MXPA99012054A - Propylene epoxidation using chloride-containing silver catalysts - Google Patents

Abstract

Direct oxidation of propylene to propylene oxide is accomplished using alkaline earth metal compound-supported silver catalysts containing an inorganic chloride promoter and a potassium promoter derived from a potassium salt containing a nitrogen oxyanion or precursor thereof.

Description

EPOXIDATION OF PROPYLENE USING SILVER CATALYST OÜE CONTAINS CHLORIDE FIELD OF THE INVENTION This invention relates to a process for the direct oxidation of propylene in propylene oxide in the vapor phase using molecular oxygen. In particular, the invention relates to the use of compositions comprising metallic silver supported on certain alkaline earth metal containing compounds to selectively catalyze the formation of epoxides. The realization of the catalyst is improved by incorporating an inorganic chloride promoter such as silver chloride together with a potassium promoter derived from a potassium salt comprising potassium cation and a nitrogen oxyanion or a precursor thereof.

BACKGROUND OF THE INVENTION The direct oxidation of ethylene in ethylene oxide by molecular oxygen is well known and is, in fact, the method currently used for the commercial production of ethylene oxide. The typical catalyst for this purpose contains metallic or ionic silver, optionally modified with various promoters and activators. Most of these catalysts contain a porous, inert carrier or carrier such as alpha-alumina upon which silver and promoters are deposited.

A review of the direct oxidation of ethylene in the presence of supported silver catalyst is provided by Sachtler et al. In Catalyst Reviews; Science and Engineering, 23 (1 &2), 127-149 (1981). It is also well known, however, that the catalyst and the reaction conditions that are most convenient for the production of ethylene oxide do not give comparable results in the direct oxidation of high olefins such as propylene. The discovery of processes capable of providing propylene oxide by direct oxidation in the vapor phase in higher yields than those currently achieved would be very desirable. Workers in the field have recognized for many years that the efficiency of a direct propylene epoxidation process catalyzed by a supported silver catalyst can be improved by introducing relatively small amounts of both a nitrogen oxide species such as NO and a chloride Volatile organic such as ethyl chloride to the feed charge containing propylene and oxygen. See, for example, U.S. Patent No. 5,387,751 (Hayden et al.) And Canadian Patents Nos. 1,282,772 (Thorsteinson) and 1,286,687 (Habenschuss et al.). However, the addition of volatile organic chlorides in the feedstock has certain practical disadvantages. The use of an organic chloride, even at the typically used parts per million levels adds significantly to raw material costs associated with the production of propylene oxide. Measures must be implemented to recover or trap any organic chloride in the effluent leaving the epoxidation reactor. These recovery methods may involve the generation of ionic chloride species, which tend to accelerate the corrosion of building materials used in the recovery section. Additionally, the recovery of organic chloride is not always quantitative; Unreacted chlorides or non-organic hydrolysates can escape the process, contributing to air pollution. Thus, it is readily apparent that the development of direct propylene epoxidation processes that do not require the addition of organic chlorides to the feedstock but that satisfactorily give high selectivity to propylene oxide will meet a great need in the field.

SUMMARY OF THE INVENTION A process for the epoxidation of propylene is provided wherein a feed charge comprising propylene and oxygen is contacted with a particular type of silver catalyst. The catalyst is composed of (a) a support; (b) a catalytically effective amount of metallic silver; (c) an amount of promotion of an inorganic chloride promoter, and (d) a promotion amount of a potassium promoter. The support is composed of an alkaline earth metal compound selected from the group consisting of alkaline earth metal carbonates (eg, calcium carbonate), alkaline earth metal titanates, and mixtures thereof. The inorganic chloride promoter can be derived from an inorganic chloride compound such as silver chloride or an alkaline earth metal chloride. The potassium promoter is derived from a potassium salt such as potassium nitrate comprising potassium cation and a nitrogen oxyanion or precursor thereof. The process described herein is capable of producing propylene oxide at remarkably high selectivity and productivity even in the absence of organic chlorides in the feedstock.

