Vapor phase oxidation of propylene to propylene oxide

Vapor phase oxidation of propylene to propylene oxide

Field of invention

The present invention relates to a method of directly oxidizing propylene to propylene oxide in the gas phase using molecular oxygen specially supported silver catalyst.

Background of the invention

The direct oxidation of ethylene to ethylene oxide by molecular oxygen is well known. In fact, this method is currently used for the commercial production of ethylene oxide. Typical catalysts for these applications contain metallic silver or ionic silver selectively modified with a variety of promoters and activators. Most catalysts contain a porous inert support or carrier on which silver and promoters are deposited. A summary of the direct oxidation of ethylene in the presence of a supported silver catalyst is provided by Sachtler et al. in<u>Catalyst Reviews: Science and Enginering</u>, 23(1&amp;2), 127-149(1981)。

However, it is well known that the most suitable catalyst and reaction conditions for the production of ethylene oxide do not have comparable selectivity when directly oxidizing higher olefins such as propylene. It is most hoped to have a selectivity comparable to the current one, and to provide a gas-phase direct oxidation method of propylene oxide with a higher selectivity.

Summary of the invention

The present invention provides a method for epoxidizing propylene, which includes contacting at a temperature of 180°C to 350°C:

(i) A feed stream comprising propylene, which contains oxygen gas, organic halide, carbon dioxide and selective nitrogen oxide species and;

(ii) A supported silver catalyst includes

(a) Inert refractory solid support composed of alkaline earth metal carbonate;

(b) A catalytically effective amount of silver;

(c) Potassium salt to promote the dosage, which includes potassium cation and nitrogen anion or their precursors; and

(d) Molybdenum accelerator for promoting dosage.

In a particularly preferred embodiment of the present invention, potassium salt includes potassium cation and anions selected from nitrate, nitrite and other anions that can undergo substitution or other chemical reactions to form nitrate and/or nitrite anions under epoxidation conditions . In another embodiment of the present invention, the supported silver catalyst includes a potassium salt of molybdenum oxide anion, such as potassium molybdate. These salts can be used as accelerator components (c) and (d) at the same time.

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

In order to achieve the highest propylene oxide selectivity, the key to the process is to contact the supported silver catalyst with a feed stream at a temperature of 200°C to 300°C; the feed stream includes 2 to 50 v% propylene, 2 to 10 v% oxygen, 50 to 500 ppm aliphatic chloride, and 5 to 25 v% carbon dioxide; and the supported silver catalyst includes alkaline earth metal carbonate selected from calcium carbonate, barium carbonate, and strontium carbonate, 25 to 60 wt % Silver, 0.5 to 3 wt% (calculated as potassium cation) potassium nitrate, and 0.05 to 2.5 wt% (calculated as Mo) molybdenum promoter derived from molybdenum oxide anion compounds. In addition, potassium salts including potassium cations and molybdenum oxide anions can be used to replace potassium nitrate and molybdenum accelerators.

One of the major advantages of the present invention is its high selectivity, and therefore can provide relatively high yields of propylene oxide. In addition, the method can be operated continuously for a long time without significant degradation of the catalyst activity.

Detailed description of the invention

The present invention relates to a method for vapor-phase oxidation of propylene to propylene oxide, and a method for epoxidation in the presence of an oxygen-containing gas and a special type of supported silver catalyst.

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 containing a cation, and the cation is an alkaline earth metal ion, especially calcium, strontium, magnesium or barium, with calcium, strontium and barium being the most preferred. These carbonates can provide extremely high propylene oxide selectivity and have been found to be surprisingly superior to other support materials. Suitable carbonate supports are described, for example, in Canadian Patent 1,282,772. The carrier of the present invention can exist in a variety of forms. In a specific example, the carrier is wherein the carbonate is the main component of the support (that is, at least 50% by weight) or preferably substantially the only component (that is, the support is mainly composed of one or more alkaline earth metal carbonates). In other specific examples of the present invention, the inorganic support material and the solid substrate are used in combination with a secondary support or a secondary structure composed of a more conventional support material such as alumina (preferably α-alumina). The support of the latter type can be coated on the sub-structure or each relatively small particle of the sub-support using a carbonate material or on a larger unit of the three-dimensional backbone with a honeycomb structure.

