Vapor phase oxidation of propylene to propylene oxide

IS 2 158 486 T3

DESCRIPTION

Vapor phase oxidation of propylene to propylene oxide.

Field of the invention

This invention relates to a process for the direct oxidation of propylene to propylene oxide in the vapor phase using molecular oxygen and particular supported silver catalysts.

Background of the invention

The direct oxidation of ethylene to ethylene oxide by molecular oxygen is well known and, in fact, is the method currently used for the commercial production of ethylene oxide. The topical catalyst for this purpose contains metallic or ionic silver, optionally modified with various promoters and activators. Most such catalysts contain an inert porous support or carrier on which the silver and promoters are deposited. A review of the direct oxidation of ethylene in the presence of supported silver catalysts is provided by Sachtler et al.

Catalyst Reviews: Science and Engineering, 23 (1 & 2), 127-149 (1981).

Another example of direct oxidation can be found in European patent no. 0.318.815, which describes the synthesis and use of silver-based catalysts in the direct oxidation of olefins to epoxides. US patent n^°. 3,888,889 describes the production of propylene oxide by propylene oxidation in the presence of an elemental silver catalyst, in which the silver catalyst has been modified by adding a Group VB or VIB metal on its surface.

Chemical Abstracts n^°. 8658z, vol 117 (1992) refers to the epoxidation of olefins by oxygen in the presence of a silver catalyst supported by Al2O3, gaseous organic halides, members of semi-redox reaction pairs, carbon dioxide and water. Also, the German patent n^°. 2,454,972 describes a catalyst for the production of alkylene oxides, comprising silver on a heat resistant support, in which the silver has been impregnated with a solution of a decomposable silver compound.

It is also well known, however, that catalysts and reaction conditions that are best suited for the production of ethylene oxide do not give comparable selectivities in direct oxidation of higher olefins such as propylene. The discovery of direct vapor phase oxidation processes capable of providing propylene oxide with a higher selectivity than currently achievable would therefore be highly desirable. Summary of the invention

This invention provides a process for epoxidizing propylene, which comprises contacting at a temperature of 180^°C to 350^°C:

(i) a feed stream consisting of propylene, an oxygen-containing gas, an organic halide, carbon dioxide, and optionally a nitrogen oxide species, with (ii) a supported silver catalyst comprising (a ) a refractory inert solid support comprising an alkaline earth metal carbonate;

(b) a catalytically effective amount of silver;

(c) a promoting amount of a potassium salt comprising the potassium cation and nitrogen dioxide or an oil precursor; and (d) a promoter amount of a molybdenum promoter.

In a particularly desirable embodiment of the invention, the potassium salt comprises a potassium cation and an anion selected from the group consisting of nitrate, nitrite, and other anions capable of undergoing a displacement or other chemical reaction to form nitrate and / or nitrite anions. under epoxidation conditions. In another embodiment of the invention, the supported silver catalyst comprises a potassium salt of a molybdenum oxyanioon such as potassium molybdate or the like. Such salts are able to function simultaneously as promoter components (c) and (d).

Surprisingly, it has been found that a high selectivity in favor of propylene oxide can be achieved even in the absence of any species of nitrogen oxide, such as NO, in the feed stream.

To achieve the maximum selectivities in favor of propylene oxide, it is crotic to operate the process by contacting a feed stream with a supported silver catalyst at a temperature of 200^°C to 300^°C, feed stream consisting of 2 to 50 volume percent propylene, 2 to 10 volume percent oxygen, 50 to 500 ppm aliphatic chloride, and 5 to 25 volume percent dioxide of carbon, said supported silver catalyst comprising a carbonate of an alkaline earth metal selected from kaolcic carbonate, baoric carbonate and strontium carbonate, from 25 to 60 percent by weight of silver, from 0.5 to 3 percent by weight , calculated as potassium cation of potassium nitrate and 0.05 to 2.5 weight percent, calculated as Mo, of a molybdenum promoter derived from a molybdenum oxyanioon compound. Alternatively, a potassium salt consisting of the potassium cation and a molybdenum oxyanioon can be used instead of potassium nitrate and a molybdenum promoter.

An important advantage of the invention is that it is highly selective and is even capable of delivering propylene oxide in relatively high yield. Furthermore, the process can be carried out continuously for a long period of time without significant deterioration in catalytic activity.

Detailed description of the invention

The present invention is directed to a process for the vapor phase oxidation of propylene to propylene oxide, that is, an epoxidation process carried out in the presence of an oxygen-containing gas and a particular class of supported silver catalysts.

