CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/511,975, filed Oct. 16, 2003.
FIELD OF INVENTION
This invention relates to catalysts for the epoxidation of alkene, especially ethylene, to the corresponding alkylene oxide, for example, ethylene oxide, which have enhanced stability, efficiency and/or activity by incorporating sufficient amount of a zirconium component substantially as zirconium silicate.
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OF THE INVENTION
The production of alkylene oxide, such as ethylene oxide, by the reaction of oxygen or oxygen-containing gases with ethylene in the presence of a silver-containing catalyst at elevated temperature is an old and well-known art. For example, U.S. Pat. No. 2,040,782, dated May 12, 1936, describes the manufacture of ethylene oxide by the reaction of oxygen with ethylene in the presence of silver catalysts which contain a class of metal-containing promoters. In Reissue U.S. Pat. No. 20,370, dated May 18, 1937, Leforte discloses that the formation of olefin oxides may be effected by causing olefins to combine directly with molecular oxygen in the presence of a silver catalyst. (An excellent discussion on ethylene oxide, including a detailed description of commonly used manufacturing process steps, is found in Kirk-Othmer's Encyclopedia of Chemical Technology, 4th Ed. (1994) Volume 9, pages 915 to 959).
The catalyst is the most important element in direct oxidation of ethylene to produce ethylene oxide. There are several well-known basic components of such catalyst: the active catalyst metal (generally silver as described above); a suitable support/carrier (for example alpha-alumina); and catalyst promoters, all of which can play a role in improving catalyst performance. Because of the importance of the catalyst in the production of ethylene oxide, much effort has been expended to improve catalyst's efficiency in producing ethylene oxide.
The use of zirconium and or silicon components as either promoters in the ethylene oxide catalyst or as modifiers to supports (that is carriers) used for such catalysts are also known.
U.S. Pat. No. 5,703,001 describes a rhenium-free silver catalyst promoted with an alkali metal component and a Group IVB component wherein the Group IVB component is added as a compound having a Group IVB cation. Soluble zirconium compounds where the Group IVB component is a cation are preferred.
U.S. Pat. No. 5,145,824 describes a rhenium-promoted ethylene oxide silver catalyst supported on a carrier comprising alpha alumina, an added alkaline earth metal in the form of an oxide, silicon in the form of an oxide, and from zero to about 10 percent (%) added zirconium in the form of the oxide. In U.S. Pat. No. 5,145,824, the term “oxide” is used to refer to simple oxides made up of only one metal as well as complex oxides made up of the indicated metal and one or more of the other metals. The amount of alkaline earth metal used in the carrier is from 0.05 to 4 weight percent (wt. %), measured as the oxide. Similarly, U.S. Pat. No. 5,801,259 describes an ethylene oxide catalyst comprising silver and promoters on a carrier prepared by mixing alpha alumina, alkaline earth metal oxide, silicon oxide, and from zero to about 15% of zirconium in the form of the oxide. The particle sizes of the ceramic components are chosen such that the packing density of the dried carrier precursor is not greater than that of the fired carrier; thereby eliminating the need for organic burnout agents. In '824 and '259 patents, the carrier mixture is formed from a starting mixture containing alpha-alumina, and requires the addition of alkaline earth metal oxide. The addition of the zirconium oxide component is optional.
There are several examples in the prior art of carriers used for ethylene oxide catalysts which contain silicon-containing compounds. U.S. Pat. No. 6,313,325 describes a method for the production of ethylene oxide wherein the carrier of the catalyst is obtained by adding an aluminum compound, a silicon compound and an alkali metal compound to a low-alkali content alpha-alumina powder. After calcination, this mixture is thought to provide a coating layer of alkali metal-containing amorphous silica alumina on the outer surface of the alpha-alumina carrier and the inner surface of the pores thereof. Canadian patent 1,300,586 describes a catalyst using a carrier composed mainly of alpha-alumina, silica, sodium, which has measurable acidity and crystals of Al6Si2O13 which are detectable by X-ray Diffraction analysis (XRD).
Several terms are commonly used to describe some of the parameters of catalytic systems for epoxidation of alkenes. For instance, “conversion” is defined as the molar percentge of alkene fed to the reactor which undergoes reaction. Of the total amount of alkene which is converted to a different chemical entity in a reaction process, the molar percentage which is converted to the corresponding alkylene epoxide, that is alkylene oxide, is known as the “efficiency” (which is synonymous with the “selectivity”) of that process. The product of the percent efficiency times the % conversion (divided by 100% to convert from %2 to %) is the percentage “yield”, that is, the molar percentage of the alkene fed that is converted into the corresponding epoxide.