In the present invention, the particulate carbonate support material is preferred, especially when used as the sole or main component of the support. Commercially available carbonate materials suitable for the present invention can be obtained as powders, which can be converted into better granular forms by conventional methods, including the method described in Canadian Patent 1,282,772. (Detailed later) The carbonate support can then be impregnated or coated with a silver-containing compound solution, and then reduced to elemental silver.

In addition, as described below, the powdered carbonate support material can be combined with a suitable silver-containing solution, such as a conventional solution for impregnating a solid support, to form a slurry or paste. This material can then be spread on a suitable surface and dried and sintered at a suitable temperature, such as 500°C. The carbonate support thus obtained has silver supported on it in an elemental state. The support body can then be impregnated with a potassium salt and/or molybdenum accelerator solution, followed by drying. As for the alternative, the potassium salt and/or molybdenum accelerator can be dissolved in the silver-containing impregnation solution (the same as the carbonate material used to form the coating paste or slurry).

The carbonate support material can be formed into a shaped composite before or after blending silver, potassium salt and/or molybdenum accelerator, which is suitable for producing propylene oxide. The complex can be formed by any suitable technique. For example, the composite body can be formed by compressing the support material into a mold having a desired configuration. The particle size can be selected to be suitable for forming the composite body, and the main size is usually in the range of about 0.001 to about 5 mm.

When a coated catalyst, that is, a catalyst coated with a carbonate material on the secondary structure, is used, the powdered or granular carbonate material slurry can be mixed with the support material particles and then dried. Like the aforementioned main or only carbonate support material, the coated catalyst can also use silver compounds, or silver compounds, potassium salts and molybdenum promoters, or silver compounds and potassium salts, or solutions of silver compounds and molybdenum promoters to form a slurry. Then dry and braise appropriately.

The surface area of the carbonate support material is usually 0.6 to about 14 m²/g is preferably about 1.5 to 10 m²/g range. However, the surface area is higher than 14 m²/g of carbonate support material can also effectively achieve the cost of invention. The surface area is borrowed from the familiar BET method using Brunauer, Emmett and Teller in<u>J. Am, Chem.Soc. 60,</u> 309-16 (1938) Nitrogen or Krypton measurement.

The carrier material used in the present invention is generally described as porous or microporous, and typically has a water pore volume of about 0.05 to 0.80 cc/g.

The supported silver catalyst is typically used as individual particles with irregular shapes and sizes. It is true for the main or only carbonate support and carbonate-coated support. However, in some cases, the support body, especially the carbonate-coated support body, may have a special shape and size, which is particularly true for the secondary support body used in carbonate. Typically the secondary supports are shaped into aggregates or "pellets" of the size and configuration that can be used in tubular reactors. These pellets can be formed by conventional extrusion and firing techniques. The pellets are usually in the size range of about 2 mm to about 15 mm, preferably about 3 mm to about 12 mm. The size is selected in accordance with the type of reactor used. Generally, in fixed bed reactor applications, the size in the range of about 3 mm to about 10 mm is most suitable for typical commercial tubular reactors. The shape of the carrier aggregate that can be used in the present invention can vary widely and can be any shape conventionally known in the heterogeneous catalyst industry.

Surprisingly, it was discovered that only through careful selection of the supported catalyst composition can the extremely high selectivity of the desired propylene oxide product be obtained. The catalyst not only needs to contain alkaline earth metal carbonate support and silver, but also potassium salt and molybdenum promoter or other potassium salt of molybdenum oxide anion. The support body can be mainly or absolutely carbonate, and each is defined as a carbonate support body. The corresponding catalyst containing this kind of support is named carbonate supported catalyst. When the carbonate is coated on the substrate or subsupport or coexist with the substrate or subsupport, the support is named carbonate-coated support; and when the support is used as a complete value changer, the catalyst is named "carbonate-coated catalyst" ". As used herein, the term coating does not imply that the substance must form a layer or encapsulation on the second substance, but merely indicates the manufacturing process of the material.