IS 2 158 486 T3

The support material used in the present invention is selected from one of several carbonate-containing carrier materials. The carbonate used is an inorganic carbonate having a cation that is an ion of an alkaline earth metal, in particular calcium, strontium, magnesium or barium, with my preferred calcium, strontium and barium. Such carbonates are capable of providing exceptionally high selectivities in favor of propylene ioxide and have been found to be surprisingly superior to other support materials in this regard. Suitable carbonate supports are described in, for example, Canadian Patent No. 1,282,772. The vehicles of the present invention can exist in various forms. In one embodiment, the carrier is one in which carbonate is predominant (that is, at least 50% by weight) or, preferably, substantially the exclusive component of the support (that is, the support consists essentially of one or more metal carbonates. alkaline earth). In other embodiments of the invention, the inorganic support material is used in conjunction with a solid substrate, that is, a subsupport or substructure composed of one or more conventional support materials such as alumina (preferably alumina). The latter type of support may employ the carbonate material as a coating of relatively small individual particles of the substructure or sub-support, or of a large unit such as a three-dimensional armazoin having a honeycomb-like structure.

A granular form of the carbonate support material is preferred in the present invention, particularly when used as the sole or predominant component of the support. Commercially available carbonate materials that are suitable for use in the present invention can be obtained as powders, such powders can be converted into the preferred granular form by conventional methods, including those described in Canadian Patent No. 1,282,772. As I will describe in detail later, the carbonate carrier can then be impregnated, or coated, with a solution containing a silver compound that is subsequently reduced to elemental silver.

Alternatively, as described hereinbelow, the powdered carbonate support material can be combined with an appropriate silver-containing solution, such as is conventionally used to impregnate solid supports to form a slurry or paste. This material can then be spread on a suitable surface and dried or calcined at an appropriate temperature, such as about 500^°C. This results in the obtaining of a carbonate support that supports the elemental silver on it. The catalyst can then be impregnated with solutions of the potassium salt and / or the molybdenum promoter and subsequently dried. Alternatively, the potassium salt and / or the molybdenum promoter can be dissolved in the same impregnating solution containing silver used to form the paste or coating suspension with the carbonate material.

The carbonate support material, before or after incorporating the silver, the potassium salt and / or the molybdenum promoter, can be formed into a composite material in a form suitable for use in the manufacture of propylene oxide. Composite materials can be shaped by any suitable technique. For example, composites can be formed by compressing the support materials in a mold having the desired configuration. The size of the particles can be selected to be appropriate for the formation of the composite material and often their largest dimension is in the range of about 0.001 to about 5 mm.

When using coated catalysts, that is, catalysts in which the carbonate material coats a substructure, a suspension of the carbonate material, in the form of powder or granules, can be mixed with the particles of the support material and subsequently dried. As with the predominantly or exclusively carbonate support materials described above, the coated catalysts can also be prepared using a solution of a silver compound or the silver compound, a potassium salt and a molybdenum promoter, or the compound of silver and the potassium salt or the silver compound and the molybdenum promoter to form the suspension, followed by suitable drying and calcination.

The specific surface area of ​​carbonate support materials generally ranges from 0.6 to about 14 μm²/ g, preferably about 1.5 to about 10 µm²/ g. However, carbonate support materials having a specific surface area greater than 14 μm are also effective for the purposes of this invention.²/ g. Surface air is measured by the conventional BET method using nitrogen or krypton described by Brunnauer, Emmett and Teller in J. Am. Chem. Soc., 60, 309-16 (1938).

The carrier materials used in the present invention can generally be described as porous or microporous and typically have a pore volume for water of about 0.05 to 0.80 cm.³/ g.

Supported silver catalysts are typically used as individual particles of irregular shape and size. This is true for supports that are predominantly or exclusively carbonate as well as carbonate-coated supports. However, in some cases, the supports, in particular the carbonate coated supports, may have a particular shape and size, which is especially true for the substrates used with the carbonates. Typically the sub-supports are shaped as aggregates or "pills" of a size and configuration that makes them usable in tubular reactors. These pills can be made by conventional extrusion and calcination techniques. The pills generally have a size ranging from about 2mm to about 15mm, preferably from about 3mm to about 12mm. The size is chosen to be consistent with the type of reactor used. Generally, in fixed bed reactor applications sizes between about 3 mm and

ES 2 158 486 T3 about 10 mm are best suited for typical commercially used tubular reactors. The forms of the useful carrier aggregates for the purposes of the present invention can vary widely and can be any of the conventional forms used in heterogeneous catalysis.

