Method of crystallizing a zeolite on the surface of a monolithic ceramic substrate

Abstract: A method is provided of crystallizing strong-bound zeolites on the surfaces of a monolithic ceramic substrate by hydrothermally treating the substrate, in the presence of active silica, with a caustic bath to zeolitize the silica. According to various embodiments of the invention, the active silica can be present as a component of the caustic bath, in the form of a dried coating pre-deposited on the substrate, or as a separate phase of the monolithic substrate, homogeneous dispersed within the ceramic material.

Description: This invention is directed to the in situ crystallization of zeolites on monolithic ceramic honeycomb substrates. Synthetic molecular sieve zeolites are used in myriad catalytic applications, for example, by the petroleum and chemical industry in the catalytic cracking or thermo-forming operations and various heterogeneous chemical processes, by the soft-drink industry in the carbonization of soft drinks, and by both automotive and power industries to abate NO x or other harmful emissions. In these applications, the zeolite-based catalysts have taken many forms which, though serviceable, all have disadvantages, either in use or in their own production. Many catalytic operations employ a "bed" of particulate catalyst through which the work stream passes. For this kind of operation, synthetic zeolites have been crystallized directly into powdery or micro-pellet form, or pre-formed zeolites have been embedded in or coated on porous ceramic pellets or beads. In US-A-3,523,092 there are disclosed supported crystalline aluminosilicate zeolites. These may be prepared by crystallizing the aluminosilicate zeolites on the surface of the inert support material. Alumina is mentioned as support material. This is in a form in which it may be dry-mixed with a source of silica to form a mixture, which mixture can then be mixed with a solution or suspension comprising alumina and sodium hydroxide. The material, in the form of a dry or slightly damp solid powder may be held in a tankless storage room for the long aging period required for crystallization. Hydrous silica may then be added to improve the adherence of the crystalline zeolite to the support. The product is then calcined, preferably after washing. The product is in particle form and has utility as an acid type catalyst. It is of the right particle size to be used as a fluidized cracking catalyst. Such pelletized catalysts, however, although presenting high catalytic surface area to the work stream, also provide high resistance to flow, resulting in substantial pressure drop. For other operations, in which high pressure drop cannot be tolerated, zeolites have been embedded in or coated on monolithic ceramic structures, such as extruded honeycombs. Pre-formed zeolites that are coated onto a ceramic substrate by traditional "wash coat" procedures, however, suffer from several disadvantages. In service, structures can be exposed to work streams which often contain dust or particulate matter, which can cause the zeolitic coating to flake off the underlying ceramic support. Such abrading of the zeolite decreases the catalytic efficiency and ultimately makes necessary the re-coating or complete replacement of the monolith. Monolithic structures containing zeolites less susceptible to abrasion have been made by extruding pre-formed zeolites together with a porous ceramic material providing a substrate in which the zeolite is embedded within the ceramic matrix. See, for example, U.S. Patent 4,007,134, in which a honeycomb catalyst containing up to 40% by weight zeolite is prepared by extruding and firing a mixture of pre-formed zeolite crystals with a ceramic binder, such as clay. This method, however, requires that the strength of the underlying ceramic structure be sacrificed. Because of the well-known thermal instability of zeolite crystals, the extruded ceramic mass incorporating them cannot be sintered at the high temperatures, generally above about 750°C, otherwise required to densify and strengthen many ceramic materials. To avoid this problem, the subsequent in situ formation of zeolites within pre-sintered ceramic structures has also been performed. See, for example, U.S. Patent 4,157,375, in which a honeycomb preform of kaolin clay is zeolitized by hydrothermal treatment in a caustic bath to convert 29% by weight of the preform into ZSM-5 zeolite. Zeolite crystals, however, do not have the structural strength associated with the ceramic materials which normally form the monolithic honeycombs, and the generation of a zeolitic sub-matrix within the pre-existing crystalline lattice of the ceramic can weaken the structure. In US-A-3,730,910 there are described aluminosilicate and zeolitic materials which comprise aluminosilicate or zeolite surfaces on substrates, these surfaces having essentially the same shape and form as the substrate. These aluminosilicate and zeolite materials are reactively and attractively