Process for synthesis of high-silica silicate molecular sieve

Abstract: A process for synthesizing a high-silica aluminosilicate molecular sieve, includes: subjecting, to hydrothermal synthesis, a raw material containing Al, H, O and Si and, as other elements, at least Fe and having a Si/Al molar ratio of 50 or more and a Si/Fe molar ratio of 80 or less (45 or less when a high-silica aluminosilicate molecular sieve having a β type zeolite structure is produced) and then subjecting the resulting material to heat treatment in an oxidizing atmosphere at 300°C or more. The above process can synthesize a high-silica zeolite low in Al content directly without dealumination operation, and can provide process simplification and a higher yield.

Description: Background of the Invention (1) Field of the Invention The present invention relates to a process for synthesizing a high-silica aluminosilicate molecular sieve which can be preferably used as a hydrocarbon adsorbent of exhaust gas purification system. (2) Description of Related Art In order for an exhaust gas purification system for an automobile or the like to be able to exhibit its catalytic activity, the catalyst used therein must be heated to at least its lowest operating temperature by, for example, the heat of exhaust gas. Therefore, when the temperature of exhaust gas is low as seen in the cold start of engine, the harmful components (e.g. HC, CO and NOx) of exhaust gas are hardly purified. HC, in particular, is generated in a large amount during the cold start and its purification is an important task. In order to improve the efficiency of HC purification during the cold start, there have heretofore been known techniques of using, as a HC adsorbent, a molecular sieve made of a crystalline aluminosilicate (e.g. zeolite) and allowing the adsorbent to adsorb HC until the exhaust gas purification system used reaches its operating temperature. For example, Japanese Patent Application Laid-Open No. 75327/1990 discloses an exhaust gas purification system for an automobile, using a Y type zeolite or mordenite as an HC adsorbent. Japanese Patent Application Laid-Open No. 293519/-1992 proposes use of an adsorbent obtained by subjecting a H + type ZSM-5 zeolite to ion exchange with Cu and Pd, in order to alleviate the effect of water adsorption, improve HC adsorbability and widen the temperature range of adsorption. Japanese Patent Application Laid-Open No. 63392/1994 proposes, for the same purpose, use of an H-, Cu- or Pd-ion exchanged pentasil type metallosilicate as an adsorbent. With respect to the above pentasil (pentasil is a generic name for structures similar to ZSM-5) type zeolite, a relatively high-silica product can be synthesized easily and even a product having a Si/Al molar ratio of substantially infinity (containing no Al) can be synthesized by using a template. However, direct synthesis of pentasil type zeolite of high Si/Al molar ratio (>50) by using no template has heretofore been difficult. Meanwhile, β type zeolite and mordenite type zeolite are known to have large lattice strain when containing no Al, as compared with pentasil type zeolite. Therefore, in synthesizing β type zeolite and mordenite type zeolite, it has been necessary to add Al (which has an ionic radius different from that of Si) in a certain amount to alleviate said lattice strain. As a result, the β type zeolite and mordenite type zeolite synthesized had a Si/Al molar ratio of about 50 or less . EP-A-64328 discloses a method of preparing zeolites, partly including β-zeolite, using starting material containing small amount of Fe, and heat-treating after synthesis. EP-A-891809 also describes manufacture of β-zeolite containing Fe by hydrothermal synthesis followed by water-washing, drying, calcinations, ion exchange, drying and calcination. While zeolites differ in heat resistance and hydrothermal resistance depending upon the crystal structures, among zeolites of the same crystal structure, a zeolite of higher Al content has lower heat resistance and lower hydrothermal resistance. Therefore, in order to allow a zeolite to have higher heat resistance and higher hydrothermal resistance, it is necessary to reduce the Al content in zeolite. However, there has been no assurance that a zeolite having a pore structure suited for HC molecules to be adsorbed can be synthesized directly so as to have a low Al content at which desired heat resistance can be obtained. Hence, when the Al content is higher than desired, it has been necessary to remove excessive Al (dealumination operation) by steam treatment, acid treatment or the like and repeat the operation until a desired Al content is obtained. Summary of the Invention The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a process which enables production of high-silica silicate molecular sieve by directly synthesizing a low-Al content zeolite without a dealumination operation, which process can provide process simplification and a higher yield. The high-silica aluminosilicate molecular sieve synthesized by the present process can be preferably used in applications where high heat resistance or high hydrothermal resistance is required, for example, the HC adsorbent used in the exhaust gas purification system (e.g. in-line