It has been unexpectedly discovered that only exceptionally high selectivity in favor of the desired propylene oxide product is obtainable by careful selection of the supported silver catalyst composition. The catalyst must contain not only a carbonate support of an alkaline earth metal and silver, but also a potassium salt as well as a molybdenum promoter or, alternatively, a potassium salt of an oxyanioan of molybdenum. The support may be present exclusively or predominantly as carbonate, in which case it is referred to herein as "carbonate support." Corresponding catalysts that include such a support are referred to as "carbonate supported catalysts." When the carbonate is as a coating of the substrate or sub-support or in the presence of a substrate or sub-support, the support is called a "carbonate-coated support" and, when the support is used in a complete catalyst, the designation of the catalyst is a "coated catalyst. with carbonate ”. As used herein, the term "coated" is not intended to imply that a substance necessarily forms a layer on a second substance or envelops a second substance, but merely refers to the process used in the preparation of such material.

Carbonate supports and carbonate coated ones can be prepared as above or purchased commercially. The carbonate supported catalysts of the present invention can be prepared by any known method to introduce silver and / or a potassium salt such as potassium nitrate, in soluble form, and / or a molybdenum promoter, in soluble form, on a support. A preferred method of introducing silver to the carbonate support is by an impregnation process in which a solution of a soluble silver salt or complex is dissolved in a suitable solvent or "complexing / solubilizing" agent in an amount sufficient to deposit the weight. desired silver on the vehicle. The solution can be used to impregnate the support or vehicle by dipping the vehicle in the impregnating solution containing silver and forming a pasty mixture or suspension. The suspension is dried and calcined then by placing the mixture in an oven or oven at approximately 100 ° C to 120 ° C for 0.5 to 6 hours and then heating the mixture at a temperature of approximately 250 ° C to approximately 600 ° C for another 1-6 hours. The process dries the carbonate / silver mixture, removes volatile components, and reduces the silver present to its elemental form.

The required potassium salt can be introduced into the catalyst as an impregnating solution in a separate impregnation step. This can be done in any known way to impregnate a porous material. This can be conveniently done by placing the catalyst material in a container, evacuating the container and then introducing the salt solution. Alternatively, the impregnating solution can be spread on the support or it can be sprayed with the impregnating solution. 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 (eg, 120 C) in an oven for half an hour to five hours. Such a procedure is known as the "sequential" or "consecutive" method of preparation. The carbonate supported catalyst can also be prepared by a "simultaneous" or "coincident" method of preparation. In this method, the potassium salt is included in the solution containing the silver compound used to impregnate the carbonate support.

Carbonate-coated catalysts are prepared by coating a suitable subsupport or substructure material, preferably alumina and most preferably α-alumina, with a carbonate-containing suspensioan. The suspension may contain only the carbonate, in which case the carbonate-coated support is further treated as above to produce a silver catalyst or a carbonate-coated potassium nitrate or silver catalyst. Alternatively, a carbonate / silver compound suspension, or a carbonate / silver compound / potassium salt suspension, or a carbonate / silver compound / molybdenum promoter suspension, or a suspension carbonate / silver compound / potassium salt / molybdenum promoter. Thus, in a sequential procedure, particles or pellets of a suitable sub-support material, such as α-alumina, are coated with a suspension of a carbonate material and a soluble silver salt or complex dissolved in a complexing / solubilizing agent. The particles or pellets are subsequently drained and calcined in an oven at a temperature of about 250-600 ° C for about three minutes to about four hours, the duration of heating being inversely proportional to the temperature used. The catalyst is then impregnated in the manner described above with a potassium salt solution and then with a solution of the molybdenum promoter and then dried. Carbonate-coated supports can also be formed by a coincident procedure in which a carbonate suspension / silver compound / potassium salt / molybdenum promoter is used to coat particles or pellets of a suitable sub-support. After draining, the catalyst is dried at the temperature and duration indicated above for carbonate-coated catalysts prepared by the sequential procedure. The particular silver compound or salt used to form the silver-containing impregnating solution in a solvent or complexing / solubilizing agent is not particularly critical and any silver salt or compound generally known in the art that is soluble and non-reactive can be employed. with

ES 2 158 486 T3 the solvent or complexing / solubilizing agent to form an unnecessary compound. Thus, silver can be introduced into the solvent or complexing / solubilizing agent as an oxide or a salt such as nitrate, carbonate or carboxylate, for example an acetate, propionate, butyrate, oxalate, malonate, malate, maleate, lactate, citrate, phthalate, generally silver salts of higher fatty acids, and the like.