Carbonate- and carbonate-coated supports can be prepared or purchased as described above. The carbonate-supported catalyst of the present invention can be prepared by any method, including introducing silver and/or potassium salts such as potassium nitrate in a soluble form and/or a molybdenum promoter in a soluble form to the support. The standard method for introducing silver to the carbonate support is by dipping, in which a soluble salt solution or silver complex (the amount of which is sufficient to deposit the required amount of silver on the carrier) is dissolved in a suitable solvent or "complex/enhanced" Solvent". The solution can be used to impregnate the support or carrier. The carrier is immersed in the silver-containing impregnation solution to form a paste mixture or slurry. The slurry is then dried and sintered, and the mixture is placed in an oven or furnace at about 100 to about 120°C for 0.5 to 6 hours, and then the mixture is heated at a temperature of about 250 to about 600°C for another 1 to 6 hours. This procedure completes the drying of the carbonate/silver mixture, removing the volatile components and reducing the existing silver to its elemental state.

The required potassium salt can be introduced to the catalyst as an impregnation solution in the respective impregnation steps. Again, any known method of impregnating porous materials can be used. The catalyst material can be conveniently placed in a container, and the container is evacuated and then the salt solution is introduced. In addition, the support body can be sprayed or splashed with the dipping solution. Then allow the excess solution to drain or evaporate in vacuo at an appropriate temperature to remove the solvent. The catalyst is then dried in an oven at a medium temperature, such as 120°C, for half an hour to 5 hours. This kind of process is called "sequential" or "continuous" method. Carbonate-supported catalysts can also be prepared by "simultaneous" or "coincidence" preparation methods. Using this method, the potassium salt is contained in the silver-containing compound solution used to impregnate the carbonate support.

The carbonate-coated catalyst is prepared by using a carbonate-containing slurry to coat an appropriate substructure or subsupport material, preferably alumina, and most preferably α-alumina. It may only contain carbonate. In this case, the carbonate-coated support is processed as described above to produce silver, or silver and potassium nitrate, or nitrite carbonate-coated catalyst. In addition, carbonate/silver compound slurry, or carbonate silver compound/potassium salt slurry, or carbonate/silver compound/molybdenum accelerator slurry, or carbonate/silver compound/potassium salt/molybdenum accelerator slurry can be sequenced or overlapped. Production. In this way, in a sequential process, particles or pellets of appropriate secondary support materials, such as α-alumina, are coated with carbonate materials and soluble salts or silver complexes and are dissolved in the complex/solubilizer slurry. The granules or pellets are then drained and sintered in an oven at a temperature of about 250°C to about 600°C for about 3 minutes to about 4 hours. The heating time is inversely proportional to the use temperature. The only chemical agent is then impregnated with the potassium salt solution in the manner described above, followed by the molybdenum accelerator solution, and then dried. Carbonate-coated support can also be formed by a superposition process, in which a carbonate/silver compound/potassium salt/molybdenum accelerator slurry is used to coat appropriate supporter particles or pellets. After draining, the catalyst is dried at the temperature and time for preparing the carbonate-coated catalyst by sequential procedures described above. There are no special restrictions on the special silver salt or complex formed in the solvent or complex/solubilizer containing silver impregnation solution. Any silver salt or compound known in the industry can be used, which is soluble in solvent or complex. / Solubilizer without reverse dredging with solvent or complex solubilizer to form undesired products. In this way, silver can be introduced into the solvent or complex/solubilizer as oxide or salt, such as nitrate, carbonate or carboxylate such as acetate, propionate, butyrate, oxalate, malonate, apple Salts, maleates, lactates, citrates, and phthalates are usually silver salts of high-carbon fatty acids.

