WO2002051541A1 - Silver catalyst for the epoxidation of alkenes - Google Patents

Abstract

The invention relates to a catalyst containing silver, an alkaline-earth metal compound and a promoter consisting of a mixture of at least two compounds, said mixture containing at least one potassium compound and having a melting point below 340 °C. The invention also relates to a method for producing the inventive catalyst and the use of the same for the epoxidation of alkenes, especially propene.

Description

 Silver catalyst for the epoxidation of alkenes

The present invention relates to a silver catalyst, a process for its preparation and its use for the epoxidation of alkenes.

The generation of alkene oxides or epoxides, for example propene oxide or ethylene oxide, by direct oxidation or epoxidation of the alkenes in the presence of a silver-containing catalyst is well known in the art. For example, CA Patent 1,282,772 describes a process for the direct epoxidation of alkenes, for example propene, with oxygen in the presence of a silver-containing catalyst which contains an alkaline earth metal carbonate as carrier and potassium nitrate as a promoter, and a gaseous compound of a redox couple, for example nitrogen oxide.

Several approaches to improve the direct epoxidation of alkenes have been disclosed in the patent literature. US-A-5 703 254, US-A-5 686 380, WO-A-9 845 280, WO-A-9 852 931 and US-A-5 965 480 describe gold, molybdenum, rhenium, Tungsten or inorganic chloride compounds as promoters. WO-A-9 906 385 describes a magnesium promoter together with an iron co-promoter. As alternatives to the alkaline earth carbonate used as carrier in many applications, alkaline earth metal titanates, triphasic calcium phosphate, calcium molybdate and calcium fluoride are mentioned in WO-9 734 693.

EP-A-0 640 598 discloses catalysts for the epoxidation of lower olefins which consist essentially of a metal silicate as a support, whereupon silver nitrate, alkali metal nitrates and / or alkaline earth metal nitrates are deposited. The catalyst contains no elemental silver and no alkaline earth metal carbonate.

The object of the present invention was to provide a catalyst suitable for the epoxidation of alkenes and a process for its preparation create, the catalyst should enable high selectivity, especially for the epoxidation of propene.

It is proposed to achieve this object by means of a catalyst which contains silver, an alkaline earth metal compound and a promoter, the promoter being a mixture of at least two compounds, at least one of which is a potassium compound, and the mixture having a melting point below 340 ° C.

The invention thus relates to a catalyst comprising (a) silver, (b) at least one alkaline earth metal compound and (c) a promoter, which is a mixture of at least two compounds, the mixture containing at least one potassium compound and a melting point below 340 ° C. The invention also relates to a process for producing the catalyst, which is characterized in that silver (a) and the promoter (c) are applied to the alkaline earth metal compound (b), which may have been applied to a support. The invention also relates to the use of the catalyst for the epoxidation of alkenes, in particular propene, and to a process for the epoxidation of propene in the presence of a catalyst and a starting gas mixture which, in addition to propene and an oxygen-containing gas, comprises a halogen or a halogen compound and / or contains a nitrogen compound, which is characterized in that the catalyst according to the invention described above is used as catalyst. Preferred embodiments of the invention emerge from the following description, the examples and the subclaims.

In general, the alkaline earth metal compound is an inorganic compound which contains one or more alkaline earth metals, in particular calcium, strontium, magnesium or barium, calcium, strontium and barium being most preferred. Depending on the alkaline earth metal selected, the alkaline earth metal compound contains a titanate, phosphate, aluminate, molybdate, Fluoride or a combination thereof. In particular, the alkaline earth metal compound is alkaline earth metal carbonates, alkaline earth metal titanates, calcium phosphate, in particular tripasic calcium phosphate, magnesium aluminumate, calcium molybdate, calcium fluoride or mixtures of two or more of these compounds. The three-base calcium phosphate is a compound of the empirical formula Ca ₁₀ (OH) ₂ (PO ₄ ) ₆ . Calcium molybdate is CaMoO ₄ . The alkaline earth metal titanates comprise titanates of the formulas MTiO ₃ , M ₂ TiO and MTi ₂ O ₅ , where M is preferably barium, strontium, calcium or magnesium, in particular calcium or strontium. In general, the alkaline earth metal compounds can be those as described in EP-B-0 888 184 or in WO-A-9 734 693.

The most preferred alkaline earth metal compound is an alkaline earth metal carbonate, which can be selected from all known alkaline earth metal carbonates, with calcium carbonate, strontium carbonate or barium carbonate again being particularly preferred. Mixtures of calcium carbonate with strontium carbonate or calcium carbonate with barium carbonate are also possible. Calcium carbonate is most preferred.

