HIGH DENSITY CORDIERITE CERAMIC ARTICLES FORMED FROM SPECIFIED ZEOLITES

Abstract: A substantially crystalline cordierite ceramic article which is especially suited for electronic applications is formed by taking a shaped article which has been formed either from a magnesium exchanged zeolite powder or from a calcined product thereof, the zeolite selected from the group consistig of phillipsite, harmotome, gismondine, zeolite B, zeolite ZK-19 and zeolite W, and sintering the shaped article at a temperature of about 1,000°C to 1,350°C for about 1 to about 6 hours. The magnesium exchanged zeolite has a SiO₂ /Al₂O₃ ratio of about 2.3 to about 2.8 and a sodium content less than 0.5 weight percent. The ceramic article that is formed has a substantially crystalline cordierite phase and has a density of at least 90% of its theoretical density. A preferred process involves calcining the magnesium exchanged zeolite powder at a temperature of about 600-800°C to collapse the zeolite framework and give upon sintering a cordierite ceramic article which is substantially crack free. A preferred magnesium exchanged zeolite is magnesium exchanged zeolite B.

Description: Ceramic articles have many uses including catalyst supports, dental porcelain, heat exchangers, turbine blades, substrates for integrated circuits, etc. One of the areas of increasing interest is the use of ceramic articles as electronic substrate materials. To be useful as an electronic substrate, a ceramic material should have a thermal expansion that matches that of silicon metal, low dielectric constant, low electronic conductivity, high thermal conductivity and low sintering temperature. Cordierite is one ceramic material that meets most of these criteria. However, the conventional manner of preparing cordierite involves sintering at temperatures above 1250°C and the resultant material does not have as high a thermal conductivity as desired. Therefore, there is a need for a process to prepare a high density cordierite at as low a temperature as possible and which has as high a thermal conductivity as possible. The prior art reveals several ways to prepare cordierite other than the conventional method of sintering the oxides. For example, J.R. Moyer et al. in "Synthesis of Oxide Ceramic Powders by Aqueous Coprecipitation", Materials Research Society Symposium, Vol. 73, p. 117(1986) discloses preparing cordierite by coprecipitating aluminum, magnesium and silicon compounds and then firing. However, sintering must be carried out at about 1420°C in order to obtain a cordierite article with a density of at least 97% of its theoretical density. The prior art also discloses that zeolites can be used to form cordierite. Thus, D.W. Breck in ZEOLITE MOLECULAR SIEVES, John Wiley & Sons, New York (1974), pp. 493-496 states that Mg-X can be heated to form cordierite. The disclosed process involves heating the Mg-X zeolite at 1500°C to form a glass and then heating the glass above 1000°C to form cordierite. Thus, two steps are required to form cordierite. Another reference which teaches the preparation of a cordierite based ceramic article is US-A-4,814,303 to Chowdry et al. Chowdry discloses producing a monolithic anorthite, anorthite-cordierite or cordierite based ceramic article by heating the Ca, Ca/Mg and Mg forms of zeolites X, Y and A at a temperature of 900°C to 1350°C. Chowdry also discloses that one should maximize the removal of sodium present in the zeolites since sodium ions are known to significantly increase the dielectric constant and dielectric loss. Finally, EP-A-298,701 describes the preparation of a ceramic article having an anorthite phase from a calcium zeolite. The process involves a calcination to form an amorphous product which can then be shaped into an article and sintered at temperatures of 850-950°C. In contrast to this prior art, it has now been discovered that heating a magnesium exchanged phillipsite family zeolite at a temperature of 1000°-1050°C produces a ceramic article in which the only crystalline phase is cordierite and which has a density of at least 90% of the theoretical density of cordierite which is 2.51 g/cc. The phillipsite family of zeolite are those natural and synthetic zeolites which have similar framework structures and consequently similar X-ray diffraction patterns but which may have different overall crystal symmetry and chemical composition, e.g., SiO₂/Al₂O₃ ratio, cation type and content, water content, etc. The phillipsite family of zeolites are phillipsite, harmotone, gismondine, zeolite B (also known as zeolite P), zeolite ZK-19 and zeolite W. The advantage to using a Mg phillipsite family zeolite over the Mg-X, Y or A disclosed by Chowdry is that the sodium which is present in