A large number of solvents or complexing / solubilizing agents can suitably be used to form the silver-containing impregnation solution. In addition to being capable of adequately dissolving silver or converting it into a soluble form, a suitable solvent or complexing / solubilizing agent must be capable of being easily removed in later stages, either by a washing, volatilization or oxidation procedure, or by others. Similar. Preferably, the complexing / solubilizing agent should also allow the solution to provide silver in the finished catalyst in the quantum of about 25 to about 60 percent silver based on total catalyst weight. It is also generally preferred that the solvents or complexing / solubilizing agents are readily miscible with water, since aqueous solutions can conveniently be employed. Among the materials that have been found to be suitable as solvents or complexing / solubilizing agents for the preparation of solutions containing silver are tin alcohols, including glycols such as ethylene glycol, amines (including alkanolamines and alkyldiamines), and carboxylic acids such as laoctic acid, as well as aqueous mixtures of such materials.

Topically, a silver-containing solution is prepared by dissolving silver in a suitable solvent or complexing / solubilizing agent such as, for example, a mixture of water, ethylenediamine, oxaolic acid, silver oxide, and monoethanolamine. The solution is then mixed with carrier particles and drained. The particles are then suitably dried.

As indicated above, after impregnation, the vehicle particles impregnated with silver are treated to convert the silver salt or complex to silver metal and effect the silver deposit on the surface of the support. As used herein, the term "surface", applied to the support, includes not only the external surfaces of the vehicle but also the internal surfaces, that is, the surfaces that delimit the pores or the internal portion of the support particles. This can be done by treating the impregnated particles with a reducing agent such as hydrogen or hydrazine, and / or by roasting at an elevated temperature to decompose the silver compound and reduce the silver to the free metallic state. Certain solubilizing agents such as alkanolamines, alkyldiamines, and the like can also function as reducing agents.

An alternative method to obtain a silver catalyst on carbonate support, suitable for use in the process of this invention once it has been modified with the potassium salt and the molybdenum promoter, is the co-precipitation of silver carbonate and carbonate of silver. an alkaline earth metal from their respective nitrates or other water soluble salts, as described in US Pat. 2,825,701 (incorporated herein by reference in its entirety). For example, a silver-containing carbonate support can be prepared by preparing an aqueous solution containing a silver salt such as silver nitrate and an alkaline earth metal salt such as kaolic nitrate, preferably in a 1: 1 molar ratio. 1: 4, adding dropwise, while stirring, a solution of alkali metal carbonate such as potassium carbonate in water to form a coprecipitate of silver carbonate and alkaline earth metal carbonate. Subsequently the coprecipitate must be washed, dried, impregnated with the molybdenum promoter and / or the potassium salt, and / or treated with a reducing agent or calcined or subjected to a similar operation to reduce the silver to the free metallic state. The sequence of such additional steps may vary as desired. The coprecipitate may coat, or be deposited on, a different porous, granular refractory material, as described in the aforementioned patent.

Although at least a catalytically effective amount of silver must be present in the finished catalyst (meaning an amount that provides a measurable conversion of propylene to propylene oxide), preferably the concentration of silver is from about 2 percent to 70 percent. by weight, based on total catalyst weight. Most preferably, the concentration is about 25 to 60 percent by weight.

The presence of certain specific potassium salts in the supported silver catalyst has been found to significantly enhance the efficiency of said catalyst as a propylene epoxidation catalyst. The anion must be a nitrogen oxyanion (that is, an anion or negative ion containing both nitrogen and oxygen atoms) such as nitrate or nitrite, or an oil precursor (that is, an anion capable of undergoing displacement or other chemical reaction to form a nitrogen oxyanion under the conditions of epoxidation or catalyst preparation). The preferred potassium salt is potassium nitrate (KNO3).

The efficiency enhancing potassium salt can be introduced into the catalyst in any known manner. Thus, the impregnation and depositing of silver and a potassium salt can be carried out coincidentally or sequentially as described above. The preferred method is a sequential impregnation of the support, in which the initial introduction of the silver-containing solution is followed by drying of the silver-containing support and heating and / or chemical reduction of the silver. This support is then impregnated with a solution of potassium salt. The described sequential impregnation process, when a Mo promoter is present, advantageously provides a catalyst whose operational start-up is faster and which reaches steady state within 1000 minutes of exposure to the reaction stream. How I know

ES 2 158 486 T3 will be explained later in more detail, it is also preferred to introduce the molybdenum promoter into the catalyst before impregnation with the potassium salt. Sequential impregnation is also desirable when the feed stream does not contain NO or other species of nitrogen oxide. In another desirable embodiment of the invention, however, the potassium salt and the molybdenum promoter are simultaneously introduced using a potassium salt of a molybdenum oxyanioan, such as potassium molybdate.