formed on the surface of the substrate from a slurry having either (i), an aluminosilicate or zeolite forming mixture having component concentrations effective to produce the desired surface aluminosilicate or zeolite, or (ii), a silicate or an aluminosilicate solution of a composition sufficient to produce a particular aluminosilicate or zeolite with components on the surface of the substrate, or (iii), an aluminosilicate or a zeolite. The substrate may be a honeycomb. Accordingly, there remains a need for a monolithic ceramic substrate having a strongly-bound coating of zeolitic material on its surface, and preferably throughout its porous structure thus providing a structurally strong underlying ceramic material and an abrasion-resistant coating of zeolitic material to provide a catalytic surface that is readily contacted by the work stream. SUMMARY OF THE INVENTION The present invention provides a method of crystallizing zeolite on the surface of a monolithic ceramic honeycomb substrate, comprising: (a) providing a substantially sintered monolithic substrate comprising a porous ceramic material and either (i) a coating of active silica and pre-formed crystals of zeolite on the substrate, the coating being 1-45% of the weight of the coated substrate, or (ii) active silica and pre-formed crystals of zeolite dispersed throughout and embedded within a matrix of the porous ceramic material, the active silica being 1-40% by weight based on the total substrate weight; and (b) hydrothermally treating the substrate with an aqueous solution of sodium hydroxide as source of sodium oxide, optionally alumina and optionally an organic templating agent, and optionally comprising active silica, at a temperature and for a time sufficient to crystallize zeolite on the surfaces of said substrate. Thus according to the invention a monolithic ceramic honeycomb substrate is coated with a layer of active silica and pre-formed crystals of zeolite, the coating being 1-45% of the weight of the coated substrate, and then hydrothermally treated with an aqueous solution of sodium hydroxide as a source of sodium oxide and optionally alumina and optionally including active silica to crystallize the active silica to the desired zeolite and provide the zeolite on the surfaces of the substrate. The active silica and the pre-formed crystals of zeolite may alternatively or also be dispersed throughout and embedded within the ceramic matrix, the active silica being 1-40% by weight based on the total substrate. In preferred versions of the invention, the ceramic material of the monolith is cordierite, and the monolith is in the form of a thin-walled honeycomb having at least 0.62 channels per square centimeter (4 channels per square inch) of frontal surface area. The use of pre-formed crystals of zeolite according to the invention provides a zeolite on the surface of the substrate of high purity. It also eliminates the need to use organic templating agents, which are expensive and toxic, to provide nucleation sites for certain of the zeolites, notably ZSM-5 zeolite (also known as "silicalite") although they may still be used if desired. Accordingly, the present invention provides a safer and less expensive method to grow strongly-bound zeolites on the surfaces of monolithic ceramic honeycomb substrates. The ceramic materials of the monolithic honeycomb are generally porous, and as used herein, "surfaces" refers to those surfaces of the monolith substrate, including surfaces forming the pore cavities, which are normally intended to be in contact with the work stream of material to be catalyzed. It is known for example, that particular zeolites, the sodium-aluminosilicates, can be crystallized and precipitated from aqueous caustic precursor solutions containing their constituent oxides, silica and alumina. The particular composition of the precursor solution is dependent on the particular zeolite desired. These known precursor solutions form the basis for the hydrothermal treatment solutions of the present invention, and those corresponding to the most preferred zeolites according to the present invention are shown in Table I below. Other zeolites, such as other ZSM-series sodium-aluminosilicates, can also be crystallized, and are contemplated for in situ production in this invention, by known adjustment to the solution composition. Zeolite Molar Ratio of Constituents SiO₂ Al₂O₃ Na₂O H₂O X-Type 1 0.2 -0.5 1.2 -1.5 40-60 Y-Type 1 0.04-0.14 0.4 -0.6 16-24 Mordenite 1 0.04-0.09 0.11-0.15 20-25 ZSM-5 1 0.0 -0.04 0.2 -0.75 16-20 The hydrothermal treatment solution used according to the present invention to crystallize a particular zeolite on the ceramic substrate will have a composition and concentration as shown in Table I or a modification thereof corresponding to the particular zeolite to be crystallized. According to the invention active silica is pre-associated with the ceramic substrate, as described above, prior to hydrothermal treatment, and is therefore not of necessity present in the solution