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to a process for the vapor phase oxidation of propylene in propylene oxide, that is, an epoxidation process carried out in the presence of an oxygen-containing gas and a particular kind of catalyst. silver supported. The support material used in the present invention in an alkaline earth metal compound selected from alkaline earth metal carbonates, alkaline earth metal titanates and mixtures thereof. Suitable carbonates for use include inorganic carbonates having a cation, which is an alkaline earth metal ion, particularly calcium, strontium, magnesium or barium, with calcium carbonate being most preferred. Carbonate supports of alkaline earth metals are described, for example, in Canadian Patent No. 1,282,772. The alkaline earth metal titanates comprise the classes of inorganic substances which contain an alkaline earth metal such as barium, strontium, calcium, or magnesium and a titanate species. The alkaline earth metal titanates suitable in this way can correspond to the empirical formula MtiO, M2TÍO4, and Mti205 wherein M preferably = Ba, Sr, Ca or Mg. Any of the conventional methods for preparing these substances can be used. Barium titanate, for example, can be prepared by heating a mixture of the correct proportions of barium carbonate and titanium dioxide at 1300 ° C until the reaction is complete. The strontium titanate can be obtained in pure form by calcining the double precipitate of strontium-titanium oxalate from a solution of titanium tetrachloride. The calcium titanate can correspond to the compound CaTi? 3 (CAS 12049-50-2), which occurs naturally as the perovskite mineral, but which can also be synthesized by heating equimolar amounts of the oxide at 1350 ° C. The term "calcium titanate" as used herein also encompasses substances having the formula 3CaO * 2Ti? 2 (CAS 12013-80-8) and 3CaO * TiO (CAS 12013-70-6). Magnesium titanates include the metatitanate MgTi03, and the orthotitanate Mg2Ti0 and the dititanate MgTi0205. These support materials are capable of providing exceptionally high selectivities of propylene oxide and have been found to be surprisingly superior to other support materials in this regard. The carriers of the present invention can exist in several ways. In one embodiment, the carrier is one in which the alkaline earth metal compound is predominant (i.e., at least 50% by weight) or, preferably, substantially the sole component of the support (i.e. the support consists essentially of one or more alkaline earth metal compounds). In other embodiments of the invention, the inorganic support material is used in conjunction with a solid substrate, i.e., a sub-support or substructure composed of a more conventional support material, such as aluminum oxide (preferably alpha-alumina). However, the support material of the alkaline earth metal compound will normally comprise at least 25% by weight (in many embodiments, at least 35% by weight) of the finished catalyst. The surface area of the alkaline earth metal composite support material is at least 0.6 p.s./g, preferably at least 1.5 m2./g. However, support materials of alkaline earth metal compounds having relatively high surface areas (for example 50 to 100 m2 / gram) are also effective for the purposes of this invention. This result was surprising in view of the preference generally expressed in the field of direct oxidation of olefins for supports of low surface areas (typically, 0.03 to 10. The surface area is measured by the conventional method B. E. T. using nitrogen or krypton described by Brunauer., Emmett and Teller in J. Am. Chem. Soc. 60, 309-16 (1938). The support materials used in the present invention can generally be described as porous or microporous and typically have water pore volumes of about 0.05 to 0.80 cubic centimeter / gram. The supported catalyst used in the present invention can be prepared by any known method of introducing silver and / or a promoter in soluble form in a support. A preferred method of introducing silver into the alkaline earth metal support is by an impregnation process in which a solution of a silver compound (which may be a silver salt or complex) in an amount sufficient to deposit the desired weight of the Silver in the support is dissolved in a convenient solvent or "complexing / solubilizing" agent. The solution can be used to impregnate the support by immersing the support in the impregnation solution containing the silver compound and forming a slurry or mud mixture. The sludge is dried and calcined by placing the mixture in an oven at approximately 100 to 120 ° C for 0.5 to 6 hours and then heating the mixture to a temperature of from about 250 to about 600 ° C for others of 1 to 6 hours. . This procedure carries out the drying of the support / silver mixture, removes the volatile components and reduces the present silver to its elemental form. The potassium salt and the inorganic chloride compound can be introduced to the catalyst, either simultaneously or separately, as impregnation solutions in a separate step or impregnation steps. Again, this can be done in a known manner to impregnate a porous material. Conveniently, this can be accomplished by placing the catalyst material in a container, evacuating the container and thereafter introducing the solution (s). Alternatively, the support can be sprayed or splashed with the impregnating solution (s). The excess solution can be allowed to drain or the solvent can be removed by evaporation under reduced pressure at a suitable temperature. The catalyst can then be dried at a moderate temperature (for example at 120 ° C) in an oven for from half an hour to 5 hours and / or calcined at a higher temperature between the impregnation steps. This procedure is known as a "sequential" or "sequential" method of preparation. In such a mode, the support is impregnated with only the inorganic chloride compound and the silver compound, dried, calcined and then impregnated with a solution of the potassium salt, followed by drying. The supported catalyst of alkaline earth metal compound can also be prepared by a "simultaneous" or "coincident" preparation method. With this method the potassium promoter and the inorganic chloride compound are included in the solution containing the silver compound used to impregnate the support. The choice of silver compound used to form the impregnating solution containing silver in a solvent or in a complexing / solubilizing agent is not particularly critical and any silver compound generally known in the art that is soluble and does not react with the solvent or complexing / solubilizing agent to form an undesired product can be employed. Thus, the silver can be introduced to the solvent or complexing / solubilizing agent, such as an oxide or a salt, such as a nitrate, carbonate, or carboxylate, for example, an acetate, propionate, butyrate, oxalate, malonate, malate, maleate, lactate, citrate, phthalate, fatty acid ester, and the like or combinations thereof.