In order to achieve a matching impregnation, the potassium salt must be soluble in the same solvent or complexing / solubilizing liquid used with the silver impregnating solution. With the preferred sequential procedure in which silver is added first, any solvent capable of dissolving the salt that neither reacts with the silver nor dislodges from the support is suitable. Aqueous solutions are generally preferred, but organic liquids such as alcohols can also be employed. Suitable procedures for introducing the potassium salt into the solid support are well known in the art.

The required potassium salt is added in sufficient quantity to provide a further improvement in the catalytic properties (eg, selectivity, activity, conversion, stability, yield) of the supported silver catalyst compared to a catalyst that does not contain the salt. potassium (referred to here as the "promoter amount"). The precise amount will vary depending on variables such as the nitrogen oxide species and its concentration used in the epoxidation process, the concentration of other components in the feed stream, the amount of silver contained in the catalyst, the specific surface area of ​​the support. , the parameters of the process, for example, space velocity and temperature, and morphology of the support. In general, however, a suitable concentration range of the added potassium salt, calculated as cation, is from about 0.15 to about 5 percent by weight, preferably from about 0.5 to about 3 percent, on the basis of total catalyst weight. Most preferably, the salt is added in an amount of about 1.5 to about 2.5 percent by weight of K.

It has been unexpectedly found that the addition of a promoting amount of molybdenum (that is, an amount that operates effectively to provide an improvement in one or more catalytic properties of a catalyst compared to a catalyst containing no molybdenum) to a silver catalyst On a support containing a potassium salt, it improves the selectivity in favor of propylene oxide of such a catalyst. The amount of promotion observed using molybdenum significantly exceeds that obtained when transition metal compounds other than molybdenum are introduced into the catalyst. The exact form of the promoter under epoxidation operating conditions is not known. It is believed that the molybdenum promoter will not be present in the catalyst in the elemental form since the promoter is applied to the catalyst in the form of ions, salts, compounds and / or complexes and the reducing conditions generally used to reduce silver to metallic silver they are usually not sufficient to reduce molybdenum to that elemental form.

The promoter deposited on the support or present on the catalyst is believed to be in compound form, most likely in the form of an oxygen- or oxide-containing compound. In a presently preferred embodiment, the promoter is applied to the catalyst in oxyaniaonic form, that is, in the form of an oxygen-containing anion or negative ion. Examples of molybdenum anions that can be suitably applied include molybdate, dimolybdate, paramolybdate, other iso- and hetero-polymolybdates, phosphomolybdate, and the like. Anions can be prepared by reactive dissolution of various non-anioanic materials such as oxides such as MoO3, etc., as well as other materials such as carbonates, sulfates, halides, oxyhalides, hydroxyhalides, hydroxides, sulfides, etc., of molybdenum. As previously mentioned, the use of potassium salts of molybdenum oxyanions (eg, potassium molybdate) obviates the need to use different compounds to introduce the necessary potassium salt and moly promoter.

The carrier is impregnated with ions, salt (s), compound (s) and / complex (s) of the molybdenum promoter. This can be done at the same time as other catalyst components are added, or before and / or after. Preferably, the molybdenum promoter and silver are incorporated into the catalyst before adding the potassium salt.

The preferred amount of promoter compound present or deposited on the support or catalyst ranges from about 0.05 to 2.5 weight percent molybdenum (measured as the element regardless of how the promoter is present) on the basis. of the total weight of the supported silver catalyst. The degree of benefit obtained within the defined limits will vary depending on the particular properties and characteristics such as, for example, reaction conditions, catalyst preparative techniques, the specific surface area and pore structure and surface chemical properties of the support used, content silver of the catalyst and potassium content of the catalyst.

The presence of the promoter amount of molybdenum indicated and claimed herein and in the claims does not preclude the use of other activators, promoters, enhancers, stabilizers, enhancers, and the like. However, it has unexpectedly been found that the present invention is capable of operating with relatively high efficiency even in the absence of other promoters such as rhenium.

The promoter compounds, salts and / or complexes used in the preparation of the present catalysts are molybdenum compounds, salts and / or complexes that can be solubilized in an appropriate solvent. Preferably, the solvent is a water-containing solvent. Most preferably the solvent is the same solvent used to deposit the silver and the po6 salt.

