodium, lanthanum, cerium, titanium, vanadium, chromium, antimony, gallium, and niobium, respectively, each in the same molar amount as Fe 2 O 3 in terms of equivalency as oxide. The compositions of these mother crystals are represented, as the molar ratio of the oxides (in the dehydrated state), by the formula: 0.5Na 2 O·0.5H 2 O·[0.2M 2 O 3 ·0.8Al 2 O 3 ·0.25CaO]·25SiO 2 wherein M is Co, Ru, Rh, La, Ce, Ti, V, Cr, Sb, Ga, or Nb, constituting mother crystals 4B to 4L, respectively. In the same manner as with mother crystal 4A but without the addition of the ferric chloride and calcium acetate, mother crystal 4M was obtained. These mother crystals 4B to 4M were finely powdered and used in place of mother crystal 4A in the synthesis of the laminar crystalline silicate in Example 5. Repeated runs of hydrothermal synthesis using the autoclave gave laminar composite crystalline silicates 4B to 4M. Using these laminar crystalline silicates 4B to 4M, the procedure of Example 5 for the preparation of a catalyst was repeated, and powder catalysts 4b to 4m were obtained. These powder catalysts were used to coat monolithic substrates of cordierite, in the same way as in the preparation of catalyst in Example 5, and honeycomb catalysts 4B to 4M were obtained, listed in Table 4. Example 7 In the synthesis of mother crystal 4A in Example 5 calcium acetate was replaced by magnesium acetate, strontium acetate, or barium acetate, each in the same molar amount as CaO in terms of equivalency as oxide, and otherwise the same procedure with the mother crystal 4A was repeated to prepare mother crystals 4N to 4P. The compositions of these mother crystals, in terms of the molar ratio of the oxides (in a dehydrated state) were represented by: 0.5Na 2 O·0.5H 2 O·(0.2Fe 2 O 3 ·0.8Al 2 O 3 ·0.25MeO)·25SiO 2 wherein Me is Mg, Sr, or Ba. These mother crystals were finely ground and were subjected to hydrothermal synthesis using the autoclave in the same manner as with the synthesis of the laminar crystalline silicates in Example 5 to obtain laminar crystalline silicates 4N to 4P. From these silicates 4N to 4P, powder catalysts 4n to 4p and then honeycomb catalysts 4N to 4P were obtained by following the procedure of Example 5. Example 8 Divided portions of laminar composite crystalline silicate 4A obtained in Example 5 were immersed with agitation in 0.04M aqueous solutions of cupric chloride, cobalt chloride, nickel chloride, ferric chloride, and silver nitrate at 60°C for ion exchange with the respective metals. Then, in the same way as described in Example 5, powder catalysts 4q to 4u were prepared. These powder catalysts were supported by monolithic substrates in the manner described in Example 5, and honeycomb catalysts 4Q to 4U were obtained. Example 9 The laminar crystalline silicate 4A prepared in Example 5 was immersed in a 4N aqueous NH 4 Cl solution at 80°C and was stirred for 24 hours for NH 4 ion exchange. Following the ion exchange, the product was rinsed, dried at 100°C for 24 hours, and baked at 500°C for 3 hours to give H type laminar crystalline silicate 4A'. With the addition of binder, this