In one embodiment, silver oxide (1) is used. Combinations of one or more silver compounds easily reduced to metallic form under calcination conditions (for example, oxide and silver carboxylates) together with silver chloride (which is not easily reduced to silver metal and can thus serve as a source of the inorganic chloride promoter in the catalyst) can also be used with advantage. Surprisingly, the silver chloride does not need to be applied to the support in the form of a solution in order to obtain an active and selective catalyst. In this way the silver chloride may be in the form of a slurry or slurry or be combined directly with the support as a dry solid. A large number of solvents or complexing / solubilizing agents can be conveniently used to form the impregnating solution containing silver compound. In addition to adequately dissolving the silver compound or converting it into a soluble form, a suitable solvent or complexing / solubilizing agent should be capable of being easily removed in subsequent steps, either by washing, volatilization, or oxidation process, or the like. The complexing / solubilizing agent should preferably also allow the solution to provide metallic silver in this finished catalyst to the extent of preferably 10 to about 60% metallic silver, based on the total weight of the catalyst. It is also generally preferred that solvents or complexing / solubilizing agents are easily miscible with water, since aqueous solutions can be conveniently employed. Among the materials which are suitable as complexing / solubilizing solvents for the preparation of the solution containing silver compound are alcohols, including glycols, such as ethylene glycol, amines (including alkanolamines such as ethanolamine and alkyldiamines such as ethylene diamine) and Caboxylic acids, such as lactic acid and oxalic acid, as well as aqueous mixtures of these materials. Typically, a solution having silver compounds is prepared by dissolving a silver compound in a convenient solvent or complexing / solubilizing agent such as, for example, a mixture of water, ethylenediamine, oxalic acid, silver oxide and monoethanolamine. The solution is mixed with support particles and drained. After that the particles are dried conveniently. As indicated above, after impregnation, the carrier particles impregnated with silver compound are treated to convert at least a portion of the silver compound into silver metal and thereby effect deposition of the silver on the surface of the support. As used herein, the term "surface", as applied to the support, includes not only the outer surfaces of the support, but also the internal surfaces, that is, the surfaces that define the pores or the internal portion of the particles. of the support. This can be done by treating the impregnated particles with a reducing agent, such as hydrogen and hydrazine and / or roasting at an elevated temperature, to decompose the silver compound and reduce the silver to its free metallic state. Certain solubilizing agents such as alkanolamines, alkyldiamines, and the like, can also function as reducing agents. Although at least a catalytically effective amount of metallic silver must be present in the finished catalyst (which means an amount that provides a measurable conversion of propylene to propylene oxide), the metal silver concentration is preferably from about 2% to 70%. %, by weight, based on the total weight of the catalyst. More preferably, the metal silver concentration ranges from about 10 to about 60% by weight. In certain embodiments of the invention, silver in non-metallic form may also be present in the finished catalyst: for example, the catalyst may contain from 0.1 to 5% by weight of silver chloride. It has been unexpectedly discovered that the presence of potassium in the preparation of the supported silver catalyst significantly increases the efficiency of the catalyst as a propylene epoxidation catalyst. Surprisingly, other alkali metals such as cesium, which are well known as promoters in the ethylene oxide art, fail to improve catalyst performance to an appreciable extent. Potassium is introduced by means of a potassium salt, with the selection of the particular anions as counterions of the potassium cation critical to achieve the optimum performance of the catalyst. A nitrogen oxyanion such as nitrate, nitrite, or another negative ion containing both nitrogens and oxygen atoms can serve as the anion. Potassium compounds containing species capable of being converted into nitrogen oxyanions under the catalyst preparation or epoxidation conditions, (ie, which are precursors of nitrogen oxyanions), are thus suitable for use. For example, potassium carbonates can be used to prepare the catalyst, the catalyst being exposed to NO, or to another species of nitrogen oxygen at an elevated temperature during a preconditioning step or during epoxidation. Preferred potassium salts include potassium nitrate, potassium nitrite, potassium carbonate, potassium bicarbonate, and mixtures thereof. The potassium salt which increases the efficiency can be introduced into the catalyst in any known manner. In this way, the impregnation and deposit of silver and potassium salt can be carried out coincidentally or sequentially.