type exhaust gas purification system) of an internal combustion engine. According to the present invention there can be provided a process for directly synthesizing a high-silica aluminosilicate molecular sieve as described by claim 1. In this specification, including the claims, the terms Si/Al molar ratio and Si/Fe molar ratio means the ratio of the atoms of Si and Al, or Si and Fe, i.e. are the Si/Al atomic ratio and Si/Fe atomic ratio respectively. In the present process, it is preferable that the raw material contains a Na + source and the Na + source contains NaOH; and it is also preferable that the Na + source contains NaOH and NaCl and the NaOH:NaCl molar ratio is 3:4 to 6:1. In the present process, it is preferable to conduct the heat treatment in an oxidizing atmosphere at 650-1,050°C. Detailed Description of the Invention The process for synthesizing a high-silica silicate molecular sieve according to the present invention comprises subjecting, to hydrothermal synthesis, a raw material containing Al, H, O and Si and, as other elements, at least Fe and having a Si/Al molar ratio of 50 or more and a Si/Fe molar ratio of 80 or less and then subjecting the resulting material to heat treatment in an oxidizing atmosphere at 300°C or more, preferably 650-1,050°C. Herein, "hydrothermal synthesis" refers to an operation in which a raw material is heated to room temperature or higher in the presence of water or steam. It is not restricted to an operation in which an ordinary aqueous gel is heated in an autoclave. The raw material may take various forms such as liquid, gas, solid, solution, colloid and the like. The pressure employed may be atmospheric pressure or reduced pressure as in chemical vapor deposition. Zeolite refers to an inorganic molecular sieve having a regular pore structure, made of aluminosilicate, in particular. Part or the whole of the Al atoms present in the zeolite framework may be substituted by particular elements. In the present invention, a Fe compound or Fe itself is added to a raw material to synthesize a Fe-containing zeolite. In this way, there can be synthesized a high-silica aluminosilicate molecular sieve which is a zeolite of relatively uniform composition. In zeolite synthesis, since optimum synthesis conditions need be employed depending upon the addition amount of Fe compound or Fe itself and the intended pore structure of zeolite to be synthesized, it is preferable to set up the synthesis conditions for said zeolite beforehand by using at least one raw material composition. It is thought that by including Fe in the crystal structure of zeolite, the strain between crystal lattices is alleviated and crystallization of zeolite becomes easy; as a result, a zeolite of lower Al content than usual can be synthesized directly with relative ease. Thereby, the control of Al content in zeolite becomes easy and a molecular sieve having properties not seen heretofore can be synthesized. Further in the present invention, in-depth study was made to examine the effects of Si/Al molar ratio, Si/Fe molar ratio and Na compound (kind and amount) of raw material, on the specific surface area, hydrothermal resistance, etc. of aluminosilicate molecular sieve synthesized; and the levels of Si/Al molar ratio and Si/Fe molar ratio and the kind and amount of Na compound were specified. In order for the high-silica aluminosilicate molecular sieve produced by the present process to have a hydrothermal resistance required for the HC adsorbent used for purification of exhaust gas from internal combustion engine (the HC adsorbent is a primary use of the molecular sieve), it is preferable to use, in the present process, a raw material having a Si/Al molar ratio of 50 or more and a Si/Fe molar ratio of 80 or less. Since a lower Al content generally gives a higher hydrothermal resistance, the Si/Al molar ratio of raw material is preferably 50 or more and yet as large as possible. In many zeolites such as β type and the like, the presence of Fe in a certain amount or more is thought to be necessary to alleviate the lattice strain during crystallization due to the low content of Al and produce an intended pore structure. Fe shortage invites non-crystallization of corresponding amount of Si and a reduced yield. Hence, the Si/Fe molar ratio of raw material is preferably 80 or less. When the pore structure is, in particular, β type, the Si/Fe molar ratio is preferably 45 or less, more preferably 30 or less, particularly preferably 20 or less. Preferably, the raw material further contains a Na + source. The Na + source preferably contains NaOH, or NaOH and NaCl. In the latter case, the NaOH:NaCl molar ratio is preferably 3:4 to 6:1, more preferably 4:3 to 6:1. The merits and effects brought about by the presence of Fe in silicate molecular sieve are as follows. The Fe used in the invention usually has a valency of 3 which gives an oxide most stable at room temperature and which is the same as that of Al. This Fe has an ionic radius relatively close to that of Al and can substitute Al easily. Nevertheless, Fe, as compared with Al, is easily eliminated from the crystal lattices of aluminosilicate molecular sieve by calcination or ion-exchange operation. Even when Fe goes out of framework, however, Fe