the phillipsite family zeolite can be removed much more easily than the sodium in zeolite X, Y or A. This is especially true of zeolites B and W. Therefore, there is an economic advantage to using a phillipsite family zeolite, and especially zeolites B and W over zeolite X, Y or A. Additionally when zeolite X, Y or A is heated at about 1000°C one obtains a mixture of phases which includes sapphrine, spine, stuffed beta quartz and mullite, whereas when a phillipsite family zeolite is heated at 1000°C one only obtains a cordierite phase or a mixture of cordierite and minor amounts of stuffed beta quartz which is a metastable precursor of cordierite (also known as mu-cordierite.) It should be noted that when cordierite is used in this application what is meant is alpha cordierite which has the diffraction pattern set forth in JCPDS #9-472. Finally, the density of a ceramic article prepared by using a phillipsite family zeolite is much higher than that formed using zeolite X, Y and A. Accordingly, the instant invention shows unexpected results by using a phillipsite family zeolite, and especially Mg-B zeolite, instead of Mg-X, Y or A. Cracks in the finished ceramic article can be minimized according to another aspect of the instant invention by first heating a phillipsite family zeolite at a temperature of 600-800°C. This heating collapses the zeolite structure, thereby minimizing crack formation and/or propagation. The present process is different from that set forth in EP-A-298,701 in that a crack free cordierite ceramic article is prepared, whereas the '701 reference involves a process for preparing an anorthite ceramic. Accordingly, the calcination temperature (800-900°C disclosed in the '701 reference would initiate crystallization of cordierite, whereas the final sintering temperature disclosed (850°950°C) is not sufficiently high to form a high density, crack free cordierite ceramic article. Therefore, there is nothing in the ′701 reference to suggest that the process disclosed therein could be used to prepare a crack free cordierite ceramic article. Further, while Breck at pp. 493-94 of ZEOLlTE MOLECULAR SIEVES, teaches a two-step process to make cordierite from Mg-X, the steps involve heating at 1500°C to form a glass, followed by heating at 1000°C to form cordierite; there is no mention in Breck that one should collapse the zeolite structure prior to preparing a shaped article in order to minimize crack formation. In fact, Breck does not address the preparation of ceramic articles at all but speaks only about converting Mg-X zeolite to cordierite. Further, the Chowdry et al patent specifically states at column 3, lines 40-46 that the advantage to their invention is a single firing step. Clearly, the present invention is the first to recognize that precollapsing the zeolite framework minimizes cracks in the final ceramic article. This invention relates to substantially crystalline cordierite ceramic articles and processes for making them. According to one embodiment a process for producing such a ceramic article comprises sintering a shaped article at a temperature of 1000 to 1350°C for a time of 1 to 6 hours, the shaped article consisting essentially of either 1) a powder of a magnesium-exchanged zeolite having a SiO₂/Al₂O₃ ratio from 2.3/1 to 2.8/1, selected from phillipsite, harmotome, gismondine, zeolite B, zeolite ZK-19 and zeolite W and having a sodium content less than 0.5 weight percent, or 2) a material formed by calcining said powder at a temperature of 600 to 800°C, thereby forming a ceramic article characterized in that it has a crystalline cordierite phase and has a density of at least 90% of its theoretical density. The invention also includes the cordierite ceramic article formed by the above process. The invention also includes a process for preparing a substantially crystalline and crack-free cordierite ceramic article comprising calcining a powder of a magnesium exchanged zeolite, the zeolite having a SiO₂ /Al₂O₃ ratio from 2.3/1 to 2.8/1, selected from phillipsite, harmotome, gismondine, zeolite B, zeolite ZK-19 and zeolite W and having a sodium content less than 0.5 weight percent, and a particle size less than 10 micrometers at a temperature of 600-800°C for an effective time to collapse the zeolite framework and provide an amorphous powder, forming the amorphous powder into a shaped article and sintering the shaped article at a temperature of 1000° to 1350°C, for a time of 1 to 6 hours, thereby forming a ceramic article characterized in that it has a substantially crystalline cordierite phase, has a density of at least 93% of the theoretical density of cordierite and is substantially crack-free. The invention also includes the ceramic article formed by