For example, the support could be impregnated with a solution or solutions of the potassium salt and the silver compound, dried and then calcined to reduce the silver compound and generate the active supported silver catalyst. Alternatively, the potassium salt can be introduced into a catalyst that has already been impregnated with silver compound, dried and calcined. In order to perform the matching impregnation, the potassium salt must be soluble in the same solvent or complexing / solubilizing agent used with the silver compound impregnating solution. With a sequential procedure in which the silver compound is added first, any solvent capable of dissolving the salt that will not react with the silver compound nor leach it from the support is convenient. Aqueous solutions are generally preferred, but organic liquids, such as alcohols, may also be employed. Suitable methods for effecting the introduction of a potassium salt to a solid support are well known in the art. The potassium salt is used in an amount sufficient to provide a concentration of potassium promoter which results in an improvement in one or more of the catalytic properties (eg, selectivity, activity, conversion, stability, yield) of the silver catalyst supported compared to a catalyst that does not contain the potassium promoter. The precise amount will vary depending on variables such as the composition in the feedstock, the amount of metallic silver and inorganic chloride promoter contained in the catalyst, the surface area of the support, the process conditions, e.g., space velocity and temperature, and the morphology of the support. However, it has been found that a minimum of at least 0.1% by weight of the potassium promoter, calculated as a cation, based on the total weight of the catalyst must be present for the catalyst to exhibit a significant advantage over an analogous catalyst that does not contain potassium promoter. Potassium concentrations as high as 10% by weight can be used, although generally little additional benefit is realized beyond a concentration of 5% by weight. More preferably, the level of the potassium promoter is a corresponding amount of from about 0.5 to about 3% by weight of potassium. The other necessary component of the silver catalyst supported with the alkaline earth metal compound of this invention in an amount of an inorganic chloride promoter. Other promoters including metal promoters such as Mo, W, Re and the like, may also be present, but the catalyst is capable of operating at relatively high activity and selectivity even when it does not have these other substances. "Amount of promotion" means an amount that effectively works to provide an improvement in one or more of the catalytic properties of the catalyst compared to a catalyst that does not contain an inorganic chloride promoter. The exact form of the inorganic chloride promoter under epoxidation operating conditions is not known. In general, the choice of the inorganic chloride compound used as a source of inorganic chloride promoter is not considered critical although the introduction of substances known as poisons for supported silver oxidation catalysts should be avoided. Exemplary classes of inorganic chloride compounds suitable for use in the present invention, include, but are not limited to, alkali metal chlorides, (eg, potassium chloride, sodium chloride), alkaline earth metal chlorides (magnesium chlorides) , barium chloride, calcium chloride), and transition metal chlorides (eg, silver chloride, molybdenum pentachloride, tungsten pentachloride, rhenium pentachloride). In a particularly desirable embodiment of the invention, a combination of silver chloride (which functions as a source of inorganic chloride promoter) and a silver compound capable of being transformed into metallic silver under the conditions of catalyst preparation is used. The support is impregnated or otherwise combined with one or more inorganic chloride compounds. This can be done at the same time as the other catalyst components are added either before and / or after. In an advantageous and convenient embodiment of the invention, the inorganic chloride compound, the potassium salt and the silver compound are incorporated in the catalyst simultaneously. In another desirable embodiment, the potassium salt is introduced after the calcination of an impregnated support containing the silver compound and the inorganic chloride compound. The inorganic chloride compound can be incorporated into the catalyst in the form of a solution or, alternatively, as a solid. When the inorganic chloride compound is soluble in the same solvent used to form impregnation solutions of the silver compound and / or potassium salt, for example, it will often be convenient to use an impregnation technique to introduce the inorganic chloride compound into the catalyst . This impregnation can be carried out either in a sequential or coincidental manner with respect to the impregnation of the silver compound or the potassium salt. When the compound is applied as a solid, it is generally desirable to use a finely divided form of the inorganic chloride compound and / or grind, pulverize or otherwise intimately mix the catalyst components to achieve a relatively dispersed and uniform distribution of the chloride compound. inorganic with the support. It has been found that the minimum amount of inorganic chloride promoter present or deposited in the support or catalyst, necessary to measurably improve catalyst performance is 0.05% by weight of chlorine (measured as the element regardless of the form in which it is present). the promoter) based on the total weight of the supported silver catalyst. Generally speaking, the maximum amount of inorganic chloride promoter will be 2% by weight. The operation within the range of 0.1 to 1.5% by weight of chlorine is particularly advantageous. The degree of benefit obtained within the limits defined above will vary depending on the particular properties and characteristics, such as, for example, reaction conditions, prepared catalyst techniques, surface area and pore structure and chemical surface properties of the catalyst. support used, silver content of the