ES 2 158 486 T3 tasic. Preferred promoter compounds are oxyanionic compounds of molybdenum, preferably oxyanionates of ammonium and alkali metals such as potassium molybdate, cesium molybdate, rubidium molybdate, ammonium molybdate, lithium molybdate, sodium molybdate, and the like.

Propylene and an oxygen-containing gas (that is, a gas comprising molecular oxygen) are put together in a reactor in the presence of the previously described catalyst under conditions effective to effect at least partial epoxidation of the propylene. Typical epoxidation conditions include temperatures within the reaction zone of the reactor on the order of about 180 ° C to 350 ° C (more preferably 200 ° C to 300 ° C) and pressures of about 1 to about 30 atmospheres. Inlet pressures can be as low as 96.5 to 517 kPa. To achieve a satisfactorily high selectivity in favor of epoxide, it is necessary that the reactor feed stream contains carbon dioxide as well as an organic halide (described in more detail later). Optionally, a kind of gaseous nitrogen oxide (described in more detail later) can be supplied to the reaction zone, inside the reactor, introducing said species in the feed stream containing propylene (fresh and / or recycled) and molecular oxygen.

The feed stream should contain an organic halide, optionally a halogenated hydrocarbon other than propylene, such as a saturated halogenated hydrocarbon. The feed stream must additionally contain carbon dioxide; The presence of carbon dioxide, contrary to what is expected according to the previous technique, has been found to substantially improve the selectivity in favor of the propylene oxide obtained in the present process.

Examples of nitrogen oxide species suitable for optional introduction into the feed stream include at least one of NO, NO2, N2O4, N2O3 or any gaseous substance capable of forming one of the mentioned gases, in particular NO and NO2, under epoxidation conditions, and mixtures of one of the above, in particular NO, with one or more of CO, PH3, SO3 and SO2. NO is the most preferred nitrogen oxide species. Unexpectedly it has been found, however, that the inclusion of such a nitrogen oxide species in the feed stream is not necessary, since a high selectivity in favor of propylene oxide can be achieved even without such additive, particularly in embodiments in where the catalyst is prepared by a sequential impregnation procedure (that is, the Mo promoter and silver are introduced before the potassium salt).

The amount of the gaseous nitrogen dioxide species present (if any) is not critical. The optimal amount is determined, in part, by the particular potassium salt used and its concentration and by other factors mentioned above that influence the optimal amount of potassium salt. Typically, a suitable concentration of the nitrogen oxide species for propylene epoxidation is from about 0.1 to about 2,000 ppm, by volume, when N2 is used as ballast. When NO is used in propylene epoxidation, the preferred concentration is about 5 to about 2,000 ppm, more preferably about 20 to 500 ppm, by volume, with N2 ballast. However, as previously explained, the concentration of the nitrogen oxide species can be essentially zero.

The "oxygen-containing gas" used in the reaction can be defined as including pure molecular oxygen, ataomic oxygen, any kind of transient radical derived from atoamic or molecular oxygen capable of existing under epoxidation conditions, mixtures of another gaseous substance with al minus one of the above, and substances capable of forming one of the above substances under epoxidation conditions. Such oxygen-containing gas is typically introduced into the reactor as air, commercially pure oxygen or another substance which under epoxidation conditions exists in a gaseous state and forms molecular oxygen.

The gaseous components that are supplied to the reaction zone or that region of the reactor in which the reactants and the catalyst are brought together under the epoxidation conditions, are generally combined before being introduced to the reactor. If desired, however, alternatively such components may be introduced separately or in various combinations. The feed stream having the particular composition previously described can thus be formed before its individual components enter the reaction zone or at the same time they enter 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. A brief description of several of the reactor parameters that can be used in the present invention is presented below.

In addition to propylene and oxygen (and optionally a species of nitrogen oxide), the feed stream must also contain a performance enhancing organic halide (preferably an aliphatic, eg alkyl, halide). Preferably the organic halide is a volatile compound, that is, a substance that predominantly exists in gaseous form under the conditions of temperature and pressure present in the reaction zone. The normal boiling point of the organic halide is preferably less than about 100^° Atmospheric pressure. Compounds containing 1 to 10 carbon atoms are preferred. Most preferably the aliphatic halide is a chloride species. The term aliphatic halide includes saturated halides and unsaturated halides such as ethylene dichloride, ethyl chloride, vinyl chloride, methyl chloride, and methylene chloride. Ethyl chloride is preferably used as the organic halide. Mixtures of different organic halides can be used. The amount of organic halide to use will vary depending on a variety of factors, including the concentration

ES 2 158 486 T3 of the propylene being oxidized, the particular potassium salt and the nitrogen oxide species and their respective concentrations, as well as other factors mentioned above that influence the optimal amount of potassium salt and nitrogen oxide species . However, a suitable range of organic halide concentrations in propylene oxidation typically is about 0.1 to about 2,000 ppm, more preferably about 50 to 500 ppm, by volume, of the feed stream. Furthermore, a hydrocarbon, in particular a saturated hydrocarbon such as methane, propane or ethane, may be included in the feed stream. The feed stream may also contain a ballast or diluent such as nitrogen or other inert gas, particularly when air is used as the oxygen-containing gas. Variable amounts of water vapor may also be present.