The “activity” of a catalyst can be quantified in a number of ways, one being the mole percent of alkylene epoxide contained in the outlet stream of the reactor relative to that in the inlet stream (the mole percent of alkylene epoxide in the inlet stream is typically, but not necessarily, zero percent) while the reactor temperature is maintained substantially constant, and another being the temperature required to maintain a given rate of alkylene epoxide production. That is, in many instances, activity is measured over a period of time in terms of the molar percent of alkylene epoxide produced at a specified constant temperature. Alternatively, activity may be measured as a function of the temperature required to sustain production of a specified constant mole percent of alkylene epoxide. The useful life of a reaction system is the length of time that reactants can be passed through the reaction system during which results are obtained which are considered by the operator to be acceptable in light of all relevant factors.
Deactivation, as used herein, refers to a permanent loss of activity and/or efficiency, that is, a decrease in activity and/or efficiency which cannot be recovered. As noted above, production of alkylene epoxide product can be increased by raising the temperature, but the need to operate at a higher temperature to maintain a particular rate of production is representative of activity deactivation. Activity and/or efficiency deactivation tends to proceed more rapidly when higher reactor temperatures are employed. The “stability” of a catalyst is inversely proportional to the rate of deactivation, that is, the rate of decrease of efficiency and/or activity. Lower rates of decline of efficiency and/or activity are generally desirable.
To be considered satisfactory, a catalyst must have acceptable activity and efficiency, and the catalyst must also have sufficient stability, so that it will have a sufficiently long useful life. When the efficiency and/or activity of a catalyst has declined to an unacceptably low level, typically the reactor must be shut down and partially dismantled to remove the catalyst. This results in losses in time, productivity and materials, for example silver catalytic material and alumina carrier. In addition, the catalyst must be replaced and the silver salvaged or, where possible, regenerated. Even when a catalyst is capable of regeneration in situ, generally production must be halted for some period of time. At best, replacement or regeneration of catalyst requires additional losses in production time to treat the catalyst and, at worst, requires replacement of the catalyst with the associated costs. It is therefore highly desirable to find ways to lengthen the useful life of a catalyst.
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OF THE INVENTION
One aspect of the present invention is a catalyst for the manufacture of alkylene oxide by the vapor-phase epoxidation of alkene, said catalyst containing impregnated silver and at least one efficiency-enhancing promoter on a refractory solid support, said support incorporating a sufficient amount of zirconium component to enhance at least one of catalyst activity, efficiency and stability as compared to a similar catalyst which does not contain the zirconium component, said zirconium component being present in the support substantially as zirconium silicate.
Another aspect of the present invention is the catalyst described above wherein the refractory solid support is alpha-alumina, particularly having a unique morphology consisting of interlocking platelets.
Yet another aspect of the present invention is the process for the manufacture of alkylene oxide, such as ethylene oxide or propylene oxide, by the vapor-phase epoxidation of alkene using the improved catalyst of this invention.
While the present invention should be understood as being unconstrained by any particular theory, it is believed that the zirconium silicate (commonly referred to as zircon), added as an ingredient with other raw materials used to form the carrier support, survives the rigors of the calcining process without being oxidized or otherwise undergoing a substantial chemical change, and thereby becomes an integral part of the modified carrier, ultimately contributing to the favorable and unexpected characteristics observed in catalysts of the present invention employing such modified carriers.
A key distinguishing feature of the present invention is the use of zirconium silicate with other raw materials to modify the inert, refractory solid support (such as alpha-alumina) used as a carrier in a manner described herein, prior to depositing silver thereon with a well known promoter (and other optional additives) to convert the carrier to a catalyst. Zirconium silicate is employed in such a way and in sufficient amount that its presence in the modified carrier ultimately enhances the activity, efficiency and/or stability of the resultant catalyst of the present invention. Zirconium silicate remains substantially the same chemically throughout various preparation steps (including multiple calcining or roasting steps involving relatively high temperatures noted herein) for making the catalyst of the present invention, from its initial introduction as a part of raw materials for the modified carrier to the finished catalyst.
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OF THE INVENTION
Alkylene oxides made using the catalysts of this invention are characterized by the structural formula
wherein R1 and R2 are lower alkyl, for example, methyl or ethyl or, preferably, hydrogen. Most preferably, the alkylene oxide is ethylene oxide. The alkylene oxides are made from the corresponding alkene, that is, R1HC═CHR2. The following discussion is presented in terms of and with reference to ethylene oxide and ethylene for the sake of simplicity and illustration. However, the scope and range of the present invention is generally applicable to catalysts for the epoxidation of suitable alkenes.