catalyst, and potassium content of the catalyst. The presence of the indicated and claimed amounts of optional promoters of inorganic chloride promoter in this specification and claims does not prevent the use of other activators, promoters, enhancers, stabilizers, boosters, and the like. In the epoxidation process of this invention, propylene and an oxygen-containing gas (ie, a gas comprising molecular oxygen) are put together in a reactor in the presence of the catalyst previously described under conditions effective to carry out at least the partial oxidation of propylene to the corresponding epoxide. Typical epoxidation conditions include temperatures within the reactor zone in the range of about 180 to 320 ° C (more preferably 200 to 300 ° C, most preferably 220 to 280 ° C) and pressures of from about 1 to 60. atmospheres In order to favor the high selectivity of epoxide, it is desirable that the feed charge contains carbon dioxide. A gaseous species of nitrogen oxide (described in more detail hereinafter) is also desirably supplied to the reaction zone within the reactor by introducing these species to the feed charge containing propylene (new and / or recycled) and molecular oxygen, particularly when the catalyst is prepared by a process other than one in which potassium nitrate is added to the catalyst after calcination. Although an organic halide such as ethyl chloride may also be present in the feedstock, satisfactory results may be obtained in the absence of any organic halide. Thus, in a preferred embodiment of the invention, the concentration of organic halide in the feedstock is essentially 0 (ie, <1 ppm). Examples of nitrogen oxide species, suitable for introduction into the feedstock, include at least one of NO, NO2, N2O4, N2O3 or any gaseous substance capable of forming one of the aforementioned gases, particularly NO and N02. , under epoxidation conditions, and mixtures of one of the above, particularly NO, with one or more of CO, PH3, S03 and S02. It is NOT the most preferred nitrogen oxide species. The amount of gaseous nitrogen oxide species present is not critical, although it will often be advantageous, depending on the specific catalyst composition and the selected epoxidation conditions, to expose the catalyst to the nitrogen oxide species either before use (as described above). one step of preconditioning) or while it is being used in the epoxidation process. The optimum amount, is determined in part, by the particular potassium salt, the inorganic chloride compound and the optional metal promoters used and the concentrations thereof, and by other factors noted above that influence the optimum amount of salt of potassium and inorganic chloride promoter. Typically, a convenient concentration of nitrogen oxide species for the epoxidation of propylene is from about 0.1 to about 2,000 ppm by volume, when N2 is used as a ballast. The "oxygen" used in the reaction can be defined as including pure molecular oxygen, atomic oxygen and transient radical species derived from atomic or molecular oxygen capable of existing under epoxidation conditions, mixtures of another gaseous substance with at least one of the above , and substances capable of forming one of the above under epoxidation conditions. Typically oxygen is introduced into the reactor either as air, as commercial pure oxygen or another substance which under epoxidation conditions exists in both a gaseous state and in molecular oxygen forms. The feedstock may also contain a ballast or diluent, such as nitrogen, or another inert gas, particularly when air is used as the oxygen source. Variant amounts of water vapor may also be present. It is also desirable to include carbon dioxide as a component of the feedstock in the epoxidation process of this invention. The presence of carbon dioxide, within certain limits, has been found to provide a surprising improvement in catalyst performance within the scope of the invention. In particular, the selectivity to propylene oxide will generally increase as the concentration of carbon dioxide in the feedstock increases. Desirable improvements are generally observed using 1 to 60% volume of C02 in the feedstock, with 5 to 50% by volume of C02 being preferred. The concentration of carbon dioxide in the feedstock can be advantageously varied during the operation of the propylene epoxidation process described herein. For example, it has been found that upon starting the process using a new catalyst charge, the selectivity can often be significantly increased over an extended period of time by having carbon dioxide in the feedstock. As soon as the catalyst has been sufficiently conditioned to reach the desired level of performance, the C02 feed can be discontinued or interrupted. The gaseous components that are supplied to the reaction zone, or to the region of the reactor where the reactants and the catalyst are put together under epoxidation conditions, are generally combined before being introduced to the reactor. If desired, however, these components can alternatively be introduced separately or in various combinations. The feed charge having the particular composition previously described can be formed before or at the moment in which the individual components of it enter the reaction zone. The feed charge can use or incorporate a recycle stream from the reactor. The use of the term "feed loading" herein does not mean limiting the present process to the mode wherein all the gaseous components are combined before the introduction of said components into the reaction zone. The reactors in which the process and catalyst of the present invention are employed can be of any type known in the art. Below is a brief description of various reactor parameters that can be used in the present invention. Volume in% Component (or ppm) for Propylene Oxide Propylene approximately 2 to approximately 50%. oxygen about 2 to about 10%. organic halide 0 up to about 2,000 ppm, more preferably, < 1 ppm, more preferably, 0 nitrogen oxide species 0 up to approximately 2,000 ppm Other hydrocarbon other than propylene from 0 to about 80% carbon dioxide 0 to 60%, more preferably 5 to 50% nitrogen or other ballast gas residue.