itself. The treatment solution need not contain silica but has all other ingredients in the same proportions, relative to each other and to the amount of pre-associated active silica. Nevertheless, additional active silica can be in the solution as well, providing the desirable effects. The conditions of hydrothermal treatment according to the invention will vary according to the zeolite to be crystallized. Generally, crystallization of the X-type and Y-type zeolites on the ceramic surface requires that the ceramic be in contact with the treatment solution for about 10-110 hours at a temperature of about 90-100°C. Production of mordenite or ZSM-5 zeolites takes place upon treatment for about 45-75 hours at a temperature of about 140-175°C, with the treatment preferably taking place in an enclosed bomb at autogenous pressure. In addition, the nucleation of mordenite or ZSM-5 zeolites can be facilitated by the presence of an organic templating agent, such as a tetra-alkylammonium bromide, in the treatment solution. The templating agent is used in an amount of about 0.3-0.6 mole of agent per mole of active silica used in the method. A preferred templating agent is tetra-propyl ammonium bromide (TPA-Br). The "active silica" of this invention is silica which, after calcining, has a surface area of at least 25 m²/g, preferably at least 100 m²/g. The silica generally has a pre-calcining surface area of at least about 200 m²/g. (As used herein, "calcining" means heating a material to a temperature sufficiently high to substantially eliminate any volatiles but below that at which the material begins to lose substantial porosity and surface area.) Examples are the silicas of about 1-10 µm (microns) or sub-micron particle size, such as CABOSIL EH-5 colloidal silica (an amorphous silica having a pre-calcining surface area of about 400 m²/g and a medium crystalline size of about 0.007 µm (micron)) available from Cabot Corporation, QUSO G-761 silica (a hydrophilic silica having a mean crystallite size of about 0.015 µm (micron)) available from PQ Corporation, and LUDOX-HS colloidal silica available from E.I. du Pont de Nemours & Company. Colloidal silica derived from gels can also be used, examples of which are Grace-81 silica gel (600 m²/g surface area) available from the Davison Chemical Division of W.R. Grace and Company. Active silica precursors, which generate the active silica itself upon firing, can also be used, particularly in embodiment C. Examples are silicone resins such as those disclosed in U.S. Patent No. 3,090,681 issued to Weyer. The most preferred silicon resin is a hydroxyl functional silicone resin available from the Dow-Corning Company as resin QC-2230. The monolithic ceramic can be prepared by directly mixing, extruding, and firing/sintering in conventional manner a composition of the component oxides, or oxide precursors as explained in U.S. Patent No. 3,885,997. However a preferred source of a pre-formed ceramic structure of such composition is a standard cordierite monolith from which magnesium and aluminum species have been selectively leached, leaving a silica-rich phase. Standard crystalline cordierite has an oxide composition, by weight, of about 51% silica, 35% alumina, and 14% magnesia. By contacting a cordierite monolith with a strong (e.g. 1-5 normal) mineral acid for about 0.25-20 hours, the aluminum and magnesium species can be selectively leached from the monolith, for a total weight loss of about 4-32%, to produce the silica-rich oxide composition described above. The procedure for acid-leaching a crystalline cordierite article is described in more detail in U.S. Patent No. 3,958,058 (issued May 18, 1976, to Elmer). The monolithic ceramic honeycomb substrate on which zeolite is to be crystallized can be composed of any porous ceramic, porous metal, or glass-ceramic material. Examples are cordierite, mullite, zircon, fused quartz, fused silica, spinel, feldspar, titania, alumina, zirconia, lithium aluminosilicates and kaolin clay. Preferred are those materials which contain silica, and more preferred are those which contain both silica and alumina. The most preferred ceramic material is cordierite, and the most preferred monolith for use in this embodiment is a cordierite monolith which has been acid-leached as described above. The sintered ceramic substrate is coated with active silica prior to the hydrothermal treatment. This coating can be applied by any of the conventional techniques by which high surface area oxides or catalytic metal oxides have been coated on ceramic substrates in the past. In a preferred application, the active silica and a silica-based bonding agent, such as Silbond H-5 prehydrolyzed ethylorthosilicate solution (Stauffer Chemical Company), are dispersed in water, an organic solvent, or a mixture thereof to form a slurry, which is then applied to the ceramic substrate by spray-coating or dip-coating. The slurry is preferably applied in several layers, with each layer permitted to air-dry prior to application of the succeeding