following the process set forth above. This invention relates to a process for preparing a substantially crystalline cordierite ceramic article from a magnesium exchanged phillipsite family zeolite and to the articles so formed. Zeolites are well known microporous three-dimensional framework structures. In general the crystalline zeolites are formed from corner sharing AlO₂ and SiO₂ tetrahedra and are characterized as having pore openings of uniform dimensions, having a significant ion-exchange capacity and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal pores or voids of the crystal without displacing any atoms which make up the permanent crystal structure. Zeolites can be represented on an anhydrous basis, by the formula M 2/n O:Al₂O₃:XSiO₂ where M is a cation having the valence n, X is generally equal to or greater than 2. In naturally occurring zeolites, M can be Li, Na, Ca, K, Mg and Ba. The M cations are loosely bound to the structure and frequently can be completely or partially replaced with other cations by conventional ion exchange techniques. The zeolites which can be used in this invention are the phillipsite family or phillipsite group of zeolites which are phillipsite, harmotome, gismondine, zeolite B, zeolite ZK-19 and zeolite W. For a detailed explanation of the structural similarities among these zeolites and a list of references where specific structural information of these zeolites may be found, the reader is referred to U.S.-A-4,344,851. The important feature of this family of zeolites is the absence of framework sites which can be either irreversibly occupied by cations such as sodium or potassium or sites which, when occupied effectively, shield other sites from the exchanging cation. Of these zeolites, zeolite B and W are preferred, with zeolite B especially preferred. In the description which follows, zeolite B will be used to exemplify the process. However, this is not to be construed as limiting the invention in any way to zeolite B. Zeolite B is a synthetic zeolite having the formula Na₂O:Al₂O₃:xSiO₂ where x ranges from 2 to 5. The synthesis of zeolite B is described in U.S.-A-3,008,803 and essentially entails forming a mixture of sodium aluminate (NaAlO₂), sodium silicate, sodium hydroxide and colloidal silica, and heating this mixture at a temperature of 60-150°C, under autogenous pressure for a time of 12 to 96 hours. The resultant product is isolated, washed and dried. The amount of the reactants can be varied such that the SiO₂/Al₂O₃ ratio is from 2 to 5 and preferably from 2.3 to 2.8. Another method of preparing zeolite B or P is that found in EP-A-315,282. Since the presence of sodium ions in the resultant cordierite ceramic article is detrimental to the desired physical properties, it is necessary to maximize removal of the sodium cations in the zeolite B. Two techniques are generally used to remove the sodium cation. One technique is a multiple ion exchange with the magnesium cation while the other technique involves pre-exchanging the zeolite with a cation such as NH₄⁺ followed by ion exchange with the magnesium ion. Ion exchange is conveniently carried out by contacting the zeolite with an aqueous solution of the metal ion to be exchanged. Thus a dilute (about 1 molar) aqueous solution of magnesium nitrate (Mg(NO₃)₂ 6H₂O) is prepared and the pH of the solution is adjusted to about 7.5 with magnesium hydroxide (Mg(OH)₂). The volume of solution which is prepared is that amount which provides from 5 to 10 times the amount of magnesium ion needed to fully ion exchange the sodium or other alkali metals in the zeolite. The contacting of the magnesium nitrate solution with the zeolite can conveniently be carried out in a batch process. Accordingly, the solution is mixed with the zeolite powder and the mixture is refluxed for about 2 hours. Next the mixture is filtered, thereby isolating the zeolite powder. This procedure is repeated with a fresh batch of solution until the sodium level is less than 0.5 weight percent and preferably less than 0.2 weight percent. These sodium levels can be achieved after only six repetitions of the ion exchange procedure. Alternatively, the magnesium exchange can be carried out using a continuous process employing methods well known in the art such as placing the zeolite in a column and flowing the magnesium solution through the column or using a basket centrifuge. A continuous process has the advantage of allowing a more efficient utilization of the magnesium solution. The zeolite may be formed into a desired shape by means well known in the art. A typical method of forming a shaped article involves placing the zeolite powder into a metal die and then pressing the powder at pressures of 500 to 50,000 psi (3,440 to 344,000 kPa). It has been found that the particle size of Mg-B zeolite powder affects the characteristics of the ceramic article. A Mg-B zeolite powder with smaller particles can pack better, thereby forming a ceramic article with higher density. It has been found that a powder which contains smaller particles crystallizes more completely to cordierite in a shorter period of time. Accordingly, it is preferred that the Mg-B zeolite powder have an average particle size smaller than 10 micrometers and preferably smaller than 5 micrometers. If the average particle size of the Mg-B zeolite powder is not less than 10 micrometers, the powder may be milled using conventional milling means such as ball milling, attrition milling and impact milling. It is also desirable to add a binder to the powder as an aid in forming the shaped article. The binder may be selected from those well known in the art such as polyvinyl alcohol, and polyethylene glycol. If a binder is added, the amount which is to be added is up to 15 weight. In a preferred embodiment, the magnesium B zeolite is treated prior to forming the shaped article. This treatment involves calcining the magnesium B zeolite, in the form of a powder, at a temperature of 600 to 800°C and preferably 650° to 750°C for a time of 1 to 2 hours. This calcination collapses the zeolite framework and produces an amorphous powder which provides green (i.e. unsintered) ceramic articles of higher density than the original zeolite. The effect of this calcination step is that cracks in the finished ceramic article are minimized or eliminated. Having formed the Mg-B zeolite in a desired shape, the shape is now sintered at a temperature of 1000°C to 1350°C and preferably at a temperature of 1000°C to 1050°C for a time of 1 to 6 hours. The resultant product obtained after sintering has been found to be composed of a substantially crystalline cordierite phase. By substantially crystalline is meant that at least 95% of the material is crystalline. No other crystalline phases have been observed except for minor amounts of mu-cordierite which were occasionally formed at lower sintering temperatures and larger particle sizes. It has also been found that the ceramic article has a density of at least 90% of the theoretical density of cordierite (2.51 g/cc) and a dielectric constant, measured at 1 MH z of 5 to 7.5. In order to fully illustrate the instant invention, the following examples are set forth. It is to be understood that the examples are only by way of illustration. EXAMPLE 1 This example shows the preparation of MgB zeolite from NaB zeolite. In a container 723.6g of Mg(NO₃)₂·6H₂O were dissolved in 2.0 liter of distilled water and the pH of the solution was adjusted to 7.5 by adding about 0.5g of Mg(OH)₂. To this solution there were added 250g of NaB zeolite (prepared according to the procedure in U.S.-A-3,008,803) whose chemical and physical characteristics were: 25.87 wt. % Al₂O₃, 41.0 wt.% SiO₂,15.40 wt.% Na₂O, 18.30 wt.% LOI and a particle size of 40-50 micrometers. The chemical formula expressed as the ratio of the oxides on an anhydrous basis was determined to be: 0.98 Na₂O: l.0 Al₂O₃: 2.69 SiO₂. The resulting slurry was heated to reflux while stirring for 2 hours. The zeolite powder was isolated by filtration, after which the powder was reexchanged five more times, each time with equal amounts of freshly prepared Mg(NO₃)₂ solution (adjusted to pH 7.5 as above), followed by another filtration. Finally, the powder was washed with a sufficient amount of distilled water to lower the level of nitrate in the filtrate to less than 10 ppm (Quantofix (TM) test paper). The resulting powder was dried at room temperature. Elemental analysis revealed the presence of: 24.40 wt. % Al₂O₃, 38.40 wt. % SiO₂, 10.10 wt. % MgO, 0.27 wt. % Na₂O and 26.90 wt.% LOI. This sample was designated sample A. The chemical formula expressed as the ratio of the oxides on an anhydrous basis was determined to be: 0.018 Na₂O: 1.0 Al₂O₃: 2.67 SiO₂: 1.05 MgO. EXAMPLE 2 A MgB zeolite powder was prepared as per example 1 with the following modifications. The solution used for the exchange consisted of 3722g of Mg(NO₃)₂ · 6H₂O dissolved in 14.5 liters of water and adjusted to a pH of 7.5 with Mg(OH)₂. To this solution there were added 2268 of NaB zeolite powder (prepared according to the procedure in EP-A-315,282) whose chemical and physical characteristics were: 26.68 wt.% Al₂O₃, 38.2 wt.% SiO₂, 16.4 wt% Na₂O, 18.86 wt.% LOI and a particle size of 2-10 micrometers. Six separate exchanges were carried out as per example 1 after which analysis of the sample showed: 27.8 wt.% Al₂O₃, 39.70 wt.