Although the present invention can be used with any type and size of vapor phase epoxidation reactor, including both fixed bed and fluidized bed reactors known in the art, it is contemplated that the present invention will find a wider application in standard fixed bed. , multitubular reactors, such as those now in use as reactors of ethylene oxide. These generally include cooled wall reactors as well as uncooled or adiabatic wall reactors. The lengths of the tubes will typically vary from about 1.5 to about 18 meters but will frequently be in the range of from about 4.5 to about 13.5 meters. The tubes may have internal diameters of from about 1.27 centimeters to about 6.35 centimeters and are expected to typically be from about 2.03 to about 3.81 centimeters. A plurality of tubes packed with catalysts arranged in parallel within a convenient cover can be employed. GHSV generally ranges from about 500 to about 10,000 hr "1. Typically the GHSV values vary from about 800 to about 3,000 hr at pressures from about 1 to about 60 atmospheres, commonly from about 1.1 to about 30 atmospheres. be sufficient to convert from 0.5 to 70% preferably from 5 to 30% of the propylene.

EXAMPLES Example 1 This Example demonstrates the preparation and use of a supported silver catalyst according to the invention. Calcium carbonate (25.7 grams) is combined with silver chloride (1.8 grams), silver oxide (35.6 grams), ammonium molybdate (0.6 grams), ethylene diamine (20.56 grams), oxalic acid (20.60 grams), ethanolamine ( 7.20 grams), and distilled water (27.90 grams) and the resulting mixture was mixed in mortar for 4 hours. After drying for one hour at 110 ° C, the impregnated support was calcined at 300 ° C for 3 hours. After that, a solution of potassium nitrate (2.8 grams) in water (80 milliliters) was mixed with the calcined material for 20 minutes in a rotary evaporator. The final supported silver catalyst was obtained by drying at 110 ° C for 2 hours. The elemental composition of the catalyst was 54% by weight of silver, 19% by weight of calcium, 1.1% by weight of potassium, 0.55% by weight of Molybdenum, 0.94% by weight of nitrogen, and 0.50% by weight of chlorine. The epoxidation performance of the catalyst was evaluated in a fixed-bed reactor at 1200 hr "1 GHSV, 250 ° C, 4 mol% of propylene and 8 mol% of oxygen without nitrogen oxide species, alkyl halides or dioxide carbon in the feed load The results obtained are summarized in the following table.

Compared to catalysts of similar composition calcined at 500 ° C (see Example 2 below), the catalyst described above disintegrates more rapidly and appears to have a better overall performance.

Example 2 The procedure of Example 1 was repeated, except that the calcination temperature was increased to 500 ° C. The obtained catalyst had the following elemental composition: Element O, or Weight Mo 0. 49 Cl 0. 70 Ca 1? ¡0 Ag 4S 1.0 N 0. 21 K 0. 57 The epoxidation performance of the catalyst thus prepared was evaluated in a fixed bed reactor at 2.1 kilogram / square centimeter using a feed charge containing 8 by volume of propylene, 8% by volume of oxygen, and varying the amount of gaseous additives . The results obtained are shown in the following table.

These runs show that the catalyst of this invention is capable of selectively catalyzing the formation of propylene oxide even in the absence of ethyl chloride or nitric oxide in the feedstock. Introducing these additives into the feedstock does not substantially alter the behavior of the catalyst.