A variety of solvents or complex/solubilizers are suitable for forming the silver-containing impregnation solution. In addition to dissolving silver properly or turning silver into a soluble form, proper solvents or complex solubilizers need to be easily removed with subsequent steps, and removed by washing, evaporation or oxidation procedures. The preferred complex/solubilizer can also provide silver in the finished catalyst to the extent of about 25 to about 60% silver based on the total weight of the catalyst. It is also generally preferable that the solvent or the complex solubilizer is easy to be miscible with water because the aqueous solution is convenient to use. Solvents or complex/solubilizing materials suitable for preparing silver-containing solutions, such as alcohols containing glycols such as ethylene glycol, amines (containing alkanolamines and alkyl diamines), and carboxylic acids such as lactic acid and the like Aqueous mixture of materials.

Typically, silver-containing solutions are prepared by dissolving silver in a suitable solvent or a complex solubilizer such as water, ethylenediamine, oxalic acid, silver oxide, and a mixture of monoethanolamine. The solution is then mixed with the support particles and drained. The granules are then dried appropriately.

As mentioned above, the silver-impregnated carrier particles are treated to convert the silver salt or complex into silver metal so that the silver is deposited on the surface of the support. As used herein, the term "surface" applied to the support body includes not only the outer surface of the carrier but also the inner surface, that is, the pores and the inner surface of the support body particles are defined. A reducing agent such as hydrogen or can be used to treat the impregnated particles and/or baking at elevated temperature to decompose the silver compound and reduce the silver to its metal free state. Certain solubilizers such as alkanolamines, alkanediamines, etc. can also be used as reducing agents.

Once the catalyst is modified with potassium salt and molybdenum accelerator, another method to obtain a carbonate-supported silver catalyst suitable for the method of the present invention is to co-precipitate silver carbonate and alkaline earth metal carbonate from various nitrates or other water-soluble salts, such as It is described in US Patent No. 2,825,701 (and hereby for reference). For example, the preparation method of the silver-containing carbonate support body is by preparing an aqueous solution containing silver salt such as silver nitrate and alkaline earth metal salt such as calcium nitrate, preferably with a molar ratio of 1:1 to 1:4, and adding alkali metal carbonate dropwise during stirring. For example, potassium carbonate forms a co-precipitation of carbonate and alkaline earth metal carbonate in aqueous solution. Then co-precipitation is washed, dehydrated, impregnated with molybdenum promoter and/or potassium salt and/or treated with reducing agent or sintering to reduce silver to a metal free state. The sequence of these additional steps can be changed if necessary. The co-precipitation can be coated or deposited on the different granular porous refractory materials described in the aforementioned patents.

Although at least a catalytically effective amount of silver needs to be present in the finished catalyst (indicating the amount of propylene converted into propylene oxide in a measurable manner), the silver concentration is preferably about 2% to 70% by weight based on the total weight of the catalyst. Furthermore, the silver concentration is about 25 to 60 wt%.

It is found that the presence of certain specific potassium salts in the supported silver catalyst can significantly promote the efficiency of the propylene epoxidation catalyst around the catalyst. The anion needs to be a nitrogen oxide anion (that is, an anion containing nitrogen and oxygen atoms) such as nitrate and nitrite or their precursors (that is, replacement or other chemical reactions can be carried out under epoxidation or catalyst preparation conditions to form nitrogen and oxygen Anion of anion). Potassium Nitrate (KNO₃) Is the preferred potassium salt.

The efficiency promoting potassium salt can be introduced to the catalyst by any known means. In this way, the impregnation and deposition of silver and potassium salts can be performed simultaneously or sequentially as described above. The preferred method is to sequentially immerse the support body, in which the silver-containing solution is initially introduced, followed by drying the silver-containing support body and heating and/or chemically reducing the silver. Then this support body is immersed in a potassium salt solution. When the molybdenum accelerator is present in the aforementioned sequential impregnation process, it is better to obtain a catalyst that breaks more quickly and can reach a stable state within 1000 minutes of flow. (Detailed description later) It is also preferable to introduce the molybdenum promoter to the catalyst before impregnation with the potassium salt. Sequential impregnation is also desirable when the feed stream does not contain NO or other nitrogen oxide species. However, in another preferred embodiment of the present invention, the potassium salt and the molybdenum accelerator are simultaneously introduced using the potassium salt of molybdenum oxide anion, such as potassium molybdate.

