PREPARATION OF MONOLITHIC CATALYST SUPPORTS HAVING AN INTEGRATED HIGH SURFACE AREA PHASE

Abstract: A method of preparing a monolithic catalyst support having an integrated high surface area phase as provided. A plasticized batch of ceramic matrix material intimately mixed with high surface area powder is formed into the desired shape for the monolith and then heated to sinter the ceramic. The resulting monolith has a strong substrate of the ceramic matrix material and a high surface area phase provided by the high surface area powder extruded with the batch.

Description: This invention is directed to monolithic ceramic catalyst supports and particularly to supports which contain a high surface area phase incorporated within the ceramic matrix itself. The conventional ceramic monolithic catalyst consists of a ceramic support with a coating of high surface material upon which the catalyst is actually deposited. In particular, the ceramic support is normally prepared by sintering a mold of clay or other ceramic material at a high temperature to impart density and strength. This procedure normally results in a very small surface area, and consequently the ceramic must be coated with another material having a higher surface area, as well as specific chemical characteristics on which to actually deposit the catalyst. This procedure of depositing a high surface area "wash coat" on the low surface area ceramic wall is disclosed, for example, in U.S. Patent Nos. 2,742, 437 and 3,824,196. Catalyst supports of this kind suffer from several disadvantages. In service, the supports are exposed to a flow of gases which often contain dusts or particulate matter, which can cause the high surface area coating to flake off the underlying ceramic support. This phenomenon can also occur where the support is exposed to thermal cycling because the wash coat and the underlying ceramic material often have different thermal expansion coefficients. Furthermore, catalysts deposited on the high surface area wash coat are susceptible to poisoning, such as by lead or phosphorous in service in automobile converters, and therefore must be periodically regenerated or replaced. U.S. Patent 4,294,806 discloses the preparation of monolithic supports by extrusion of an alumina ceramic material into the shape of a honeycomb, calcining the material, and then sintering only the front portion. This procedure is said to make the support more abrasion resistant. However, the bulk of the support remains unsintered, so that even though it retains high surface area, the support would lack high strength. U.S. Patent 4,151,121 discloses the preparation of a catalyst by dispersing zeolite and a high surface area alumina (on which a catalytic metal is supported) in a hydrogel of a porous oxide matrix material (such as alumina, clay, silica-alumina and the like) to form a composite mixture. The composite is spray dried, washed free of salts, and then flash dried. This method produces catalyst materials in which the high surface material is embedded within a matrix, and thereby somewhat protected from abrasion or poisoning. However, the method is not suitable for the preparation of catalyst support structures that are in monolithic form, the kind normally used in the services where these problems are most prevalent or most severe. British Patent 1,064,018 discloses tubular catalyst support prepared by forming a paste of alpha-alumina, active alumina, and hydrargillite (a high surface area alumina trihydrate), extruding the paste to form tubular elements, and firing the elements. In JP-A-55111843 there is described a method of manufacturing a catalyst and a catalyst support. According to the disclosure a skeleton composition is made up of to 10-90 wt % of rehydratable alumina and 10-90 wt % cordierite and is formed into a shape. The total of the two components in the composition amounts to at least 50 wt %. After shaping the formed body is rehydrated and sintered at a temperature within the range of 1100-1350°C. The composition may contain as additional skeleton materials which will not damage the various properties which are to be achieved by the composition of the invention α-alumina, silica, aluminium hydrates, clay, talc, bentonite, silicide, zeolite, spodumene, titania, zirconia, silica sol, mullite and activated charcoal. These materials are, like cordierite, to form the structural portion of the skeleton material. The rehydratable alumina of the composition according to the invention is partially or completely coated with a rehydration prevention agent before it is contacted with water or water-containing material. This is to ensure that rehydration takes place only after the body has been formed. The catalyst and catalyst substrate according to the disclosure is said to have superior thermal resistance and compression strength as well as a high specific surface. The only reference to means for obtaining a high specific surface is the addition of organic crystalline cellulose or of synthetic resin, which presumably burn out on sintering. It is an object of the present invention to provide a monolithic support having a high surface area which is not easily abraded and which supports catalysts in a manner that resists poisoning. It is a further object of the invention to provide a monolithic support which has good mechanical properties while retaining the porosity and high surface area necessary for proper catalytic functioning. These and other objects are met by the invention to be described. Summary of the invention The present invention provides a method of preparing a monolithic ceramic support for a