layer. After the final layer is applied, the coated substrate is dried at 90-100°C for 18-24 hours, and then fired at 350-550°C for about 3-10 hours to drive off any organic material. Sufficient active silica material should be applied to the substrate in this manner to provide a dried coating of active silica weighing 1-45%, preferably 15-30%, of the weight of the coated substrate. According to the invention pre-formed crystals of the desired zeolite are added to the coating to "seed" the further zeolite growth preferably in an amount of 1-15% by weight, advantageously 5-10% by weight based on the weight of the active silica. The seed crystals are incorporated into the coating by mixing the crystals with the active silica and bonding agent in the slurry. The presence of the seed provides nucleation sites for zeolite growth, and eliminates the need to use templating agent for the production of the ZSM-series and mordenite zeolites. For example, mordenite zeolite often fails to crystallize to any substantial degree unless templating agent or, preferably, seed is present. The presence of seed is further preferred to provide the desired zeolite in purer form. It has been found that in the absence of the associated seeds, the growth of X-type or Y-type zeolites on the coated substrates is often accompanied by crystalline impurities (such as Na-A zeolite and/or sodalite). The coated monolith is immersed in an aqueous solution of alumina, sodium oxide (or sodium hydroxide), and optionally templating agent at a temperature and for a time sufficient to crystallize the desired zeolite. Additional active silica can also be present in the solution. After crystallization, the monolith is washed with hot distilled water and is then dried at about 100°C for 16-24 hours. According to the invention the monolithic ceramic honeycomb substrate on which zeolite is to be crystallized can contain active silica dispersed throughout and embedded within the ceramic material itself, which material forms a matrix in which the active silica is embedded. The ceramic can be any metal, sinterable ceramic, or glass-ceramic material. Examples of such materials are cordierite (preferably pre-reacted cordierite), mullite, zircon, quartz, spinel, feldspar, titania, fused silica, alumina, zirconia, lithium aluminosilicate, kaolin clay, metals of the kind disclosed in U.S.-A-4758272, silicon carbide silicon nitride and mixtures thereof. Preferred are those ceramic materials which contain one or both of silica and alumina in the crystalline lattice. Most preferred is cordierite. The substrate, containing the active silica, can be prepared by conventional techniques. The sinterable ceramic matrix material, and active silica in an amount of 1-40% by weight, preferably 10-30% by weight, based on the total ceramic plus silica weight, are admixed into a substantially homogeneous body. The ceramic material is preferably in particulate form of a size finer than 200 mesh (U.S. standard). Optionally a temporary binder is admixed with the other components as well, which binder can be any of those commonly used in ceramic catalyst support manufacture. The preferred binder is methyl cellulose, an example of which is Methocel® A4M from the Dow Chemical Company. Up to about 3% 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. Normally, the dry ingredients are first pre-mixed dry and then combined with any wet ingredients. Conventional equipment is used to form a homogeneous or substantially homogeneous mixture. Ultimately, the batch mix is formed into the honeycomb shape of the monolithic support, preferably by extrusion through a die or injection molding. A thin-walled honeycomb having at least 0.62 more preferably at least 15.5, and most preferably at least 31, through-and-through channels per square centimeter 4, 100 and 200 channels per square inch of frontal surface area and a wall thickness of about 0.0762-6.35mm (3-250 mils.) is preferred Finally, the shaped monolith is heated to a temperature and for a time sufficient to sinter the ceramic material. Generally, the shape is dried at about 100-120°C, and then fired/sintered at 500-850°C for 1-12 hours. Preferably 5-80% by weight, based on the weight of the active silica, of pre-formed crystals of the desired zeolite can be incorporated into the monolith to "seed" the further zeolite growth. The seed crystals are admixed with the other components during the formation of the monolith. The presence of the seed provides nucleation sites for zeolite growth, and permits growth of certain of the zeolites to proceed without the use of organic templating agent. For example, the ZSM-5, mordenite, and Y-type zeolites might fail to crystallize to any substantial degree unless seed is present. The presence of seed further provides zeolites, particularly the ZSM-5 and mordenite zeolites, with little or no crystalline impurity. The monolith is immersed in an aqueous solution of sodium oxide (or sodium hydroxide), and optionally alumina and templating agent, at a temperature