Example 3 A silver catalyst supported on calcium carbonate was prepared according to the invention using an elemental composition (calculated by the proportions of reagents used) corresponding to 50% by weight of silver, 0.05% by weight of Molybdenum (derivative of pentachloride of molybdenum), 0.6% by weight of chlorine (derived from molybdenum pentachloride), 2% by weight of potassium (derived from potassium nitrate, added to the catalyst after calcination). The effects of having gaseous promoters in the feedstock and of pretreating them with a gaseous organic chloride (ethyl chloride) before using it in the oxidation of propylene were studied for the catalysts of the previous description. The pretreatment with ethyl chloride was carried out in Run A under the following conditions: 500 ppm EtCl, 5% by volume of oxygen, nitrogen residue, 250 ° C, 2.1 kilogram / square centimeter, 1200 hr "1 GHSV, 20 hours. No pretreatment with ethyl chloride was carried out in Run B. The results obtained are summarized in Table 1. Runs A-2 and B-2 were continuations of Runs Al and Bl, respectively, where the feed of Carbon dioxide was discontinued after a period of time.All runs were performed at 250 ° C, 2.1 kilogram / square centimeter and 1200 hr "1 GHSV using a feed charge containing 10% by volume of propylene and 6% by weight. volume of oxygen. A comparison of Run A-1 with Run B-1 and Run A-2 with Run B-2 shows that pretreatment with ethyl chloride has little or no effect on the behavior of the catalyst; the use of the modified catalyst with an inorganic chloride promoter makes it possible to eliminate the pretreatment step of the catalyst.

Pretreatment Co2, Conversion Time Selectivity oxide Productivity PO Run EtCl% Vol current - propylene hour% prolylene% kg / m3 hr A-l Yes 12 16 5 49 8.0 A- 2 Yes 0 64 9 35 9.6 B-l NO 16 88 5 47 8.0 B-2 NO 0 114 11 36 12.8 fifteen Example 4 A silver catalyst supported on calcium carbonate having an elemental composition corresponding to 50% by weight of silver, 2% by weight of potassium (from KNO3, added sequentially after calcination) and 0.6% by weight was prepared Chlorine weight (from AgCl). Using a feed charge containing 10% by volume of propylene, 5% by volume of oxygen and 200 ppm of NO at 250 ° C and a GHSV of 1200 hr "1, the conversion of propylene was 11%, the selectivity of propylene oxide was 30%, and the productivity of propylene oxide was 9.76 kg / m3 hr. To demonstrate the benefits of including an organic chloride promoter in the catalyst, a comparative epoxidation run was performed under the same conditions using a supported silver catalyst prepared without AgCl or any other source of chloride (40% by weight of silver, 2% by weight of potassium, added as KNO3 by co-impregnation) The behavior of the catalyst was inferior in every respect to the modified catalyst with AgCl: 4.8% of propylene conversion, 3.6% of propylene oxide selectivity, 0.48 kg / pr hr of propylene oxide productivity.

Example 5 The preparation of a supported silver catalyst promoted by modified tungsten with an inorganic chloride compound is illustrated in this example. Calcium carbonate (34.0 grams) is combined with silver chloride (1.3 grams), silver oxide (25.75 grams), ammonium tungstate oxide (0.5 grams), ethylenediamine (20.56 grams), oxalic acid (20.6 grams), ethanolamine (7.20 grams) and distilled water (27.9 grams) and the resulting mixture was milled for 4 hours. After drying one hour at 110 ° C, the impregnated supported was calcined at 300 ° C for 3 hours. After that, a solution of potassium nitrate (3.2 grams) in water (80 milliliters) was mixed with the calcined material for 20 minutes in a rotary evaporator. The final supported silver catalyst was obtained by drying at 110 ° C for 2 hours. The elemental composition by analysis was as follows: 41% by weight of silver, 22% by weight of calcium, 2.1% of potassium, 0.7% by weight of Tungsten, 0.5% by weight of chlorine, and 0.96% by weight of nitrogen. The behavior of the epoxidation of the catalyst was evaluated in a reactor mixed rearwardly at 232 ° C, 1200 hr, GHSV, and 2.1 kilogram / square centimeter using a feed charge containing 4% by volume of propylene, 8% by volume of oxygen and 10 ppm NO. The propylene conversion was 20%, the selectivity to propylene oxide was 42%, and the productivity of propylene oxide was 10.4 kg / m3 hr (3500 ppm PO in the output stream). A fixed bed run using the same catalyst at 288 ° C, 4800 hr "1 GHSV and 2.1 kilogram / square centimeter (the same feed charge composition, except with 50 ppm NO) yielded 22% propylene conversion, 29 % selectivity of propylene oxide, and a propylene oxide productivity of 32 kg / pr hr (2500 ppm propylene oxide in output stream).