catalyst, which support has a high surface area phase intimately mixed with, and incorporated into, the ceramic material itself. The method comprises (a) providing a substantially homogeneous body comprising an admixture of (i) a first phase sinterable ceramic matrix material, in particulate form finer than 200 mesh, selected from cordierite, mullite, alpha-alumina, lithium aluminosilicate, mixtures of the foregoing materials, optional addition of aluminum titanate to the foregoing materials, fired chrome ore-TiO₂-MgO, fired R x O y -TiO₂-Chrome ore where R x O y is selected from Al₂O₃, Al₂O₃+Fe₂O₃, Al₂O₃+ZnO and Al₂O₃+Fe₂O₃+ZnO, and fired R x O y -TiO₂-MgO where R x O y is Al₂O₃, Al₂O₃+ZnO, Fe₂O₃, Fe₂O₃+ZnO and mixtures of these, and (ii) a second phase high surface area support material having a crystallite size no larger than 0.2 microns and a surface area of at least 40 m²/g selected from silica, spinel, titania, zeolite, zirconia, a mixture of these oxides, transition metal sulfide, a mixture of transition metal sulfides, and a mixture of such oxide(s) and transition metal sulfide(s); (b) forming the admixture into a shaped monolithic body; and (c) heating the shaped body at a temperature sufficient to sinter the first phase material and retain the second phase with a crystalline size no larger than about 0.5 micron. The monolithic support prepared in this manner comprises a ceramic matrix, as a first phase, sintered to a desirable level of strength, and a second high surface area phase well dispersed within the ceramic matrix on which to actually support catalyst. The catalyst support has a surface area of at least 5 m²/g. The second phase of the catalyst support has a crystallite size no larger than about 0.5 microns. It has been recognized that the ceramic, although sintered, is itself porous and that the high surface area material, even though within the walls of the ceramic, is accessible to the target gas stream and provides suitable surface area and extended catalyst life. The embedded high surface area material, upon which catalytically active materials are deposited, is protected from abrasion, and it is thought that the ceramic acts as a filter, by reaction or adsorption, to eliminate poisons before they can contact and adversely affect the catalyst itself. Another advantage of the monolithic supports of this invention, compared to those heretofore used, is the lower weight attributable to replacement of the denser ceramic material with the lighter high surface area phase and the elimination of the conventional washcoat. In those applications requiring the catalyst to be thermally activated and to function quickly, such as in automotive catalytic convertors, the reduced thermal mass in the present monolithic permits the "light off" temperature to be reached quickly. Detailed Description of the Invention In the method of the present invention, a sinterable, ceramic matrix material and a high surface area material are combined into a single plasticized batch which is formed into a desired shape for the monolithic support. In this manner, the high surface area phase is incorporated into the monolith itself, eliminating the heretofore required step of coating a pre-formed sintered ceramic, which itself normally has low porosity and surface area, with an additional high surface area substance on which catalyst is actually supported. Accordingly, the present invention provides a monolithic support having strength, due to the sintered ceramic phase, and available surface area, due to the embedded high surface area materials as a second phase. The high surface area materials suitable for use in the present invention are selected oxides and transition metal sulfides, generally in fine powder form, having a crystallite size of 0.2 microns or smaller and a surface area of at least 40 square metres per gram of weight (m²/g), preferably at least 100 m²/g, and most preferably at least 200 m²/g. This surface area may be present in the material naturally or may manifest itself after calcining. The practice of this invention contemplates either case. (As used herein, "calcining" means heating a material to a temperature below that at which the material begins to shrink or sinter). With respect to the oxides, they are silica, spinel, titania, zirconia, or zeolite. Mixtures of the oxides can also be used, as well as mixtures thereof with transition metal sulfide(s). Spinels useful in the present invention are the magnesium aluminate spinels heretofore used as catalyst supports, including spinel solid solutions in which magnesium is partially replaced by such other metals as manganese, cobalt, zirconium, or zinc. Preferred spinels are magnesium aluminate spinels having 1-7 percent by weight alumina in excess of 1:1 MgO.Al₂O₃ spinel; that is, those having about 72.0-73.5 weight percent Al₂O₃ (balance MgO). Spinels of this kind are available on order from Baikowski International Corporation of Charlotte, North Carolina, or can be prepared by co-precipitation or wet-mixing precursor powders of alumina and magnesia, followed by drying and calcining. Such a procedure is described in U.S. Patent 4,239,656, the disclosure of which is hereby incorporated by reference. As a supplement to this disclosure, however, it has been found that calcining of the spinels should normally not exceed 1300°C. for 2-2.5 hours. Calcining temperatures below 1200°C. are preferred. Suitable alumina precursor powders for preparation of the spinels are commercially available as Kaiser SA hydrated alumina or Conoco CATAPAL SB alumina (boehmite alpha-alumina monohydrate). Magnesium oxide component