For superposition impregnation, the potassium salt must be soluble in the same solvent or complex increasing solution used for the silver impregnation solution. It is preferable to add silver first in the sequence of procedures. Any solvent that can dissolve silver is suitable for solvents that neither react with silver nor dope with silver filter in the support. Aqueous solutions are generally preferred, but organic liquids such as alcohols can also be used. The proper procedure for introducing the potassium salt to the solid support is well known in the industry.

The required potassium salt is sufficient to provide the amount of improvement in one or more of the catalytic properties (such as selectivity, activity, conversion rate, stability, yield) of the supported silver catalyst compared to the catalyst without potassium salt (herein referred to as "promotion Dosage"). The correct amount will be determined based on the following variables, such as the nitrogen oxide species used in the epoxidation process and its concentration, the concentration of other components in the feed stream, the silver content of the catalyst, the surface area of the support, and the process conditions such as space velocity and temperature and support Body shape. However, it is usually calculated as a cation to add the potassium salt to an appropriate concentration based on the total weight of the catalyst of about 0.15 to about 5%, preferably 0.5 to about 3% by weight. The optimal amount of salt added is about 1.5 to about 2.5 wt% K.

Surprisingly, it has been found that adding a promoting amount of molybdenum (that is, an effective amount that can provide one or more catalytic properties improvement compared to a catalyst containing no molybdenum) to prepare a potassium-containing salt supported silver catalyst can improve the selectivity of reduced oxygen propane. The degree of promotion observed with molybdenum significantly exceeds the degree of introduction of transition metals other than molybdenum into the catalyst. It also oxidizes the correct position of the accelerator under the operating conditions. The phase instrument molybdenum promoter does not exist on the catalyst in elemental form. The reason is that the promoter is applied to the catalyst in the form of ionic salt compounds and/or complexes. Generally, the reducing conditions used to reduce silver to metallic silver are not sufficient to reduce molybdenum to form. Element state.

The accelerator deposited on the support or in the catalyst is in the form of a mixed material, preferably in the form of an oxygen-containing or oxygen-containing anion compound. In the most preferred embodiment, the accelerator is applied to the catalyst in the form of cationic anions, that is, in the form of oxygen-containing anions. Examples of molybdenum anions that can be suitably applied include molybdate, dimolybdate, trimermolybdate, other iso- and heteropolymolybdate, molybdate phosphate, and the like. Anions can be passed through a variety of non-anionic materials such as oxides such as MoO₃And other materials such as molybdenum magnetite, sulfate, halide, oxyhalide, hydroxy halide, hydroxide, sulfide, etc. are prepared by reactive dissolution. The use of the potassium salt of molybdenum oxide anion (such as potassium molybdate) as mentioned above can avoid the need to use different compounds to introduce the required potassium salt and molybdenum promoter.

The carrier is impregnated with molybdenum promoter ions, salts, compounds, and/or complexes. It can be carried out at the same time or before and/or after the addition of other components of the catalyst. Preferably, the molybdenum accelerator and the salt are mixed with the catalyst before the potassium salt is added.

The preferred amount of accelerator compound present or deposited on the support or catalyst is about 0.05 to 2.5 wt% Mo (measured as an element, regardless of the form of the accelerator) based on the total weight of the supported silver catalyst. Within the aforementioned range The effects obtained vary with special properties and characteristics, such as reaction conditions, catalyst preparation technology, surface area and pore structure and surface chemistry of the support used, the silver content of the catalyst and the potassium content of the catalyst.

The molybdenum content mentioned and requested in the scope of this specification and patent application does not exclude the use of other activators, accelerators, enhancers, stabilizers, modifiers, etc. However, it was unexpectedly found that this discovery can be operated at a very high efficiency without the presence of other accelerators such as rhenium.

The promoter compounds, salts, and/or complexes used in the preparation of the catalyst are molybdenum compounds, salts) and/or complexes that can be dissolved in a suitable solvent. The preferred solvent is an aqueous solvent. A more preferred solvent is the same solvent used to deposit silver and potassium salts. Preferred accelerator compounds are molybdenum oxyanion compounds, preferably ammonium and alkali metal oxyanions, such as potassium molybdate, cesium molybdate, rubidium molybdate, ammonium molybdate, lithium molybdate, sodium molybdate and the like.