The alkaline earth metal compound can be applied to or be present on an inert carrier material, wherein suitable carrier materials can be selected from aluminum oxide, silicon dioxide, silicon carbide, titanium dioxide and / or zirconium dioxide. Aluminum oxide is particularly preferred as the inert carrier material, which can be any known aluminum oxide. It is particularly preferably α-aluminum oxide or a highly annealed aluminum oxide with a specific surface area <10 m ² / g. The promoter is a mixture which, according to the invention, has a melting point below 340 ° C., preferably a melting point below 320 ° C., in particular below 300 ° C. These are preferably melting point values as described in J. Phys. Chem. Ref. Data, Vol. 1, No. 3, 1972 or the melting point is determined preferably wise as stated therein. The potassium compound of the promoter can be any potassium compound, preferably potassium nitrate or a potassium compound which forms or can form potassium nitrate under the epoxidation conditions. In addition to the potassium compound, the mixture of the promoter preferably contains a compound which comprises a metal other than potassium, selected from alkali metals (other than potassium), alkaline earth metals, lead, silver and / or thallium. Suitable promoter mixtures which are in the melting point range defined above can easily be determined by the person skilled in the art by routine experiments and / or can be found in phase diagrams, as described, for example, in J. Phys. Chem. Ref. Data, Vol. 1, No. 3, 1972. In addition to the potassium compound, the promoter mixture contains a further compound, in principle any compound which combines with the potassium compound to form a mixture having a melting point in the defined range is suitable for this. Preferred compounds are those which are chemically largely stable under the reaction conditions and do not catalyze any undesired reactions. Such compounds are in particular sodium nitrite, sodium nitrate, lithium nitrate, lead nitrate, barium nitrate, silver nitrate, cesium nitrate, rubidium nitrate and thallium nitrate, sodium nitrate, sodium nitrite and lithium nitrate being particularly preferred. In addition to binary mixtures, mixtures of the potassium compound with two or more of the above-mentioned further compounds are also possible. In a preferred embodiment of the invention, the promoter is a mixture of potassium nitrate and a compound selected from sodium nitrate, sodium nitrite, lithium nitrate, lead nitrate, barium nitrate, silver nitrate, cesium nitrate, rubidium nitrate and / or thallium nitrate. If the mixture of the promoter is a mixture of potassium nitrate and potassium nitrite, the proportion of potassium nitrate is at least 55 mol%, in particular 70 to 90 mol%, and the remainder in mol% is potassium nitrite.

The catalyst according to the invention preferably contains as constituents in the following amounts, in each case based on 100 percent by weight of catalyst: 1-50% by weight, more preferably 5-35% by weight, most preferably 10-25% by weight, silver; 1-50% by weight, more preferably 5-30% by weight, most preferably 10-25% by weight, alkaline earth metal compound; 0-95% by weight, more preferably 20-90% by weight, most preferably 30-80% by weight, carrier material; 0.1-20% by weight, more preferably 1-15% by weight, most preferably 2-10% by weight, potassium compound; and 0.01-20% by weight, more preferably 0.05-15% by weight, most preferably 0.1-10% by weight of the further compound of the mixture of the promoter.

The catalyst can also contain further promoters or constituents, in particular for increasing selectivity and / or activity. Such promoters are the compounds of molybdenum, magnesium, iron, tungsten, gold, rhenium or inorganic chlorides disclosed in the publications mentioned at the outset. Molybdenum, tungsten and / or rhenium compounds, in particular molybdenum compounds, are preferred. The content of such further constituents and / or promoters is 0-5 parts by weight, in particular 0-2 parts by weight, in each case based on 100 parts by weight of catalyst.

The shape of the catalyst is not limited. It can be designed in any form, preferably it is designed in a form which is suitable for use in fixed bed, fluidized bed or fluidized bed reactors. The catalyst is expediently designed as a full catalyst or as a supported catalyst. If it is designed as a supported catalyst, the alkaline earth metal compound is present on a support material, the support material being one of the above-mentioned support materials, in particular aluminum oxide.

The production of the catalyst is not particularly limited. All methods are suitable in which the constituents of the catalyst are brought into contact with one another in a suitable manner, the order of contact being arbitrary. It is preferably carried out in such a way that the silver and the promoter onto the alkaline earth metal compound, which may be applied to a support. has been brought to be applied. If there is a carrier, for example aluminum oxide, the other components are applied to it. Preferred production processes are described in CA Patent 1,282,772, a promoter as defined above being used instead of the potassium salt mentioned therein.