Example 6 The preparation of a supported silver catalyst promoted by rhenium modified with an inorganic chloride compound is illustrated by this example. Calcium carbonate (12.85 grams) was combined with silver chloride (0.9 grams), silver oxide (17.80 grams), ammonium perrenate (0.41 grams), potassium carbonate (1.55 grams), ethylene diamine (10.28 grams), acid oxalic (10.30 grams), ethanolamine (3.20 grams) and distilled water (14.0 grams) and the resulting mixture was milled for 4 hours. After drying for one hour at 110 CC, the impregnated supported was calcined at 300 ° C for 3 hours. The elemental composition of the catalyst by analysis was as follows: 50% by weight of silver, 0.5% by weight of Rhenium, 2% by weight of potassium, and 0.6% by weight of chlorine. The epoxidation behavior of the catalyst was evaluated in a fixed bed reactor at 250 ° C, 1200 hr "1 GHSV and 2.1 kilogram / square centimeter using a feed charge containing 10% by volume of propylene, 5% by volume of oxygen , and 200 ppm of NO The conversion of propylene was 10.9%, the selectivity of propylene oxide was 42.4%, and the productivity of propylene oxide was 14.4 kg / m3 hr (4740 ppm of propylene oxide in the output current).

Claims (16)

1. A process for propylene epoxidation comprising contacting a feed charge comprising propylene and oxygen with a supported silver catalyst composed of: (a) a support containing an alkaline earth metal compound selected from the group consisting of carbonates of ferrous alkali metals, alkaline earth metal titanates, and mixtures thereof; (b) a catalytically effective amount of metallic silver; (c) an amount of promotion of an inorganic chloride promoter; and (d) a promotion amount of a potassium promoter derived from a potassium salt comprising potassium cation and a nitrogen oxyanion or precursor thereof.

2. The process of Claim 1 wherein the inorganic chloride promoter is derived from an inorganic chloride compound selected from the group consisting of alkali metal chlorides, alkaline earth metal chlorides, transition metal chlorides, and mixtures thereof. same.

3. The process of Claim 1 wherein the feedstock essentially has no organic halide.

4. The process of Claim 1 wherein the alkaline earth metal compound is selected from the group consisting of calcium carbonate, calcium titanate, and mixtures thereof.

5. The process of Claim 1 wherein the feedstock additionally comprises nitrogen oxide species.

6. The process of Claim 1 wherein the feedstock additionally comprises carbon dioxide. The process of Claim 1 wherein the potassium salt is selected from the group consisting of potassium carbonate, potassium bicarbonate, potassium nitrate, potassium nitrite, and mixtures thereof. 8. The process of Claim 1 wherein the contact is carried out at a temperature of from 180 ° C to 320 ° C. The process of Claim 1 wherein the supported silver catalyst has a corresponding elemental composition from 10 to 60% by weight of silver, from 0.05 to 2% by weight of chlorine, and from 0.5 to 10% by weight of potassium and comprises at least 25% by weight of the alkaline earth metal compound. The process of Claim 1 wherein the additionally supported silver catalyst comprises a promoter amount of a promoter metal selected from the group consisting of Mo, Re, W and mixtures thereof. The process of Claim 1 wherein the supported silver catalyst is prepared by impregnating the support with a silver compound, an inorganic chloride compound, and the potassium salt and thereafter the impregnated support is calcined under the effective conditions to reduce at least a portion of the silver compound to metallic silver. The process of Claim 1 wherein the supported silver catalyst is prepared by impregnating the support with a silver compound and an inorganic chloride compound, calcining the impregnated support under effective conditions to reduce at least a portion of the silver compound to metallic silver, and impregnate the calcined support with the potassium salt. The process of Claim 1 wherein the inorganic chloride promoter is derived from a transition metal chloride selected from the group consisting of silver chloride, molybdenum pentachloride, and mixtures thereof. 14. A process for propylene epoxidation comprising contacting a feed charge comprising propylene and oxygen at a temperature of from 200 ° C to 300 ° C with a supported silver catalyst consisting of: (a) a support which consists of calcium carbonate; (b) a catalytically effective amount of metallic silver; (c) a promoter amount of an inorganic chloride promoter derived from an inorganic chloride compound selected from the group consisting of alkali metal chlorides, alkaline earth metal chlorides, transition metal chlorides and mixtures thereof; and (d) an amount of promotion of a potassium promoter derived from a potassium salt selected from the group consisting of potassium carbonate, potassium bicarbonate, potassium nitrate, potassium nitrite, and mixtures thereof. The process of Claim 14 wherein the inorganic chloride compound is a transition metal chloride selected from the group consisting of silver chloride, molybdenum chloride, and mixtures thereof. 16. A supported silver catalyst comprising (a) at least 25% by weight of a support consisting of an alkaline earth metal compound selected from the group consisting of ferrous alkali metal carbonates, and alkaline earth metal titanates, and mixtures thereof; (b) from 10 to 60% by weight of silver; (c) from 0.05 to 5% by weight of chlorine; Y (d) from 0.5 to 10% by weight of potassium.