In commercially useful catalysts for the production of ethylene oxide, the carrier upon which the silver and promoters reside must have a physical form and strength to allow proper flow of gaseous reactants, products and ballast through the reactor while maintaining physical integrity over catalyst life. Significant catalyst breakage or abrasion is highly undesirable because of the pressure drop and safety problems such degradation can cause. The catalyst must also be able to withstand fairly large temperature fluctuations within the reactor. The pore structure and chemical inertness of the carrier are also important factors that must be considered for optimum catalyst performance. Refractory materials, particularly alpha-alumina, have been successfully used as the carrier for ethylene oxide catalysts. Other porous refractory carrier or materials may also be used as long as they are relatively inert in the presence of the reactant feeds introduced for epoxidation and the product epoxide, and are able to withstand preparation conditions when converted into catalyst. For example, carriers may be composed of alpha-alumina, silicon carbide, silicon dioxide, zirconia, magnesia, various clays and mixtures thereof.
The catalyst of the present invention which is useful for the production of an alkylene oxide, such as ethylene oxide, from alkene, such as ethylene, is supported on a zircon-modified carrier. Zircon, a naturally occurring material which is also known as zirconium silicate, has the chemical formula of ZrSiO4. Zircon may also be prepared synthetically, following a number of well-known procedures such as that given in R. Valero, B. Durand, J-L. Guth, T. Chopin, “Hydrothermal Synthesis of Porous Zircon in Basic Fluorinated Medium,” Microporous and Mesoporous Materials, Vol. 29 (1999) p. 311-318. In general, the carriers are made up of an inert, refractory support, such as alpha-alumina, having a porous structure and relatively high surface area, which has been modified by the presence of zirconium silicate introduced with the other raw materials used to produce the carrier. In preparing a catalyst of the present invention, silver is deposited throughout the pores of the carrier and reduced to silver metal. Promoters, such as alkali salts, can be added with the soluble silver mixture impregnated into the carrier or added in a separate step. These promoters are generally associated with silver, although they may also be present on the carrier. The promoters act to improve catalyst efficiency, activity and/or stability.
The raw materials for the carrier must be of sufficient purity so that there is limited reaction between any components thereof and the zirconium silicate to be added during the preparation of the carrier in accordance with the teachings of the present invention. Limiting such reaction ensures that the added zirconium silicate remains substantially unchanged chemically throughout the processing of the carrier and the conversion of the carrier into the catalyst. Even the partial decomposition of zirconium silicate to zirconium oxide (ZrO2) is a particularly undesirable reaction, which decreases significantly the benefits from the addition of zirconium silicate to the carrier. At higher zirconium silicate concentrations, the presence of zirconium silicate may be easily ascertained by the use of X-ray diffraction analysis of the fired carrier. At lower zirconium silicate concentrations, zirconium silicate may not be detectable by the same analysis. However, the presence of zirconium and silicon may be detected using elemental analyses, such as X-ray fluorescence. In any case, the beneficial effect on catalyst performance and life are the primary indicator of the presence of zirconium silicate, especially at lower zirconium silicate concentrations.
In addition, the zircon itself must be of sufficient purity so that any impurities therein do not promote decomposition of zircon to zirconia during the preparation of the carrier. Impurities in zircon comprise primarily the inorganic compounds of transition metals (excluding zirconium and halfnium, which naturally occurs with zirconium), and are preferably limited to not more than 1.5 wt. %. More common inorganic compounds of transition metals occurring as impurities in zircon are oxides of transition metals. Two of the common oxide impurities are titania and iron oxides.
In the present invention, the zircon is mixed with the other raw materials for the carrier prior to the final firing at high temperature. The zircon may be incorporated in any number of ways, including the adding of the zircon in the form of powder or flour to the other dry raw materials, followed by mixing and adding of liquid raw materials. The order of addition of the zircon to the other raw materials is not critical.
Suitable shapes for the carrier of this invention include any of the wide variety of shapes known for such catalyst supports, including pills, chunks, tablets, pieces, pellets, rings, spheres, wagon wheels, toroids having star shaped inner and/or outer surfaces, and the like, of a size suitable for employment in fixed bed reactors. Conventional commercial fixed bed ethylene oxide reactors are typically in the form of a plurality of parallel elongated tubes (in a suitable shell) about 1 to 3 inches O.D. and 15-45 feet long filled with catalyst. In such fixed bed reactors, it is desirable to employ carrier formed into a rounded shape, such as, for example, spheres, pellets, rings, tablets and the like, having diameters from about 0.1 inch to about 0.8 inch.