% SiO₂, 11.00 wt% MgO, 0.17 wt% Na₂O and 22.30 wt% LOI. The chemical formula expressed as the ratio of the oxides on an anhydrous basis was determined to be: 0.01 Na₂O: 1.0 Al₂O₃: 2.42 SiO₂: 1.0 MgO. This sample was designated sample B1. EXAMPLE 3 A magnesium B zeolite was prepared by first preparing the ammonium form of the B zeolite and then by exchanging the ammonium ion with magnesium ion. First 203g of (NH₄)₂SO₄ were dissolved in 1.5 liters of distilled water. This solution was added to 150g of NaB zeolite (prepared as in Example 1) whose chemical analysis was - 25.30 wt.% Al₂O₃, 41.20 wt.% SiO 2, 15.50 wt.% Na₂O, 17.90 wt.% LOI: particle size 40-50 micrometers. The chemical formula expressed as the ratio of the oxides on an anhydrous basis was determined to be: 1.01 Na₂O: 1.0 Al₂O₃: 2.76 SiO₂. The resulting slurry was heated to reflux with stirring for 2 hr. The zeolite powder was isolated by filtration, after which the powder was reexchanged as above two additional times, each time with freshly prepared solutions. The powder was then washed with 4 liters of distilled water and dried at room temperature. 100g. of this NH₄⁺ exchanged powder was further exchanged four more times, each time refluxing for two hours with fresh solutions containing 325 g. of Mg(NO₃)₂·6H₂O dissolved in 1.5 liters of distilled H₂O. The powder was then washed nitrate-free with distilled water and dried at room temperature. Elemental analysis of the powder revealed the presence of 27.20 wt.% Al₂O₃, 41.20 wt.% SiO₂, 8.62 wt.% MgO, 0.26 wt.% Na₂O, 0.80 wt.% N, and 22.30 wt.% LOI. The chemical formula expressed as the ratio of the oxides on an anhydrous basis was determined to be: 0.016 Na₂O: 1.0 Al₂O₃: 2.57 SiO₂: 0.8 MgO: 0.11 (NH₄)₂O. EXAMPLE 4 This example shows the preparation of ceramic pellets using magnesium B zeolite. Four pellets were formed by placing about 1 gram portions of sample A into 0.5 inch (1.27 cm) diameter pellet dies. Pellets 1 and 2 were pressed at 68,800kPa (10,000 psi) while pellets 3 and 4 were pressed at 344,000 kPa (50,000 psi). The four pellets were heated at 4° C/min. up to 806°C, held there for 8 hours and then heated at 1°C/min up to 1050°C and held there for 4 hours. The sintered pellets 1, 2, 3 and 4 had densities of 1.73, 1.70, 1.96 and 1.99 g/cc respectively (The respective percents of theoretical density were 69, 68, 78 and 79%). The pellets were ground into a fine powder and analyzed by X-ray diffraction which showed the presence of cordierite and a minor amount (about 10%) of mu-cordierite. EXAMPLE 5 A 50 g portion of sample A was milled for 18 hours in a jar mill containing alpha alumina grinding media. The mill was filled with sufficient water to cover the media. The particle size of the powder after milling was between 4-5 micrometers. Two pellets were formed by placing 1 gram portions of the ground magnesium B zeolite powder into 1.27 cm (0.5 inch) pellet dies and pressing at 68,800 kPa (10,000 psi). The pellets were removed from the dies and heated at 6°C per minute to 1050°C and held at 1050°C for 4 hours. The resulting pellets each contained a large radial crack. The densities of the two pellets were 2.46 g/cc and 2.44 g/cc (measured geometrically) or 98% and 97% respectively of cordierite's theoretical density (2.51 g/cc). One of the pellets was ground into a fine powder. X-ray diffraction analysis of the powder indicated that the material was well crystallized and the only crystalline species present was cordierite (JCPDS #9-472). EXAMPLE 6 A portion of sample B1 was used to prepare four ceramic pellets according to the following procedure. Four pellets were made by pressing approximately 1 g each of sample B powder in steel dies. Pellets 1 and 2 were pressed at 68,800 kPa (10,000 psi) and pellets 3 and 4 were pressed at 344,000 kPa (50,000 psi) The four pellets were heated at 6°C per minute to 1050°C and held at 1050°C for 4 hours. All four pellets were extensively cracked. Pellet 2 had broken into several pieces, making geometric density measurement impossible. Pellets 1,3 and 4 had densities of 2.37, 2.31 and 2.40 g/cc respectively (The respective percents of theoretical density were 94, 92 and 90%). Pellets 1 and 3 were ground into a fine powder. X-ray diffraction analysis of the two powders revealed that both were highly crystalline and pure cordierite. EXAMPLE 7 This example shows the preparation of a ceramic pellet using a magnesium zeolite B that has been calcined prior to sintering. A 5 gram portion of sample B was heated in air at 6°C/min. up to 750°C and held there for 1 hour, followed by cooling to room temperature. The calcined magnesium B zeolite was used to form two pellets by placing about 1 gram portions into 1.27 cm (0.5 inch) steel dies and pressing at 68800 kPa (10,000 psi.) The pellets were removed from the dies and heated