Propylene and oxygen-containing gas (that is, a gas including molecular oxygen) are placed in the reactor, and coexist with the aforementioned catalyst under conditions that can effectively complete the at least partial oxidation reaction of propylene. Typical oxidation conditions include an internal temperature of about 180 to 350°C (more preferably 200 to 300°C) and a pressure of about 1 to about 30 atmospheres in the reaction zone of the reactor. The inlet pressure can be as low as 96.5 to 517 KPa gauge pressure (14 to 75 psig). In order to achieve a satisfactory high selectivity to the oxides, it is important that the feed stream sent to the reactor contains carbon dioxide and organic halides (detailed later), and gaseous nitrogen oxide species (detailed later) can be selected The properties are supplied to the reaction zone in the reactor by introducing the species to a feed stream containing propylene (fresh and/or recycled) and molecular oxygen.

The feed stream needs to contain organic halides, selective halogenated hydrocarbons other than propylene, such as saturated halogenated hydrocarbons. The feed stream additionally needs to contain carbon dioxide; it was found that, contrary to the expectations of the prior art, the presence of carbon dioxide can substantially improve the selectivity of propylene oxide obtained in this process.

Examples of nitrogen oxide species suitable for selective introduction of feed streams include NO, NO₂, N₂O₄, N₂O₃Or any one of the aforementioned gases can be formed under epoxidation conditions, especially NO and NO₂Gaseous substances, and any of the foregoing, especially NO and CO, PH₃, SO₃And SO₂Any one of one or more mixtures. NO is the best nitrogen oxide species. However, it was unexpectedly found that the feed stream does not need to contain these nitrogen oxide species, because even if it is not added, a high propylene oxide selectivity can be achieved. In particular, the catalyst system is impregnated sequentially (in potassium salt). Previously introduced molybdenum accelerator and silver) preparation.

The amount of gaseous nitrogen oxide species (if present) is not particularly limited. The optimal amount is partly determined by the special potassium salt used and its concentration and other factors mentioned above that will affect the optimal amount of potassium salt. Typically, the appropriate concentration of the nitrogen oxide species of epoxidized propylene is about 0.1 to about 2000 ppm by volume when nitrogen is used as the ballast. When NO is used in the epoxidation reaction of propylene, the preferred concentration is about 5 to 2000 ppm, more preferably 20 to 500 ppm by volume, and N is used₂Ballast. However, as mentioned above, the concentration of nitrogen oxide species can be substantially zero.

The "oxygen-containing gas" used in the reaction can be defined as containing pure molecular oxygen, atomic oxygen, any atom or molecular oxygen-derived transition group that can exist under epoxidation conditions, another gaseous substance and any of the foregoing can be epoxidized A mixture of substances formed under conditions. These oxygen-containing gases are typically air, commercially available pure oxygen or other substances that can exist in a gaseous state under epoxidation conditions and form molecular oxygen are introduced to the reactor.

The area of the reactor where the gaseous components or reactants and catalysts supplied to the reaction section are placed under epoxidation conditions are usually combined before being introduced to the reactor. However, if necessary, these ingredients can be introduced separately or in different combinations. Such a feed stream having the aforementioned special composition can be formed before or when the individual components enter the reactor section. The reactor used in the method and catalyst of the present invention can be of any type known in the industry. A brief description of several reactor parameters that can be used in the present invention is presented below.