does not seem to promote the destruction of zeolite structure, differently from Al. Rather, the Fe out of the zeolite framework is presumed to stay in the pores and function as an adhesive, thereby suppressing the destruction of crystal structure. It is further presumed that the Fe staying in the pores makes a solid solution with eliminated Al, thereby preventing the movement of Al and the destruction of crystal structure. For the above reasons, in the present process, heat treatment is conducted in an oxidizing atmosphere at 300°C or higher, preferably 650-1,050°C so that Fe goes out of the zeolite framework, whereby a high-silica aluminosilicate molecular sieve can be produced. Further, a previous heat treatment in a temperature range in which a product is actually used can contribute to stabilization of a crystal structure of a high-silica aluminosilicate molecular sieve. Thus, the present process for synthesis of a high-silica aluminosilicate molecular sieve can directly synthesize a zeolite low in Al content without dealumination operation and therefore can provide process simplification and a higher yield. Further, the high-silica aluminosilicate molecular sieve synthesized by the present process can maintain its crystal structure up to at least a high temperature to which a zeolite (aluminosilicate) having about the same Si/Al molar ratio is stable. Therefore, the present high-silica aluminosilicate molecular sieve can be preferably used in applications where high heat resistance or high hydrothermal resistance is required, for example, the HC adsorbent used in the exhaust gas purification system (e.g. in-line type exhaust gas purification system) of internal combustion engine. The preferable pore structure of the high-silica aluminosilicate molecular sieve synthesized by the present process is determined in view of the following matters when the molecular sieve is used particularly as an HC adsorbent for purification of exhaust gas from internal combustion engine. Incidentally, in this specification, types of zeolite are indicated by typical names, which includes corresponding isomorphisms or the like. First, the pore structure preferably has a high framework density [in the case of zeolites, the framework density is represented by the number of T atoms (the atoms surrounded by oxygen atoms and forming a TO 4 type tetrahedron) in 1 nm 3 ]. For example, pentasil type zeolite and ferrierite type zeolite have high framework densities for the pore diameters. When a molecular sieve is used for HC adsorption, the pore diameter of the molecular sieve is required to have a size through which the HC molecules to be adsorbed can enter the molecular sieve. The size, however, is preferably close to the size of the HC molecules to be adsorbed because it allows strong adsorption of HC molecules by molecular sieve. β type zeolite and mordenite type zeolite have pores larger than those of pentasil type zeolite and can adsorb even HC components of larger molecular sizes. β type zeolite is known to have excellent adsorption capacity for C 6-8 aromatic hydrocarbons. Chabazite type and sigma-1 type having small pore diameters are suitable for adsorption for C 2-3 alkenes. Coke formed from HC may deposit in the pores of molecular sieve. When the pores are linear and have no branches and when the two locations of one-dimensional pore are plugged by coke, the portion of the pore between the two plugged portions has no adsorbability. Therefore, the molecular sieve of the present invention preferably has a two-dimensional or three-dimensional pore structure. All of the above-mentioned pentasil type, ferrierite type, β type, mordenite type, chabazite type and sigma-1 type have a two-dimensional or higher pore structure. β type is particularly preferable because it can adsorb even larger HC components than pentasil type can, has a large HC adsorption capacity, and can be produced so as to have sufficient heat resistance by the present process. The present invention is hereinafter described specifically by way of Examples. However, the present invention is not restricted to the Examples. Examples 1-3 and Comparative Examples 1-4 [1. Synthesis of high-silica β type aluminosilicate molecular sieves and evaluation of heat resistances thereof] An aqueous silica gel containing Fe, Al and TEA cation [(CH 3 CH 2 ) 4 N + ] was treated in an autoclave at 135°C at an autogenous pressure for 6 days to give rise to crystallization. The resulting solids were filtered, washed with water, dried and subjected to heat treatment in air at 540°C for 3.5 hours to remove TEA and have Fe go out of the zeolite framework of aluminosilicate molecular sieve to obtain a high-silica aluminosilicate. Then, the aluminosilicate was subjected to ion exchange with NH 4 + ion and calcinated to remove Na + ion to prepare a Fe-containing β type silicate molecular sieve (Examples 1-3 and Comparative 1-2). In Comparative Example 3, synthesis was conducted using a raw material containing substantially no Fe and having a Si/Al molar ratio of 15. The results are shown in Table 1. Si/Fe molar ratio Si/Al molar ratio Yield (%) Example 1 45 150 40 Example 2 25 150 66 Example 3 18 150 88 Comparative Example 1 75 150 27 Comparative Example 2 22 48 89 