at 6°C per minute to 750°C then at 1°C per minute to 1050°C, then held at 1050°C for four hours. The two pellets were crack-free and had densities of 2.46 and 2.52 g/cc. (The respective percents of theoretical density were 98 and 100%) One of the pellets was ground to a fine powder. X-ray diffraction analysis of the powder revealed the presence of pure well-crystallized cordierite. EXAMPLE 8 Two 0.8 gram portions of sample B1 were calcined as in Example 7 and each portion was combined with two drops of an aqueous solution of 2 wt. % polyvinyl alcohol and 1 wt. % polyethylene glycol and ground in a sapphire mortar and pestle. Each portion was placed in a 1.27 cm (0.5 inch) steel die and pressed at 68,800 kPa (10,000 psi). The pellets were removed from the dies and heated at 6°C per minute to 600°C, held at 600°C for 0.5 hours, then heated at 6°C per minute to 1050°C, and held at 1050°C for four hours. The two sintered pellets were crack-free and had densities of 2.41 and 2.38 g/cc (The respective percents of theoretical density were 96 and 95%). One of the pellets was ground to a fine powder. X-ray diffraction analysis of the powder revealed the presence of pure well-crystallized cordierite. EXAMPLE 9 A magnesium B zeolite powder was prepared as in Example 2. A 150 g portion of this magnesium B zeolite powder was calcined at 750°C for one hour. Three different size pellets were prepared by placing 1 g of the calcined powder in a 0.5 inch (1.27 cm) die, 7g in a 1 inch (2.54 cm) die and 30 g in a 2.25 inch (5.72 cm) die and pressing at 68,800 kPa (10,000 psi). The pellets were removed from the dies and heated at 4°C/min up to 600°C and held there for 1 hour, followed by heating at 4°C/min. up to 1,000°C and held there for 4 hours. The densities of the 1/2″, 1″ and 2-1/4″ pellets were 2.39, 2.35 and 2.47 g/cc respectively (The respective percents of theoretical density were 95, 93 and 98%). X-ray diffraction analysis showed the crystalline phase to be substantially cordierite (at least 95%) with a minor amount of mu-cordierite. EXAMPLE 10 This is a comparative example showing the preparation of magnesium X zeolite from NaX zeolite. A solution was prepared by adding 201 g of MgCl₂ to 1400 mL of distilled water. To this solution there were added 200 g of NaX zeolite obtained from Union Carbide Corporation, identified as Linde 13X and whose analysis showed 24.30 wt. % Al₂O₃, 35.30 wt. % SiO₂, 14.80 wt. % Na₂O and 26.10 wt. % LOI. The resulting slurry was heated at reflux, while stirring, for 1 hour, after which the powder was isolated by filtration. The powder was reexchanged five more times as above, each time with 1400 mL of freshly prepared MgCl₂ solution. After the final exchange and filtration, the powder was washed until chloride-free and dried at 100°C. Elemental analysis revealed the presence of 3.6 wt. % Na₂O. 160 grams of the above MgX powder was further exchanged 6 more times, each time by refluxing the powder in a fresh solution of 158 g of MgCl₂ dissolved in 1400 mL of water and filtering between each exchange. The powder was then washed chloride-free and dried at 100°C. 149 grams of the above powder was further exchanged five more times, the first four times with fresh solutions of 365 g of MgCl₂ in 1500 mL of H₂O, and the fifth time in a solution of 365 g of MgCl₂and 85 mL of 6 M HCl in 1200 mL of H₂O. The powder was then washed chloride-free and dried at 100°C. Elemental analysis of the powder revealed the presence of 25.70 wt. % Al₂O₃, 36.70 wt. % SiO₂, 7.70 wt. % MgO, 1.70 wt. % Na₂O, and 27.30 wt. % LOI. The chemical formula expressed as the ratio of the oxides on an anhydrous basis was determined to be: 0.11 Na₂O: 1.0 Al₂O₃: 2.42 SiO₂: 0.76 MgO. This sample was designated sample C. This experiment shows that even after 17 exchanges, the sodium level in the MgX zeolite is on order of magnitude greater than the sodium level in MgB zeolite which was exchanged only 6 times. EXAMPLE 11 This example shows the preparation of MgX zeolite using magnesium sulfate. A solution was prepared by adding 65.2 g of Mg(SO₄)₂·7H₂O to 550 mL of distilled water. The pH of the solution was adjusted to 7.2 by addition of a small quantity of Mg(OH)₂. 100 g of NaX zeolite (Linde 13X, Union Carbide Corp.) was added to the solution and the resulting slurry was heated with stirring at reflux for 2 hours. The zeolite powder was isolated by filtration, after which the powder was reexchanged four more times, each time with equal amounts of freshly prepared Mg(SO₄)₂ solution. After the final exchange and filtration, the powder was washed with 3 liters of distilled water. The resulting powder was dried at room temperature. Elemental analysis revealed the presence of 24.2 wt. % Al₂O₃, 34.9 wt. % SiO₂, 