In addition to propylene and oxygen (and selective nitrogen oxide species), the feed stream also needs to contain performance enhancing organic halides (preferably aliphatic halides such as alkyl halides). The organic halide is preferably a volatile compound, that is, it exists in a gaseous state under the temperature and pressure conditions of the reactor section. The normal boiling point of organic halides is preferably less than about 100°C at atmospheric pressure. Compounds containing 1 to 10 carbon atoms are preferred. The best aliphatic halide is the chloride species. The term aliphatic halides includes saturated and unsaturated halides such as methylene chloride, ethyl chloride, vinyl chloride, methyl chloride and methylene chloride. It is preferable to use ethyl chloride as the organic halogen compound. Mixtures of different organic halides can be used. The amount of organic halide can vary depending on many factors, including the concentration of propylene to be oxidized, the specific potassium salt and nitrogen oxide species and their concentration, and the aforementioned other factors that affect the optimal amount of potassium salt and nitrogen oxide species. However, the appropriate concentration range of the organic halide of propylene oxide is typically about 0.1 to about 2,000 ppm of the feed stream, more preferably about 50 to 500 ppm by volume. In addition, specially saturated hydrocarbons such as methane, propane or ethane can be contained in the feed stream. The feed stream may also contain ballasts or diluents such as nitrogen or other inert gases, especially when air is used as the oxygen-containing gas. There may also be unequal amounts of water vapor.

Carbon dioxide is another essential component in the feed stream of the epoxidation process of the present invention. The presence of carbon dioxide within certain limits can provide a surprising improvement in propylene oxide selectivity. Reasonable selectivity promotion is usually seen in the feed stream with 1 to 30 v% CO₂, With 5 to 25 v% CO₂Better.

The most suitable feed stream components are shown in the following table

Although the present invention can use any size and type of gas-phase epoxidation reactor, including fixed bed and fluidized bed reactors known in the industry, it is expected that the present invention is most widely used in standard fixed bed and multitubular reactors. Those used as ethylene oxide reactors. Usually include wall-cooled and adiabatic or non-wall-cooled reactors. The tube length is in the range of about 1.52 to about 18.3 m (about 5 to about 60 feet), but is often in the range of about 4.57 to about 137 m (about 15 to about 45 feet). The inner diameter of the tube ranges from about 12.7 to about 63.5 mm (about 0.5 to about 2.5 inches), and is expected to be typically from about 20.3 to about 38.1 mm (about 0.8 to about 1.5 inches). It is possible to use multiple tubes filled with catalyst and arranged in a suitable shell in parallel. GHSV is usually about 500 to about 10,000 hr^-1The scope. Typical GHSV value is about 800 to about 3,000 hr^-1, The pressure is usually about 111 to about 507 KPa (about 1.1 to about 5 atmospheres) in the range of about 101 to about 3040 KPa (about 1 to about 30 atmospheres). The contact time needs to be sufficient to convert 0.5 to 70%, preferably 5 to 30% of propylene.

Instance

The preferred preparation method of the supported silver catalyst suitable for the method of the present invention is as follows: Step I: Place a 0.471 (16 ounce) jar containing a teflon-coated stir bar on the stirring plate. Add 41.12 g of ethylenediamine to the bottle and then add 40.80 g of distilled water. Mix thoroughly and then slowly add 41.20 g of oxalic acid and let it dissolve completely. Slowly add 71.20 g of silver (I) oxide and let it dissolve completely. Add 14.40 g ethanolamine and 1.20 g diammonium molybdate and mix thoroughly. Add 15.0 g of distilled water and 51.4 calcium carbonate. Add 10 mixed stones, cover the flask and place on a ball mill for 4 hours, dry at 100°C for 1 hour and then sinter at 300°C for 4 hours.

Step II: Grind the solid obtained in Step I into powder. Add 160 ml of distilled water to a 500 ml one-neck round bottom bottle. Dissolve 6.2 g of potassium nitrate in water and add 120 g of the ground solid from step I. Mix on a rotary evaporator for 20 minutes, then apply vacuum and heat to 60°C. Rotary evaporation is continued until the contents of the flask are clearly dry. The obtained solid was dried at 110°C for 2 hours. Then the obtained catalyst was pelletized and sieved to a 14×30 mesh.

Example 1

According to the present invention, a supported silver catalyst comprising 54% Ag of calcium carbonate support, 2% K (added as potassium nitrate) and 0.5% Mo (added as molybdic acid, diammonium salt) was prepared. The supported silver catalyst (2 cc) was loaded in a tubular reactor and tested under the following operating conditions: 10% propylene, 5% oxygen, 200 ppm ethyl chloride, 75 ppm nitrogen oxide, 10% carbon dioxide and the balance nitrogen, GHSV = 1200hr^-1, 207 KPa gauge pressure (30 prig) total pressure, 245°C. The propylene conversion rate is 3.2%, and the propylene oxide selectivity is 58-59%.