and for a time sufficient to crystallize the desired zeolite. Because of the extent to which the active silica is dispersed throughout the ceramic material, zeolites will be crystallized within the ceramic structure itself as well as on the surfaces of the material. Additional active silica can also be present in the solution. Alumina need not be present in the treatment solution for the production of zeolites, which have low alumina contents, such as ZSM-5 zeolites, when the ceramic material itself contains alumina in its crystalline structure. Following the hydrothermal treatment, the monolith is washed with hot distilled water and is then dried at about 100°C for 16-24 hours. Where the active silica is applied as a coating, the sintered ceramic substrate may be a monolithic substrate having an oxide composition consisting essentially of 45-75% by weight silica, 8-45% by weight alumina and 7-20% by weight magnesia and after the coating process this may be contacted with a strong mineral acid. Similarly, a coating of active silica can be applied to the sintered monolithic substrate in which active silica and pre-formed crystals of zeolite have been dispersed and embedded according to the invention. The above-described embodiments of this invention provide monolithic ceramic substrates having essentially pure zeolites strongly bound to the surfaces of the substrate. The zeolites are not easily abraded, and the monolith itself provides a strong and thermally stable underlying support that is particularly well suited for catalytic applications in which physical or thermal shock will be encountered. The following examples are illustrative, but not limiting, of the invention. EXAMPLE 1 A series of Celcor® cordierite honeycomb monoliths (Corning Glass Works) were acid-leached in 1.5 N HNO₃ solution at 95°C for 6 hours. Leaching of the specimens were followed by washing for 10 minutes in 0.15 N HNO₃ (95°C), further washing for 10 minutes in distilled water (95°C) and drying at 100°C for 16-20 hours. Coating slurries were prepared by admixing 40 weight parts of silica gel (ball-milled Grace-81), 150 weight parts Silbond H-5 (prehydrolyzed ethyl-orthosilicate) solution, and either 5% by weight, based on the silica weight, of silicalite (pentacil structure) zeolite crystals (S-115, Union Carbide Co.) or 10% by weight, based on silica weight, of mordenite zeolite crystals (LZM-5, Union Carbide Co.). The leached honeycombs were provided with a dried coating of active silica from the coating slurries by the following procedure. Coating slurries of 40 weight parts of Grace-81 silica gel, either "as received" (200 particle size) or "ball-milled" (about 7µ particle size), and 150 weight parts of Silbond H-5 pre-hydrolyzed ethyl-orthosilicate solution were prepared by combining the ingredients and mixing for 10-15 minutes. The honeycombs were dipped into one or the other of the slurries and then removed. The channels of the honeycombs were cleared with compressed air, and the honeycombs allowed to dry in air for 30-40 minutes. This coating/drying procedure was repeated 3-4 times for each honeycomb to achieve high silica loading. The coated honeycombs were then oven dried for 24 hours at 100°C, followed by firing at 500°C for four hours. In each case, the honeycomb had a coating weighing 20-30% of the weight of the uncoated honeycomb. The coated honeycombs were immersed in various treatment solutions to crystallize either mordenite or silicalite zeolite. For each honeycomb, crystallization was conducted in a 125-ml Teflon® cup enclosed in a steel bomb at 160-170°C and at autogenous pressure. After crystallization, the honeycombs were washed with hot distilled water, dried at 100°C for 16-24 hours, and the major crystalline phases in the honeycomb identified by XRD. Table 1 gives details of the experiments. EXAMPLE 2 A series of honeycomb substrates were prepared by standard mixing, extruding, and firing techniques from fired (amorphous) silica, active silica (Grace-81 silica gel, ball-milled to 8N average particle size), silicon resin (Dow-Corning QC-2230 series, 46% by weight SiO₂), and either silicalite (pentacil structure) zeolite seed crystals (S-115, Union Carbide Co.) or mordenite zeolite seed crystals (LZM-5, Union Carbide Co.). Four different honeycomb specimens, representing two for each kind of zeolite seeding, were prepared according to the following compositions: Honeycomb Specimen Fused Quartz Active Silica Silicon Resin Mordenite Seed Silicalite Seed 1 68.0 18.8 8.2 - 5.0 2 64.4 17.9 7.7 - 10.0 3 64.4 17.9 7.7 10.0 - 4 77.2 5.1 7.7 10.0 - The honeycombs were immersed in various treatment solutions to crystallize mordenite or ZSM-5 zeolite. For each honeycomb, the crystallization was conducted in a 125-ml teflon cup enclosed in a steel bomb at 160-170°C and at autogenous pressure. After crystallization, the honeycombs were washed with hot distilled water, dried at 100°C for 24 hours, and characterized by XRD. Details of the experimental procedures are given in Table 2.