powders found to be suitable are magnesium hydroxide slurry, about 40 weight percent MgO, available from Dow Chemical Company, or hdyrated magnesium carbonate. High surface area silicas useful as the high surface area phase are the amorphous silicas of about 1-10 microns or sub-micron particle size such as CABOSIL EH-5 colloidal silica, available from Cabot Corporation. Silica precursors, such as an aqueous suspension of colloidal silicate, can also be used. High surface area titanias suitable for use are also commercially available, such as P25 TiO₂ available from DeGussa Corporation. Titania precursors such as hydrolyzed titanium isopropoxide can also be used. The use of zeolites to provide high surface area in various catalytic and molecular sieving operations is well known. Readily-available zeolites useful in the present invention include the crystalline aluminosilicate zeolites with the art-recognized designations A, X, and Y, and silicalite. Zeolites A, X, and Y, and their methods of preparation, are disclosed in U.S. Patents 2,882,243; 2,882,244; and 3,130,007; respectively. Disclosures of these patents is incorporated by reference. Silicalite is described in NATURE (271), No. 5645 (1978). Transition metal sulfides, such as cerium sulfide, nickel sulfide, iron sulfide, titanium sulfide, and chromium sulfide, or mixtures can be combined with cordierite, mullite, alpha-alumina, lithium aluminosilicates or mixtures. When the high surface area material a spinel, it is preferred to add up to about 20 percent by weight (based on the weight of spinel, of a rare earth oxide. The preferred rare earth oxides are those of the "cerium subgroup", that is, elements of atomic number 57-62, particularly cerium and lanthanum. Cerium oxide is most preferred. Particularly useful spinels, for example, are those in which about 1 to 20 percent by weight, based on the total spinel weight of cerium oxide is present. Cerium oxide is incorporated by adding, for example, cerium acetate, cerium carbonate, or cerium nitrate to the other precursor powders during the spinel preparation. The transition metal sulfides preferable for use in the present invention are cerium sulfide, nickel sulfide, iron sulfide, titanium sulfide, and chromium sulfide. Mixtures of these can also be used. The preferred high surface area materials are silica and the magnesium aluminate spinels The ceramic material, which forms the high-strength matrix phase of the monolith, is comprised of the well known sinterable materials capable of providing mechanical strength and good thermal properties in monolithic supports as heretofore prepared by those skilled in the art, and is selected from cordierite,mullite, alpha-alumina, and lithium aluminosilicates. Mixtures or these can also be used to the extent that the chosen materials are compatible and will not degrade each other, as those skilled in the art will recognize. The cordierite can be in the precursor or "raw" form, as in U.S. Patent 3,885,977, which becomes true cordierite upon heating, but it is preferably pre-reacted. The use of raw cordierite is disclosed in the U.S. Patent 3,885,977. When raw cordierite is used, it is preferred that up to 10% by total weight of B₂O₃ be added to the raw batch to promote the actual cordierite formation and to impart strength. The ceramic material can contain substantial amounts of certain components which causes intracrystalline and inter-crystalline microcracking to occur. Such microcracking enhances the thermal shock resistance of monolithic supports based on these ceramics and is therefore desirable when the monoliths, in service,may be exposed to rapid changes in temperature. Ceramic materials which contain such component, and are therefore contemplated for use within the present invention are disclosed in U.S. Patents 3,528,831; 3,549, 400; and 3,578,471; all issued to I.M. Lachman. A preferred microcracking agent for addition to the ceramic material is aluminum titanate, which is normally incorporated into the ceramic matrix as a "solid solution" with the basic ceramic material. An aluminum titanate solid solution with mullite is disclosed in U.S. Patent 4,483,944 to Day, et al. The disclosures of the four above-mentioned patents are incorporated herein by reference. The monolithic supports are prepared by mixing the sinterable ceramic materials with the high surface area materials described above and, optionally, a binder. Generally about 10-50 parts by weight of the high surface area material will be combined with 50-90 parts by weight of the ceramic material. Preferably, 1-30 parts by weight of binder will also be used. Any binder material conventially used in ceramic catalyst support manufacture is suitable. Examples are disclosed in: "Ceramics Processing Before Firing," ed. by George Y. Onoda, Jr. & L.L. Bench, John Wiley & Sons, New York "Study of Several Groups of Organic Binders Under Low-Pressure Extrusion," C.C. Treischel & E.W. Emrich, Jour. Am. Cer. Soc. , (29), pp. 129-132, 1946 "Organic (Temporary) Binders for Ceramic Systems," S. Levine, Ceramic Age , (75) No. 1, pp. 39+ January 1960 "Temporary Organic Binders for Ceramic Systems," S. Levine, Ceramic Age , (75) No. 2, pp 25+ February 1960 Preferred are methyl cellulose or a silicone resin. The silicone resins preferred for use are Dow Corning Corporation's Q6-2230 silicone resin or those described in U.S. Patent 3,090,691 to Weyer. The most preferred binder is methyl cellulose, available as Methocel R A4M from the Dow Chemical Company. It is preferred to use at least some methyl cellulose in addition to silicone resin as a binder. Up to about 1 