7.15 wt. % MgO, 4.37 wt. % Na₂O, and 29.1 wt. % LOI. The chemical formula expressed as the ratio of the oxides on an anhydrous basis was determined to be: 0.11 Na₂O: 1.0 Al₂O₃: 2.42 SiO₂: 0.76 MgO. This sample was designated sample D. EXAMPLE 12 This example shows the preparation of MgX zeolite by first forming the ammonium form of X zeolite. In a container there were mixed 17.28 g of NH₄NO₃ and 1 liter of distilled water. To this solution there were added 50 g of NaX zeolite (chem. analysis - 24.2 wt. % Al₂O₃, 36.6 wt. % SiO₂, 14.6 wt. % Na₂O, and 24.6 wt. % LOI) was added to the solution and the resulting slurry was heated with stirring at reflux for 2 hours. The zeolite powder was isolated by filtration, then was exchanged four more times by refluxing in freshly prepared solutions of 109.5 g of MgCl₂ in 1 liter of H₂O. The product was washed with 2 liters of distilled H₂O. Elemental analysis of the powder revealed the presence of 24.0 wt. % Al₂O₃, 35.0 wt. % SiO₂, 6.57 wt. % MgO, 3.15 wt. % Na₂O, and 31.7 wt. % LOI. The chemical formula expressed as the ratio of the oxides on an anhydrous basis was determined to be: 0.22 Na₂O: 1.0 Al₂O₃: 2.47 SiO₂: 0.69 MgO. This sample was designated sample E. EXAMPLE 13 In a container, there were mixed 60.9 g of MgCl₂and 640 mL of distilled water. The pH of the solution was adjusted to 7.5 by addition of a small quantity of Mg(OH)₂. 50 g of NaX zeolite (same NaX as in Example 12) was added to the solution and the resulting slurry was heated with stirring at reflux for 2 hours. The zeolite powder was then isolated by filtration, after which the powder was reexchanged five more times, each time with equal amounts of freshly prepared pH adjusted (to pH 7.5) MgCl₂ solution. After the final exchange and filtration, the powder was washed with 9 liters of distilled water. The resulting powder was dried at room temperature. Elemental analysis revealed the presence of 23.7 wt. % Al₂O₃, 33.9 Wt. % SiO₂, 9.11 wt. % MgO, 1.76 wt. % Na₂O, and 30.0 wt. % LOI. The chemical formula expressed as the ratio of the oxides on an anhydrous basis was determined to be: 0.12 Na₂O: 1.0 Al₂O₃: 2.43 SiO₂: 0.97 MgO. This sample was designated sample F. EXAMPLE 14 A series of experiments were conducted to compare the densities and phases of the ceramic articles obtained when magnesium X or magnesium B zeolite was used to prepare the ceramic article. One gram portions of samples B1, C, D, E, F and G were placed in 1.27 cm (0.5 inch) steel dies and pressed at 68800 kPa (10,000 psi). A fresh set of pellets was prepared for each of the following sintering experiments. I. All five pellets were simultaneously heated at 4°C/min. to 1050°C and held there for 3 hours. II. All five pellets were simultaneously heated at 4°C/min. to 950°C and held there for 3 hours. III. All five pellets were simultaneously heated at 4°C/min. to 1250°C and held there for 4 hours. IV. All five pellets were simultaneously heated at 4°C/min. to 1350°C and held there for 3 hours. The densities and phases of the pellets from experiments I-IV were determined and are reported respectively in Tables I-IV. Abbreviations for phases given in the tables are as follows: Co = cordierite JCPDS #9-472; theoretical density = 2.51 g/cc; Am = amorphous; Sa = sapphirine JCPDS #21-549; Qu = stuffed beta quartz also known as mu-cordierite. Each table contains numbered columns under a "Phases" heading. These numbers represent the proportion of any one phase relative to the total product as determined by relative X-ray peak intensities and have the following values: 1 = greater than 60%; 2 = 40%; 3 = 20%; 4 = 10% and 5 = trace. When only one product is listed and appears in the "1" column, the product was determined to be phase pure by X-ray diffraction. The results presented in Tables I-IV show the advantages which MgB zeolite has over MgX zeolite in preparing ceramic articles. First the MgB zeolite forms a pure well crystallized cordierite or cordierite plus minor amounts of mu-cordierite ceramic with a density greater than 91% (>2.28 g/cc) of the theoretical density of cordierite. In contrast, none of the MgX samples form single phase cordierite ceramics of greater than 85% (>2.13 g/cc) of theoretical density. Further, in most cases the major phase of the ceramic pellet derived from the MgX zeolite powders was an amorphous phase. Accordingly, none of the pellets derived from the MgX zeolite samples are suitable for specialty ceramic applications such as electronic applications. Finally, the MgX zeolite samples contain high levels of Na₂O whereas the MgB samples have very low levels of Na₂O making the MgX derived ceramic unsuitable for electronic applications. Clearly MgB zeolites show unexpected advantages over MgX.

