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Catalysts having enhanced stability, efficiency and/or activity for alkylene oxide production

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Title: Catalysts having enhanced stability, efficiency and/or activity for alkylene oxide production.
Abstract: A catalyst for the manufacture of alkylene oxide, for example ethylene oxide, by the vapor-phase epoxidation of alkene containing impregnated silver and at least one efficiency-enhancing promoter on an inert, refractory solid support, said support incorporating a sufficient amount of zirconium component (present and remaining substantially as zirconium silicate) as to enhance at least one of catalyst activity, efficiency and stability as compared to a similar catalyst which does not contain the zirconium component. ...

USPTO Applicaton #: #20090291847 - Class: 502227 (USPTO) - 11/26/09 - Class 502 
Catalyst, Solid Sorbent, Or Support Therefor: Product Or Process Of Making > Catalyst Or Precursor Therefor >Halogen Or Compound Containing Same >And Group Iv Metal (i.e., Ti, Zr, Hf, Ge, Sn Or Pb)

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The Patent Description & Claims data below is from USPTO Patent Application 20090291847, Catalysts having enhanced stability, efficiency and/or activity for alkylene oxide production.

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This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/511,975, filed Oct. 16, 2003.


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.


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.



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.



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.

There are many well-known methods of preparing carriers suitable for use in ethylene oxide catalysts. Some of such methods are described in, for example, U.S. Pat. Nos. 4,379,134; 4,806,518; 5,063,195; 5,384,302, U.S. Patent Application 20030162655 and the like. As long as the carrier materials and method of preparation do not substantially decompose zircon, these methods can be employed to prepare the zircon modified carrier of the present invention. For example, an alpha-alumina support of at least 95% purity (exclusive of zirconium component) can be prepared by compounding (mixing) the raw materials, extrusion, drying and a high temperature calcination. In this case, the starting raw materials usually include one or more alpha-alumina powder(s) with different properties, a clay-type material which may be added as binder to provide physical strength, and a burnout material (usually an organic compound) used in the mix to provide desired porosity after its removal during the calcination step. The levels of impurities in the finished carrier are determined by the purity of the raw materials used, and their degree of volatilization during the calcination step. Common impurities may include silica, alkali and alkaline earth metal oxides and trace amounts of metal and/or non-metal-containing additives.

Another method for preparing a carrier of this invention having particularly suitable properties for ethylene oxide catalyst usage comprises mixing zirconium silicate with boehmite alumina (AlOOH) and/or gamma-alumina, peptizing the aluminas with a mixture containing an acidic component and halide anions (preferably fluoride anions) to provide peptized halogenated alumina, forming (for example, by extruding or pressing) the peptized halogenated alumina to provide formed peptized halogenated alumina, drying the formed peptized halogenated alumina to provide dried formed alumina, and calcining the dried formed alumina to provide pills of modified alpha-alumina carrier.

The modified alpha-alumina carrier prepared by the method described above preferably has a specific surface area of at least about 0.5 m2/g (more preferably from about 0.7 m2/g to about 10 m2/g), a pore volume of at least about 0.5 cc/g (more preferably from about 0.5 cc/g to about 2.0 cc/g), purity (exclusive of zirconium component) of at least 99 weight percent alpha-alumina, and median pore diameter from about 1 to about 50 microns. In this case, the modified alpha-alumina carrier comprises particles each of which has at least one substantially flat major surface having a lamellate or platelet morphology which approximates the shape of a hexagonal plate (some particles having two or more flat surfaces), at least 50% of which (by number) have a major dimension of less than about 50 microns.

In the finished carrier of the present invention, including those prepared by the two particular methods described above as a way of illustration, zirconium silicate is present in an amount which is preferably in the range of from about 0.01 to about 10.0% by weight, more preferably from about 0.1 to about 5.0% by weight, and most preferably from about 0.3 to about 3.0% based on the total weight of the finished modified alumina carrier.

While the invention is not constrained by any particular theory, the raw materials used to manufacture the carrier should not contain large amounts of reactive calcium compounds in order to minimize the reaction of these species with the added zirconium silicate, resulting in the formation of less beneficial species, particularly zirconia (ZrO2, also called zirconium oxide). The cumulative concentration of calcium compounds in carrier raw materials should be limited so that the fired carrier (excluding zirconium component) contains less than 2000 ppmw calcium, preferably less than 350 ppmw calcium.

In addition, certain other alkaline earth metal compounds may also promote the decomposition of zirconium silicate to zirconia. The cumulative concentration of alkaline earth metal compounds in carrier raw materials should be limited so that the fired carrier (excluding zirconium component) contains less than 500 ppmw alkaline earth metal (excluding calcium compounds), measured as the alkaline earth metal oxide.