percent by weight, based upon total mixture weight, of a surfactant, such as sodium stearate, can also be used to facilitate mixing and flow for subsequent processing. The mixing step should be performed in a liquid, such as water, which acts as a further plasticizer. When the binder is a silicone resin, it is preferred to use isopropyl alcohol in addition to water. Normally, the dry ingredients are first pre-mixed and then combined with the liquid plasticizer and any wet ingredients. The most preferred ceramic materials for use in this invention are the pre-reacted cordierite and mullite, including mullite with a microcracking agent. The ceramic material should be in particulate form, preferably of a size finer than 200 mesh (U.S. Standard) and most preferably finer than 325 mesh (U.S. Standard). With such characteristics, the ceramic material can normally be sintered at temperatures below those at which the surface area of the incorporated porous oxides or sulfides would be adversely affected. The monoliths are prepared by combining the components to form a homogeneous or substantially homogeneous mixture. Conventional mixing equipment can be used, but the use of a mix muller is preferred. To effect further mixing, the batch can subsequently be extruded through a "noodling" die one or more times. The noodling die can form, for example, ribbon-like or tubular shapes, or shapes having circular or polygonal cross-section. Ultimately, the batch is formed into the desired shape of the monolithic support, preferably by extrusion through a die, but another method, for example, is injection molding. The method of this invention is particularly well suited to the preparation of supports in the shape of, for example, thin-walled honeycombs and wagon-wheels. Finally, the shapes are heated to a temperature and for a time sufficient to sinter the ceramic material. Optionally, this heating/sintering step can be preceeded by drying the shapes at about 100°-120°C. The heating/sintering generally takes place at 700°C-1300°C, although when silicone resin is used as a binder for the ceramic matrix, particularly when the ceramic has a high alumina content, temperatures as low at 500°C may be sufficient. Temperatures below about 1100°C are preferred. When the high surface area support material is a zeolite, temperatures below 800°C are preferred. With the retention of high surface area by the embedded material, despite the temperatures used to sinter the ceramic, the monolithic support preferably has an overall surface area of at least 5-10 square meters per gram, more preferably at least 20 m²/g, and most preferably at least 40 m²/g. Although some sintering of the embedded material may take place, it is expected that the crystallite size of this material will grow no larger than about 0.5 microns. Crystallite size can be determined by scanning or transmission electron microscopy. The monolithic supports of this invention may have some catalytic activity of their own by virtue of the chemistry and structure of the high surface area phase. The support may further carry additional catalytically active ingredients dispersed throughout, but generally more concentrated at the high surface area sites provided by the embedded oxide and sulfide materials. These additional catalytic ingredients can be incorporated into the monolith by methods known in the art. Preferably, these ingredients will be deposited onto the high surface phase after fabricating and sintering the final structure. The monolithic supports of this invention are useful in most applications in which it is necessary to catalytically convert undesirable components in a gas stream prior to the stream's further processing or exhaustion to the atmosphere. The supports have good thermal shock resistance, particularly when the ceramic matrix phase is microcracked, and are therefore useful in applications in which they might be exposed to rapid and frequent changes in temperature. Capability to withstand thermal shock makes the supports of this invention particularly well suited for catalyzing the conversion of truck or automotive exhaust gasses to less noxious forms. The following examples are illustrative, but not limiting, of the invention. Example 1 A mixture of the following ingredients was prepared: 80 parts by weight of pre-reacted cordierite (particle size finer than 200 mesh), 20 parts by weight of CABOSIL fumed silica, 6 parts by weight methyl cellulose, 0.6 part sodium stearate. The mixture was dry blended by rolling overnight, after which it was charged to a mix muller and mulled with sufficient distilled water to produce a well-plasticized batch. The batch was extruded through a spaghetti die two times and then through a honeycomb die to form a shape having 31 square cells per square centimetre (200 square cells per square inch) with a wall thickness of 381 µm (15 mils). The honeycombs were steam dried and then fired in electrically heated furnaces, in air, at 50-100 o C/hr. to a maximum temperature of 800-1200 o C with a six hour to hold at the maximum temperature. A short hold at 300 o C is generally used to burn out the binder. Properties of the honeycombs, according to firing temperature, are shown in the table below. Heating Temp (°C) Linear Shrinkage (%) Density (g/cc) Open Porosity (%) Mean Pore Size (Microns) BET Surface Area (m²/g) 700 0 1.06 52.1 0.1 41.9 900 0 1.06 51.6 0.2 33.8 1030 5 1.22 46.3 2.0 4.7 1100 7 1.23 43.5 2.5 0.7 1300 5 1.28 44.3 3.0 0.3 1400 5 1.41 36.4 3.8 0.3 All samples fired at temperatures of 1000°C. and above exhibited significant strength and could be easily handled without breaking.