Claims: A substantially crystalline ceramic article, characterized in that it has a substantially crystalline cordierite phase and a density of at least 90% of its theoretical density and has been formed by a process comprising sintering at a temperature of 1000 to 1350°C for a time of 1 to 6 hours a shaped article consisting essentially of 1) a powder of a magnesium-exchanged zeolite having an SiO₂/Al₂O₃ ratio from 2.3/1 to 2.8/1, selected from phillipsite, harmotome, gismondine, zeolite B, zeolite Zk-19 and zeolite W and having a sodium content less than 0.5 weight percent, or 2) a material formed by calcining such a powder at a temperature of 600 to 800°C. A substantially crystalline and crack-free ceramic article, characterized in that it has a substantially crystalline cordierite phase and a density of at least 93% of its theoretical density and has been formed by a process comprising calcining a powder of a magnesium-exchanged zeolite having an SiO₂/Al₂O₃ ratio from 2.3/1 to 2.8/1, selected from phillipsite, harmotome, gismondine, zeolite B, zeolite ZK-19 and zeolite W and having a sodium content less than 0.5 weight percent, at a temperature of 600 to 800°C for a time effective to collapse the zeolite framework and provide an amorphous powder, forming the amorphous powder into a shaped article and sintering the shaped article at a temperature of 1000 to 1350°C for a time of 1 to 6 hours. An article as claimed in claim 1 or 2, characterized in that the sodium content of the magnesium-exchanged zeolite is less than 0.2 weight percent. An article as claimed in any of claims 1 to 3, characterized in that the powder of magnesium-exchanged zeolite has an average particle size of less than 10 micrometers. An article as claimed in any of claims 1 to 4, characterized in that the sintering step is carried out at a temperature of 1000 to 1050°C. An article as claimed in any of claims 1 to 5, characterized in that the magnesium-exchanged zeolite is magnesium-exchanged zeolite B or magnesium-exchanged zeolite W. A process for producing a substantially crystalline ceramic article, comprising sintering at a temperature of 1000 to 1350°C for a time of 1 to 6 hours a shaped article consisting essentially of 1) a powder of a magnesium-exchanged zeolite having an SiO₂/Al₂O₃ ratio from 2.3/1 to 2.8/1, selected from phillipsite, harmotome, gismondine, zeolite B, zeolite ZK-19 and zeolite W and having a sodium content less than 0.5 weight percent, or 2) a material formed by calcining such a powder at a temperature of 600 to 800°C. A process as claimed in claim 7, including the step of calcining a powder of a magnesium-exchanged zeolite having an SiO₂/Al₂O₃ ratio from 2.3/1 to 2.8/1, selected from phillipsite, harmotome, gismondine, zeolite B, zeolite ZK-19 and zeolite W and having a sodium content less than 0.5 weight percent, at a temperature of 600 to 800°C for a time effective to collapse the zeolite framework and provide an amorphous powder. A process as claimed in claim 8, in which the powder is calcined at a temperature of 650 to 750°C for a time of 1 to 2 hours.

Inventors: BEDARD, ROBERT L., FLANIGEN, EDITH M.
Applicants: UOP
Application year: 1990
Main classification: C04B35
IPC codes: C04B35, C01B33