Carbon dioxide is another essential component of the feed stream in the epoxidation process of this invention. The presence of carbon dioxide within certain laminations has been found to provide a surprising improvement in selectivity in favor of propylene oxide. A desirable enhancement of selectivity is generally observed by using 1 to 30% by volume CO2 in the feed stream, with 5 to 25% by volume CO2 being preferred.

The components of the feed stream will be very adequately present in the amounts indicated in the following table.

<td>Component</td><td>% by volume (or ppm) for propylene oxidation</td>

<td>Propylene</td><td>From about 2 to about 50%</td>

<td>Oxygen</td><td>About 2 to about 10%</td>

<td>Organic halide</td><td>From about 0.1 to about 2,000 ppm, more preferably, about 50 to 500 ppm</td>

<td>Rust species</td><td></td>

<td>nitrogen</td><td>0 to about 2,000 ppm</td>

<td>Hydrocarbon that</td><td></td>

<td>not propylene</td><td>0 to about 5%</td>

<td>Carbon dioxide</td><td></td>

<td>not</td><td>1 to 30%, more preferably 5 to 25%</td>

<td>Nitrogen or other</td><td></td>

<td>diluent gas</td><td>Rest</td>

Although the present invention can be used with any size and type of vapor phase epoxidation reactor, including fixed bed and fluidized bed reactors known in the art, it is contemplated that the present invention will find a very wide application in standard reactors of multitubular fixed bed, such as those now used as reactors for ethylene oxide. Generally, these reactors include both the cooled wall reactors as well as the adiabatic or non-cooled wall reactors. Typically the lengths of the tubes can vary from about 1.52 m to about 18.3 m, but often the lengths will be in the range of about 4.57 m to about 13.7 m. The tubes can have an internal diameter of from about 12.7mm to about 63.5mm and are typically expected to have a diameter of from about 20.3mm to about 38.1mm. A plurality of catalyst packed tubes arranged in parallel within a suitable envelope may be employed. GHSVs generally range from about 500 to about 10,000 h^-1. Typically GHSV values ​​vary from approximately 800 to approximately 3,000 h^-1 at pressures from about 101 KPa to about 3040 KPa (about 1 to 30 atmospheres), commonly from about 111 KPa to about 507 KPa (about

1.1 to about 5 atmospheres). The contact time should be sufficient to convert 0.5 to 70%, preferably 5 to 30% of the propylene.

Examples

A preferred method of preparing a supported silver catalyst suitable for use in the process of the invention is as follows:

Stage I

A 0.47 L wide-mouth jar containing a Teflon-coated stir bar is placed on a stir plate. 41.12 g of ethylenediamine and then 40.80 g of distilled water are added to the jar. Mix well and then slowly add 41.20 g of oxalic acid and allow it to dissolve completely. 71.20 g of silver (I) oxide are slowly added and allowed to dissolve completely. 14.40 g of ethanolamine and 1.20 g of molybdic acid diamoanic salt are added and mixed well. 15.0 g of distilled water and 51.4 g of calcium carbonate are added. Add 10 stone balls to mix, cover the jar and grind with the balls for 4 hours, dry at 100 µ for 1 hour and then calcine at 300 µ for 4 hours.

Stage II

The salids obtained in step I are ground to a powder. 160 ml of distilled water are added to a 500 ml round bottom flask with one neck. 6.2 g of potassium nitrate are dissolved in the water and then 120 g of ground solids from stage I are added. It is mixed for 20 minutes on a rotary evaporator and then vacuum is applied and heated to 60 ° C. It is continued rotary evaporation until the contents of the flask appear dry. The resulting salids are dried at 110^°C for 2 hours. The resulting catalyst can then be pelleted and screened through a 14 x 30 mesh.