Claims: A method of preparing a monolithic catalyst support which comprises: (a) providing a substantially homogeneous admixture of (i) 50-90 parts by weight of a first phase sinterable ceramic matrix material, in particulate form finer than 200 mesh, selected from cordierite, mullite, alpha-alumina, lithium aluminosilicate, mixtures of the foregoing materials, optional addition of aluminum titanate to the foregoing materials, fired chrome ore-TiO₂-MgO, fired R x O y -TiO₂-Chrome Ore where R x O y is selected from Al₂O₃, Al₂O₃+Fe₂O₃, Al₂O₃+ZnO and Al₂O₃+Fe₂O₃+ZnO, and fired R x O y -TiO₂-MgO where R x O y is Al₂O₃, Al₂O₃+ZnO, Fe₂O₃, Fe₂O₃+ZnO and mixtures of these, and (ii) 10-50 parts by weight of a second phase surface area support material having a crystallite size no larger than 0.2 microns and a surface area of at least 40 m²/g selected from silica, spinel, titania, zeolite, zirconia, a mixture of these oxides, transition metal sulfide, a mixture of transition metal sulfides, and a mixture of such oxide(s) and transition metal sulfide(s). (b) forming the admixture into a shaped monolithic body; and (c) heating the shaped body at a temperature sufficient to sinter the first phase material and retain the second phase with a crystallite size no larger than about 0.5 micron. A method as claimed in claim 1 characterised in that the mixing step (a) is performed using 1-30 parts by weight of a binder material. A method as claimed in claim 2 characterised in that the second phase material has a surface area of at least 100 m²/g and is selected from silica, spinel, titania, zeolite, zirconia, and a mixture of these. A method as claimed in claim 2 or claim 3 characterised in that the binder is methyl cellulose, silicone resin, or a mixture of these. A method as claimed in claim 1 characterised in that the second phase material is spinel with up to 20 percent by weight, based on the weight of the second phase material, of rare earth oxide. A monolithic catalyst support having an overall surface area of at least 5 m²/g and comprising 50-90 parts by weight of a sintered ceramic matrix phase and 10-50 parts by weight of a high surface area support phase dispersed throughout the matrix phase wherein: (a) the ceramic matrix phase consists of cordierite, mullite, alpha-alumina, lithium aluminosilicate, mixtures of the foregoing materials, optional addition of aluminum titanate to the foregoing materials, fired chrome ore-TiO₂-MgO, fired R x O y -TiO₂-Chrome Ore where R x O y is selected from Al₂O₃, Al₂O₃+Fe₂O₃, Al₂O₃+ZnO, and Al₂O₃+Fe₂O₃+ZnO and fired R x O y -TiO₂-MgO where R x O y is Al₂O₃, Al₂O₃+ZnO, Fe₂O₃, Fe₂O₃+ZnO and mixtures of these; and (b) the dispersed support phase has a crystallite size no larger than 0.5 microns and is silica, spinel, titania, zeolite, zirconia, a mixture of these oxides, transition metal sulfide, a mixture of transition metal sulfides, and a mixture of such oxide(s) and transition metal sulfide(s). A catalyst support as claimed in claim 6 characterised in that the ceramic matrix phase is cordierite or mullite. A catalyst support as claimed in claim 6 or claim 7 characterised in that it has a surface area of at least 20 m²/g. A catalyst support as claimed in claim 6 or claim 7 characterised in that it has a surface area of at least 40 m²/g. A catalyst support as claimed in claim 6 characterised in that the dispersed support phase is spinel with up to 20 percent by weight, based on the weight of the support phase, of rare earth oxide.

Inventors: DEANGELIS, THOMAS PETER, LACHMAN, IRWIN MORRIS
Applicants: CORNING GLASS WORKS
Application year: 1986
Main classification: B01J37
IPC codes: B01J21, B01J37, B01J29, B01J35