Claims: A method of crystallizing zeolite on the surface of a monolithic ceramic honeycomb substrate, comprising (a) providing a substantially sintered monolithic substrate comprising a porous ceramic material and either (i) a coating of active silica and pre-formed crystals of zeolite on the substrate, the coating being 1-45% of the weight of the coated substrate, or (ii) active silica and pre-formed crystals of zeolite dispersed throughout and embedded within a matrix of the porous ceramic material, the active silica being 1-40% by weight based on the total substrate weight; and (b) hydrothermally treating the substrate with an aqueous solution of sodium hydroxide as source of sodium oxide, optionally alumina and optionally an organic templating agent, and optionally comprising active silica, at a temperature and for a time sufficient to crystallize zeolite on the surfaces of said substrate. The method of claim 1 wherein there is provided the coated substrate (a)(i) with the pre-formed crystals being 1-15% by weight based on the weight of the active silica. The method of claim 1 wherein there is provided the substrate (a)(ii) with the pre-formed crystals being 5-80% by weight based on the weight of the active silica. The method of claim 1 wherein there is provided the substrate (a)(ii) and further comprising, prior to the treating step (b), providing a dried coating of active silica on the substrate wherein the weight of the coating is 1-45% of the weight of the coated substrate. The method of claim 1 further comprising, prior to the treating step (b), contacting the substrate with a strong mineral acid for a time sufficient to produce a weight loss of about 4-32% in the substrate. The method of any of the preceding claims in which the active silica has a surface area of at least 25 m²/g. The method of claim 6 wherein the molar ratio of silica/alumina/sodium oxide/water in the aqueous solution is about 1/0.2-0.5/1.2-1.5/40/60, or 1/0.04-0.14/0.4-0.6/16-24, or 1/0.04-0.09/0.11-0.15/20-25, or 1/0-0.04/0.2-0.75/16-20. The method of claim 7 wherein the pre-formed crystals are X-type zeolite, or Y-type zeolite, or mordenite zeolite, or ZSM-5 zeolite, dispersed throughout and embedded within the matrix of ceramic material. The method of any of the preceding claims wherein the porous ceramic material is selected from ceramic, metal and glass-ceramic. The method of any of the claims 1 through 8 wherein the porous ceramic material consists essentially of cordierite, mullite, zircon, quartz, spinel, feldspar, titania, fused silica, alumina, zirconia, lithium aluminosilicate, kaolin clay, metal, silicon carbide, silicon nitride, or mixtures thereof.

Inventors: LACHMAN IRWIN MORRIS, PATIL MALLANAGOUDA DYAMANAGOUD, LACHMAN, IRWIN MORRIS, PATIL, MALLANAGOUDA DYAMANAGOUDA
Applicants: CORNING GLASS WORKS
Application year: 1988
Main classification: B01J29
IPC codes: C01B39, B01J29, C01B37, C04B41, B01J37, B01J35