silicate 4A' was applied to a cordierite type monolithic substrate as 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 solution (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 CuCl 2 ·2H 2 O and 32.2 g CoCl 2 ·6H 2 O in 200 cm 3 of water), and aqueous solution mixture of nickel chloride and cobalt chloride (32.2 g NiCl 2 ·2H 2 O and 32.2g 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 dried at 200°C. A 12-hour purging in a nitrogen atmosphere at 500°C gave honeycomb catalysts 4V to 4α. Comparative Example 2 Mother crystal 4M obtained in Example 6 was directly subjected to Cu ion exchange in the same manner as with the preparation of catalyst in Example 5 to obtain powder catalyst 4m'. Likewise a monolithic substrate was coated with it to yield honeycomb catalyst 4β. Experiment 3 Honeycomb catalysts 4A to 4β prepared in Examples 5 to 9 and in Comparative Example 2 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). The denitration rates of the catalysts in the initial state at reaction temperatures of 350°C and 450°C are given in Table 5. Experiment 4 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 4A to 4β were evaluated for their activities under the same conditions as used in Experiment 3. The results are also shown in Table 5. As Table 5 indicates, the honeycomb catalysts 4A to 4α prepared using laminar composite crystalline silicates according to the fourth aspect of the invention had high denitration activities after the forced deterioration as well as in the initial state, proving that they were durable. The table also shows that the ordinary silicate catalyst (honeycomb catalyst 4β) lost much of its activity after the forced deterioration, revealing that it was less durable. Honey comb catalyst No. Powder catalyst Laminar composite crystalline silicate No. Qty of active substance supported (mmol/g) No. Composition* 4A 4a Cu(0.45) 4A S·L/0.5Na 2 O·0.5H 2 O(0.2Fe 2 O 3 ·0.8Al 2 O 3 ·0.25CaO) 4B 4b Cu(0.40) 4B S·L/0.5Na 2 O·0.5H 2 O(0.2Co 2 O 3 ·0.8Al 2 O 3 .0.25CaO) 4C 4c Cu(0.35) 4C S·L/0.4Na 2 O·0.5H 2 O(0.2Ru 2 O 3 ·0.8Al 2 O 3 ·0.25CaO) 4D 4d Cu(0.40) 4D S·L/0.5Na 2 O·0.4H 2 O(0.2Rh 2 O 3 ·0.8Al 2 O 3 ·0.25CaO) 4E 4e Cu(0.45) 4E S·L/0.5Na 2 O·0.5H 2 O(0.2La 2 O 3 ·0.8Al 2 O 3 ·0.25CaO) 4F 4f Cu(0.40) 4F S·L/0.6Na 2 O·0.4H 2 O(0.2Ce 2 O 3 ·0.8Al 2 O 3 ·0.25CaO) 4G 4g Cu(0.45) 4G S·L/0.5Na 2 O·0.6H 2 O(0.2Ti 2 O 3 ·0.8Al 2 O 3 ·0.25CaO) 4H 4h Cu(0.35) 4H S·L/0.4Na 2 O·0.6H 2 O(0.2V 2 O 3 ·0.8Al 2 O 3 ·0.25CaO) 4I 4i Cu(0.40) 4I S·L/0.5Na 2 O·0.6H 2 O(0.2Cr 2 O 3 ·0.8Al 2 O 3 ·0.25CaO) 4J 4j Cu(0.45) 4J S·L/0.6Na 2 O·0.4H 2 O(0.2Sb 2 O 3 ·0.8Al 2 O 3 ·0.25CaO) 4K 4k Cu(0.40) 4K S·L/0.6Na 2 O·0.5H 2 O(0.2Ga 2 O 3 ·0.8Al 2 O 3 ·0.25CaO) 4L 4l Cu(0.50) 4L S·L/0.5Na 2 O·0.5H 2 O(0.2Nb 2 O 3 ·0.8Al 2 O 3 ·0.25CaO) 4M 4m Cu(0.40) 4M S·L/0.4Na 2 O·0.4H 2 O·0.8Al 2 O 3 4N 4n Cu(0.45) 4N S·L/0.5Na 2 O·0.5H 2 O(0.2Fe 2 O 3 ·0.8Al 2 O 3 ·0.25MgO) 4O 4o Cu(0.45) 4O S·L/0.5Na 2 O·0.5H 2 O(0.2Fe 2 O 3 ·0.8Al 2 O 3 ·0.25SrO) 4P 4p Cu(0.45) 4P S·L/0.5Na 2 O·0.5H 2 O(0.2Fe 2 O 3 ·0.8Al 2 O 3 ·0.25BaO) 4Q 4q Cu(0.40) 4A S·L/0.5Na 2 O·0.5H 2 O(0.2Fe 2 O 3 ·0.8Al 2 O 