Example 2

Example I was repeated but the propylene concentration was reduced to 5%, the carbon dioxide concentration was increased to 20% and the temperature was reduced to 240°C. The propylene conversion rate is 4.5% and the propylene oxide selectivity is 59-61%.

Comparative example 3-4

These examples verify that the supported silver catalyst removes the adverse effects of the molybdenum accelerator. A catalyst containing 43% Ag and 1.7% K (added as potassium nitrate) was prepared. Then the catalyst (2 cc) was loaded in a tubular reactor and tested under the following operating conditions: 10% propylene, 5% oxygen, 50 ppm ethyl chloride, 200 ppm nitrogen oxide and the balance of nitrogen, GHSV = 1200 hr^-1, 207 KPa gauge pressure (30 psig) total pressure, 250°C. The conversion rate of propylene was 11% and the selectivity of propylene oxide was only 33%.

The second supported silver catalyst was prepared including calcium carbonate support 54% Ag, 2% K (as potassium nitrate addition) and 0.5% Mo (as molybdic acid, diammonium salt addition). The silver catalyst was then tested under the operating conditions described above for the molybdenum-free catalyst. The propylene conversion rate was 11.2%, and the propylene oxide selectivity was 39.5%. Even the addition of a small amount of molybdenum to the catalytic aid can obviously increase the epoxide selectivity to a significant degree under comparable test conditions. According to the present invention, the selectivity can be further improved by adding carbon dioxide to the reactor feed mixture.

Comparative example 5

Including calcium carbonate support 52% Ag and 2.1% K (in KNO₃The supported silver catalyst (2 cc) added) but not containing molybdenum was tested under the test conditions of Example 1. The propylene conversion rate was 5.1%; the selectivity of propylene oxide was 50-52% (significantly lower than the observed value of Example 1 using the molybdenum-supported silver catalyst).

Comparative example 6

The supported silver catalyst containing potassium molybdate is prepared as follows. Ethylene diamine (5.14 g) distilled water (5.17 g), oxalic 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 calcium carbonate (6.45 g) are contained in a 4-ounce bottle containing 5 ceramic stones. The flask was sealed and placed in a ball mill for 4 hours. The resulting mixture was then heated at 110°C for 1 hour; then the temperature was increased at 10°C/min up to 300°C and maintained at 300°C for 3 hours. The solid thus obtained is ground into powder, granulated and sieved to 14×30 mesh. The obtained supported catalyst contained 51 wt% Ag, 1.6 wt% K and 1.8 wt% Mo.

The aforementioned catalyst was loaded in a tubular reactor and tested under the following operating conditions: 10% propylene, 5% oxygen, 50 ppm ethyl chloride, 200 ppm nitrogen oxide, the difference being nitrogen; GHSV = 1200 hr^-1; 207 KPa gauge pressure (30 psig) total pressure, 250°C. The propylene conversion rate was 13% and the propylene oxide selectivity was 40%. According to the present invention, mixing 1 to 30 v% carbon dioxide in the feed stream is expected to further improve the PO selectivity.

Example 7

This example verifies that the presence of NO or other nitrogen oxide species in the feed stream is not required to achieve apparently higher propylene oxide selectivity. According to the present invention, a supported silver catalyst including calcium carbonate support 53% Ag, 1.1% K (added as potassium nitrate) and 0.54% Mo (added as diammonium molybdate) was prepared. The catalyst (2 cc) is loaded in a tubular reactor and tested under the following operating conditions: 10 v% propylene, 5 v% oxygen, 200 ppm chloroethane, 10 v% carbon dioxide, the balance is nitrogen; GHSV= 1200hr^-1, 207 KPa gauge pressure (30 psig) total pressure, 245 °C to obtain propylene conversion rate of 2.8% and propylene oxide selectivity 58%.