Comparative Example 3 >1000 15 83 Then, each of the above-obtained molecular sieves was subjected to a durability test of 1,000°C and 4 hours in an atmosphere containing 10% of steam, in an electric furnace; and the specific surface areas (BET values) of molecular sieve before and after the test were measured; the value obtained by dividing the value after the test by the value before the test, i.e. the retention (%) of specific surface area was taken as the heat resistance of molecular sieve produced. Comparative Example 4 is a case in which the molecular sieve obtained in Comparative Example 3 was subjected to repeated steam treatment and acid washing for dealumination. The results are shown in Table 2. Si/Fe molar ratio Si/Al molar ratio Specific surface area Before test (m 2 /g) After test (m 2 /g) Retention (%) Example 1 19 62 576 351 60.9 Example 2 17 101 575 422 78.4 Example 3 16 136 578 500 86.5 Comparative Example 1 21 41 575 172 29.9 Comparative Example 2 20 45 576 251 43.6 Comparative Example 3 >1000 12 675 52 7.7 Comparative Example 4 >1000 59 574 285 49.7 Examples 4-11 [2. Synthesis of high-silica β type silicate molecular sieves and evaluation of heat resistances thereof] An aqueous NaOH solution was added to colloidal silica (SiO 2 : 30% by weight) solution to obtain a mixture. Thereto was added a solution obtained by adding an Iron nitrate nonahydrate [Fe(NO 3 ) 3 .9H 2 O] and aluminum nitrate nonahydrate [Al(NO 3 ) 3 .9H 2 O] to an aqueous solution containing 35% by weight of tetraethylammonium hydroxide (TEAOH). Thereto was added an aqueous sodium chloride (NaCl) solution. The resulting mixture was stirred until it became homogeneous, to prepare various mixtures each having a different NaOH:NaCl molar ratio as shown in Table 3. All the mixtures had the following composition although their NaOH:NaCl molar ratios were different. 21Na 2 O.Al 2 O 3 .6Fe 2 O 3 .300SiO 2 .150TEAOH.6200H 2 O As seen above, in all the mixtures, the Si/Al molar ratio was 150 and the Si/Fe molar ratio was 25. Each mixture was placed in a teflon container and heated in an autoclave at an autogenous pressure at 135°C for 6 days. The resulting solid reaction product was separated by the use of a centrifuge, washed, dried at 80°C and heat-treated in air at 540°C for 4 hours to remove the template used. The resulting powder was treated in an aqueous ammonium nitrate solution at 80°C for 1 hour to conduct ion exchange. The resulting material was separated, washed, dried and calcined to prepare various H + -type Fe-containing β type silicate molecular sieves. Then, each of the above-obtained molecular sieves was subjected to a durability test of 1,000°C and 4 hours in an atmosphere containing 10% of steam, in an electric furnace; and the specific surface areas (BET values) of molecular sieve before and after the test were measured; the value obtained by dividing the value after the test by the value before the test, i.e. the retention (%) of specific surface area was taken as the heat resistance of molecular sieve produced. The results are shown in Table 3. Molar ratio when fed Specific surface area NaOH NaCl Before test (m 2 /g) After test (m 2 /g) Retention (%) Example 4 7 0 633 420 66.4 Example 5 6 1 640 431 67.5 Example 6 5 2 636 417 65.6 Example 7 4 3 623 415 66.6 Example 8 3 4 503 341 67.8 Example 9 2 5 498 300 60.2 Example 10 1 6 480 273 56.9 Example 11 0 7 451 230 51.0 As described above, the process for synthesis of high-silica silicate molecular sieve according to the present invention can give a high-silica zeolite low in Al content directly without dealumination operation, and can provide process simplification and a higher yield. Since the above merits are based on the properties of Al and Fe in zeolite, the same merits can be obtained when zeolites other than β type are made into high-silica. The high-silica aluminosilicate molecular sieve produced according to the present process can be preferably used in applications where high heat resistance or high resistance to water and heat is required, for example, the HC adsorbent used in the exhaust gas purification system (e.g. in-line type exhaust gas purification system) of internal combustion engine.

Claims: A process for synthesizing a high-silica aluminosilicate molecular sieve having a β-zeolite structure, which comprises subjecting to hydrothermal synthesis a raw material containing Al, H, O and Si and, as another element or elements, at least Fe and having a Si/Al atomic ratio of 50 or more and a Si/Fe atomic ratio of 80 or less, subjecting the resulting material to heat treatment in an oxidizing atmosphere at 300°C or more, and thereafter subjecting the heat-treated material to ion exchange with NH 4 + ions. A process according to claim 1, wherein the raw material comprises NaOH and NaCl, the NaOH:NaCl molar ratio being in the range 3:4 to 6:1, or NaOH without NaCl. A process according to claim 1 or 2, wherein the heat treatment is conducted in an oxidizing atmosphere at 650 - 1050°C.

Inventors: HIRAMATSU TAKUYA, SUZUKI KENJI, TAKAHASHI AKIRA, YAMAMOTO AZUMA, HIRAMATSU, TAKUYA, SUZUKI, KENJI, TAKAHASHI, AKIRA, YAMAMOTO, AZUMA
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1998
Main classification: C01B39
IPC codes: B01D53, C01B39, B01J20