3 ·0.25CaO) 4R 4r Co(0.35) 4A " 4S 4s Ni(0.30) 4A " 4T 4t Fe(0.30) 4A " 4U 4u Ag(0.70) 4A " 4V Cu(0.80) A'(H type) S·L/H 2 O·(0.2Fe 2 O 3 ·0.8Al 2 O 3 ·0.25CaO) 4W Cu(0.80) A'( " ) " 4X Co(0.80) A'( " ) " 4Y Ni(0.40) A'( " ) " 4Z Cu(0.40) A'( " ) " Co(0.40) 4α Ni(0.40) A'( " ) " Co(0.40) 4β 4m' Cu(0.40) 0.4Na 2 O·0.8Al 2 O 3 (without S·L) In the composition, the symbol S·L stands for silicalite, and 25SiO 2 is omitted because it is common to all the compositions. Honeycomb catalyst No. Powder catalyst Initial (ηNOx%) activity Activity after forced (ηNOx%) deterioration No. Qty of active substance supported (mmol/g) 350°C 450°C 350°C 450°C 4A 4a Cu(0.45) 40 40 39 36 4B 4b Cu(0.40) 39 39 37 34 4C 4c Cu(0.35) 41 41 38 36 4D 4d Cu(0.40) 38 38 38 34 4E 4e Cu(0.45) 38 41 37 38 4F 4f Cu(0.40) 37 40 35 36 4G 4g Cu(0.45) 38 42 35 35 4H 4h Cu(0.35) 38 38 36 34 4I 4i Cu(0.40) 38 35 35 34 4J 4j Cu(0.45) 37 40 35 32 4K 4k Cu(0.40) 37 39 37 34 4L 4l Cu(0.50) 38 37 37 36 4M 4m Cu(0.40) 39 40 39 37 4N 4n Cu(0.45) 39 36 38 36 4O 4o Cu(0.45) 40 37 36 35 4P 4p Cu(0.45) 41 40 37 34 4Q 4q Cu(0.40) 39 36 35 33 4R 4r Cu(0.35) 39 40 36 36 4S 4s Ni(0.30) 36 39 37 36 4T 4t Fe(0.30) 36 40 37 36 4U 4u Ag(0.70) 36 39 35 33 4V - Cu(0.80) 36 36 35 33 4W - Cu(0.80) 36 38 33 34 4X - Co(0.80) 35 38 33 33 4Y - Ni(0.40) 36 39 31 32 4Z - Cu(0.40) 36 36 33 31 Co(0.40) 4α - Ni(0.40) 35 38 35 31 Co(0.40) 4β 4m' Cu(0.40) 39 42 6 12

Claims: A catalyst for exhaust gas purification comprising a laminar composite crystalline silicate having the following X-ray diffraction pattern, 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; said laminar composite crystalline silicate being formed by growing a crystalline silicate made from Si and O over a mother crystal of a crystalline silicate synthesized in advance, said mother crystal being represented, in a dehydrated state and in terms of the molar ratio of oxides, by either the chemical formula: (1±0.6)R 2 O·[aM 2 O 3 ·bAl 2 O 3 ]·ySiO 2 , or by the chemical formula: (1±0.6)R 2 O·[aM 2 O 3 ·bAl 2 O 3 ]·cMeO·ySiO 2 , respectively, wherein R is an alkali metal ion and/or hydrogen ion; M is an ion of at least one element selected from the group consisting of Group VIII elements, rare earth elements, titanium, vanadium, chromium, niobium, antimony, and gallium; Me is an alkaline earth metal; a ≥ 0, b ≥ 0, a+b = 1, and y > 12 or y/c > 12, respectively; M and Me are contained in the Si-O-Al structure of the mother crystal; said mother crystal having the same X-ray diffraction pattern as the laminar composite crystalline silicate, said crystalline silicate grown over said mother crystal having the same crystalline structure as said mother crystal, and said laminar composite crystalline silicate containing at least one metal chosen from Groups Ib and VIII of the periodic table, said metal being supported onto the silicate by ion exchange or by impregnation. A method for purifying exhaust gases containing nitrogen oxides, hydrocarbons, and carbon monoxide, comprising the use of the catalyst as claimed in claim 1.

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: B01D53, B01J29