IS 2 158 486 T3

Example 1

According to the invention, a supported silver catalyst was prepared comprising a kaolic carbonate support, 54% Ag, 2% K (added as potassium nitrate) and 0.5% Mo (added as diammonium acid salt). molobdic). Silver catalyst on support (2 cm³) was loaded into a tubular reactor and tested under the following operating conditions: 10% propylene, 5% oxygen, 200 ppm ethyl chloride, 75 ppm nitric oxide, 10% carbon dioxide and the remainder, nitrogen; GHSV 1200 h^-1, total measured pressure 207 KPa, 245^°C. A propylene conversion of 3.2% was obtained with a selectivity to propylene oxide of 58-59%. Example 2

Example 1 was repeated, except that the propylene concentration was lowered to 5%, the carbon dioxide concentration was increased to 20%, and the temperature was lowered to 240.^°C. A propylene conversion of 4.5% was obtained with a selectivity to propylene oxide of 5961%.

Comparative Examples 3-4

These examples demonstrate the detrimental effect of omitting the molybdenum promoter from the supported silver catalyst. A catalyst was prepared comprising a colic carbonate support, 43% Ag and 1.7% K (added as potassium nitrate). The catalyst (2 cm³) was then loaded into a tubular reactor and tested under the following operating conditions: 10% propylene, 5% oxygen, 50 ppm ethyl chloride, 200 ppm nitric oxide and the remainder, nitrogen; GHSV 1200 h^-1, total measured pressure 207 KPa, 250^°C. A propylene conversion of 11% was obtained with a selectivity to propylene oxide of only 33%.

A second supported silver catalyst was prepared comprising a colcic carbonate support, 54% Ag, 2% K (added as potassium nitrate) and 0.5% Mo (added as diamoonic salt of molobdic acid). The catalyst was then tested under the operating conditions described above for the Mo-free catalyst. The propylene conversion was 11.2% with a propylene oxide selectivity of 39.5%. The addition of even small amounts of molybdenum to the catalyst clearly increased the epoxide selectivity by quantum significantly under comparable test conditions. Further improvements in selectivity can be obtained by adding carbon dioxide to the reactor feed mixture according to the invention.

Comparative Example 5

A supported silver catalyst (2 cm³) that comprised a caolic carbonate support, 52% Ag and 2.1% K (added as KNO₃), but without molybdenum. The propylene conversion was 5.1%, the propylene oxide selectivity was 50-52% (significantly lower than that observed in Example 1 using a supported silver catalyst containing molybdenum).

Comparative Example 6

A supported silver catalyst containing potassium molybdate was prepared as follows. They were combined in a 120 ml jar containing 5 ceramics balls, ethylenediamine (5.14 g), distilled water (5.17 g), oxoalic acid dihydrate (5.15 g), silver (I) oxide (8 , 94 g), ethanolamine (1.87 g), potassium molybdate (0.779 g) in distilled water (1.88 g) and kaolic carbonate (6.45 g). The jar was closed and put in a ball mill for 4 hours. The resulting mixture was then heated to 110^°C for 1 hour; the temperature was then raised at the rate of 10^°C / min up to a maximum of 300^°C and held at 300^°C for 3 hours. The roasted solids obtained were ground to powder, the powder was pelleted and 14 x 30 mesh sieved. The resulting supported catalyst contained 51% by weight of Ag, 1.6% by weight of K and 1.8% by weight of Mo .

The described catalyst was charged into a tubular reactor and tested under the following conditions: 10% propylene, 5% oxygen, 50 ppm ethyl chloride, 200 ppm nitric oxide, nitrogen balance; GSVH 1200 h^-1; total measured pressure 207 KPa; 250^°C. The conversion of propylene was 13%, with a selectivity to propylene oxide of 40%. A further advantage of selectivity in favor of OP is expected with the incorporation of 1 to 30 volume percent carbon dioxide into the feed stream according to the present invention.

Example 7

This Example demonstrates that the presence of NO or other species of nitrogen oxide in the feed stream is not necessary to achieve remarkably high selectivity in favor of propylene oxide. In accordance with the present invention, a supported silver catalyst was prepared comprising a kaolcic carbonate support, 53% Ag, 1.1% K (added as potassium nitrate) and 0.54% Mo (added as potassium nitrate). diammonium salt of molobdic acid). Catalyst charged (2 cm³) in a tubular reactor and was tested under the following conditions: 10% by volume of propylene, 5% by volume of oxygen, 200 ppm of ethyl chloride, 10% by volume of carbon dioxide, remainder of nitrogen; GHSV 1200 h^-1; total measured pressure 207 KPa; 250^°C. A propylene conversion of 2.8% was achieved with a selectivity to propylene oxide of 58%.