The calcination temperature (firing temperature) of the carrier must also be controlled to limit the thermal decomposition of zircon to zirconia which occurs in the pure state at temperatures above 1540° C.

Catalysts for the production of alkylene oxide, for example ethylene oxide or propylene oxide, may be prepared on the modified supports of the present invention by impregnating the carrier with a solution of one or more silver compounds, as is well known in the art. One or more promoters may be impregnated simultaneously with the silver impregnation, before the silver impregnation and/or after the silver impregnation. In making such a catalyst, the carrier is impregnated (one or more times) with one or more silver compound solutions sufficient to allow the silver to be supported on the carrier in an amount which ranges from about 1 to about 70%, more preferably from about 5 to about 50%, most preferably from about 10 to about 40% of the weight of the catalyst.

Although silver particle size is important, the range is not narrow. Suitable silver particle size can be in the range of from about 100 to 10,000 angstroms.

There are a variety of known promoters, that is, materials which, when present in combination with particular catalytic materials, for example, silver, benefit one or more aspect of catalyst performance or otherwise act to promote the catalyst\'s ability to make a desired product, for example ethylene oxide or propylene oxide. Such promoters in themselves are generally not considered catalytic materials. The presence of such promoters in the catalyst has been shown to contribute to one or more beneficial effects on the catalyst performance, for example enhancing the rate or amount of production of desired product, reducing the temperature required to achieve a suitable rate of reaction, reducing the rates or amounts of undesired reactions, etc. Competing reactions occur simultaneously in the reactor, and a critical factor in determining the effectiveness of the overall process is the measure of control one has over these competing reactions. A material which is termed a promoter of a desired reaction can be an inhibitor of another reaction, for example a combustion reaction. What is significant is that the effect of the promoter on the overall reaction is favorable to the efficient production of the desired product, for example ethylene oxide. The concentration of the one or more promoters present in the catalyst may vary over a wide range depending on the desired effect on catalyst performance, the other components of a particular catalyst, the physical and chemical characteristics of the carrier, and the epoxidation reaction conditions.

There are at least two types of promoters—solid promoters and gaseous promoters. A solid promoter is incorporated into the catalyst prior to its use, either as a part of the carrier (that is support) or as a part of the silver component applied thereto. When a solid promoter is added during the preparation of the catalyst, the promoter may be added to the carrier before the silver component is deposited thereon, added simultaneously with the silver component, or added sequentially following the deposition of the silver component on the carrier. Examples of well-known solid promoters for catalysts used to produce ethylene oxide include compounds of potassium, rubidium, cesium, rhenium, sulfur, manganese, molybdenum, and tungsten. During the reaction to make ethylene oxide, the specific form of the promoter on the catalyst may be unknown.

In contrast, the gaseous promoters are gas-phase compounds and or mixtures thereof which are introduced to a reactor for the production of alkylene oxide (for example ethylene oxide) with vapor-phase reactants, such as ethylene and oxygen. Such promoters, also called modifiers, inhibitors or enhancers, further enhance the performance of a given catalyst, working in conjunction with or in addition to the solid promoters. One or more chlorine-containing components are typically employed as gaseous promoters, as is well known in the art. Other halide-containing components may also be used to produce a similar effect. Depending on the composition of the solid catalyst being employed, one or more gaseous components capable of generating at least one efficiency-enhancing member of a redox half reaction pair may be employed as gaseous promoters, as is well known in the art. The preferred gaseous component capable of generating an efficiency-enhancing member of a redox half reaction pair is preferably a nitrogen-containing component.

The solid promoters are generally added as chemical compounds to the catalyst prior to its use. As used herein, the term “compound” refers to the combination of a particular element with one or more different elements by surface and/or chemical bonding, such as ionic and/or covalent and/or coordinate bonding. The term “ionic” or “ion” refers to an electrically charged chemical moiety; “cationic” or “cation” being positive and “anionic” or “anion” being negative. The term “oxyanionic” or “oxyanion” refers to a negatively charged moiety containing at least one oxygen atom in combination with another element. An oxyanion is thus an oxygen-containing anion. It is understood that ions do not exist in vacuo, but are found in combination with charge-balancing counter ions when added as a compound to the catalyst. Once in the catalyst, the form of the promoter is not always known, and the promoter may be present without the counterion added during the preparation of the catalyst. For example, a catalyst made with cesium hydroxide may be analyzed to contain cesium but not hydroxide in the finished catalyst. Likewise, compounds such as alkali metal oxide, for example cesium oxide, or transition metal oxides, for example MoO3, while not being ionic, may convert to ionic compounds during catalyst preparation or in use. For the sake of ease of understanding, the solid promoters will be referred to in terms of cations and anions regardless of their form in the catalyst under reaction conditions.

It is desirable that the silver and optional one or more solid promoters be relatively uniformly dispersed on the zircon-modified carrier. A preferred procedure for depositing silver catalytic material and one or more promoters comprises: (1) impregnating a porous zircon-modified carrier according to the present invention with a solution comprising a solvent or solubilizing agent, silver complex and one or more promoters, and (2) thereafter treating the impregnated carrier to convert the silver salt to silver metal and effect deposition of silver and the promoter(s) onto the exterior and interior pore surfaces of the carrier. Silver and promoter depositions are generally accomplished by heating the carrier at elevated temperatures to evaporate the liquid within the carrier and effect deposition of the silver and promoters onto the interior and exterior carrier surfaces. Impregnation of the carrier is the preferred technique for silver deposition because it utilizes silver more efficiently than coating procedures, the latter being generally unable to effect substantial silver deposition onto the interior surfaces of the carrier. In addition, coated catalysts are more susceptible to silver loss by mechanical abrasion.

The silver solution used to impregnate the carrier is preferably comprised of a silver compound in a solvent or complexing/solubilizing agent such as the silver solutions disclosed in the art. The particular silver compound employed may be chosen, for example, from among silver complexes, silver nitrate, silver oxide or silver carboxylates, such as silver acetate, oxalate, citrate, phthalate, lactate, propionate, butyrate and higher fatty acid salts. Silver oxide complexed with amines is a preferred form of silver for use in the present invention.

A wide variety of solvents or complexing/solubilizing agents may be employed to solubilize silver to the desired concentration in the impregnating medium. Among those disclosed as being suitable for this purpose are lactic acid; ammonia; alcohols, such as ethylene glycol; and amines and aqueous mixtures of amines.

For example, Ag2O can be dissolved in a solution of oxalic acid and ethylenediamine to an extent of approximately 30% by weight. Vacuum impregnation of such a solution onto a carrier of approximately 0.7 cc/g porosity typically results in a catalyst containing approximately 25% by weight of silver based on the entire weight of the catalyst. Accordingly, if it is desired to obtain a catalyst having a silver loading of greater than about 25 or 30%, and more, it would generally be necessary to subject the carrier to at least two or more sequential impregnations of silver, with or without promoters, until the desired amount of silver is deposited on the carrier. In some instances, the concentration of the silver salt is higher in the latter impregnation solutions than in the first. In other instances, approximately equal amounts of silver are deposited during each impregnation. Often, to effect equal deposition in each impregnation, the silver concentration in the subsequent impregnation solutions may need to be greater than that in the initial impregnation solutions. In further instances, a greater amount of silver is deposited on the carrier in the initial impregnation than that deposited in subsequent impregnations. Each of the impregnations may be followed by roasting or other procedures to render the silver insoluble.

The catalyst prepared on the zircon-modified carrier may contain alkali metal and/or alkaline earth metal as cation promoters. Exemplary of the alkali metal and/or alkaline earth metals are lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium and barium. Other cation promoters include Group 3b metal ions including lanthanide series metals. In some instances, the promoter comprises a mixture of cations, for example cesium and at least one other alkali metal, to obtain a synergistic efficiency enhancement as described in U.S. Pat. No. 4,916,243, herein incorporated by reference. Note that references to the Periodic Table herein shall be to that as published by the Chemical Rubber Company, Cleveland, Ohio, in CRC Handbook of Chemistry and Physics, 46th Edition, inside back cover.

The concentration of the alkali metal promoters in the finished catalyst is not narrow and may vary over a wide range. The optimum alkali metal promoter concentration for a particular catalyst will be dependent upon performance characteristics, such as catalyst efficiency, rate of catalyst aging and reaction temperature.

The concentration of alkali metal (based on the weight of cation, for example cesium) in the finished catalyst may vary from about 0.0005 to 1.0 wt. %, preferably from about 0.005 to 0.5 wt. %. The preferred amount of cation promoter deposited on or present on the surface of the carrier or catalyst generally lies between about 10 and about 4000, preferably about 15 and about 3000, and more preferably between about 20 and about 2500 ppm by weight of cation calculated on the total carrier material. Amounts between about 50 and about 2000 ppm are frequently most preferable. When the alkali metal cesium is used in mixture with other cations, the ratio of cesium to any other alkali metal and alkaline earth metal salt(s), if used, to achieve desired performance is not narrow and may vary over a wide range. The ratio of cesium to the other cation promoters may vary from about 0.0001:1 to 10,000:1, preferably from about 0.001:1 to 1,000:1. Preferably, cesium comprises at least about 10, more preferably, about 20 to 100, percent (weight) of the total added alkali metal and alkaline earth metal in finished catalysts using cesium as a promoter.

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