In a preferred embodiment of the invention, the promoter mixture is applied by impregnation, in particular on an aluminum oxide support coated with silver and alkaline earth metal compound, in particular alkaline earth metal carbonate. Preferably, this coated support is made by applying silver in the form of a soluble salt or complex to the support material before, after or together with the alkaline earth metal compound, and then calcining the wet support to convert the silver compound to elemental silver. In order to obtain a silver compound in soluble form, a complexing agent such as ethanolamine, oxalic acid and / or ethylene diamine can be added to a silver compound such as silver (I) oxide or silver (I) oxalate in a suitable manner, which or which can also act as a reducing agent at the same time. The promoter mixture is advantageously applied as a solution, especially in water.

For an unsupported catalyst, the promoter mixture is applied by impregnation to a catalyst precursor consisting of silver and alkaline earth metal compound, in particular alkaline earth metal carbonate. This precursor is preferably prepared by mixing silver in the form of a soluble salt or complex with the alkaline earth metal compound and then calcining the material in order to convert the silver compound into elemental silver. The material can then be processed into moldings by measures known to the person skilled in the art which are suitable for use in a reactor. Suitable measures are tableting, extrusion or build-up agglomeration. The promoter mixture can be applied before or after shaping. The same materials as described above can be used as starting materials for the preparation of the catalyst. Mixtures of different alkaline earth metal compounds and / or carrier materials can be used. Mixtures of various soluble silver salts and / or silver complexes can also be used to apply the silver.

In a preferred embodiment of the invention, the catalyst is prepared as follows: First, ethylenediamine and oxalic acid are dissolved in water. Silver (I) oxide and ethanolamine are added to this solution. After the silver (I) oxide has been dissolved, a calcium compound, in particular calcium carbonate, in the form of a powder is added as the alkaline earth metal compound and the suspension obtained is mixed. The carrier material, insofar as it is used, is stirred into the suspension. The moist material is then calcined, for example at a temperature of 100 to 400 ° C. After cooling, it is impregnated with a solution of the potassium compound and the further compound of the promoter in water and then dried. If the catalyst contains additional promoters, such as molybdenum compounds, or constituents, these are preferably added to the silver salt solution in soluble form.

The catalyst according to the invention is generally suitable for the epoxidation of alkenes, very good results being achieved in particular for propene, ethylene, butadiene or styrene. The epoxidation is carried out as a gas phase reaction in the presence of the catalyst and a starting mixture which, in addition to the alkene and an oxygen-containing gas, contains a halogen or a halogen compound. Acyclic and cyclic alkenes with up to 12 carbon atoms can be used as alkene. Air, air enriched with oxygen or pure oxygen is expediently used as the gas containing oxygen. The halogen is preferably chlorine, while preferred halogen compounds are an organic halide, for example Ethyl chloride, methyl chloride, methylene chloride, vinyl chloride or ethylene dichloride. In addition to the constituents mentioned, the starting mixture can furthermore contain a nitrogen compound, for example NO, NO ₂ , N ₂ O ₄ and / or N ₂ O ₃ , with NO and NO _{2 being} particularly advantageous. Nitrogen, methane, ethane, propane, butane, water vapor and carbon dioxide can also be present. Preferred epoxidation temperatures are 100 to 300 ° C, in particular 220 to 270 ° C. The pressure during the epoxidation is preferably in the range from 1 to 30 bar, in particular from 1 to 20 bar. The starting mixture preferably contains the constituents in the following proportions, in each case based on 100% by volume of starting mixture: 1 to 10% by volume of alkene, in particular propene; 5 to 50 vol% oxygen; 0 to 1000 ppm nitrogen oxide (s); 0 to 1000 ppm halogen or halogen compounds; and the rest carbon dioxide and / or gas inert under the reaction conditions, for example nitrogen, methane, ethane and / or water vapor.

Reactors suitable for epoxidation are known to the person skilled in the art. Fat bed, fluid bed or fluidized bed reactors are particularly suitable.

A particularly preferred embodiment of the invention relates to the epoxidation of propene in the presence of the catalyst according to the invention and a starting gas mixture which contains propene, an oxygen-containing gas, in particular air, nitrogen oxide (NO and / or NO ₂ ) and a halogen compound, preferably further Nitrogen is present.

The catalyst according to the invention enables the production of alkene oxides, in particular propene oxide, with very good selectivity and at the same time high conversion. The invention thus provides an extremely effective system for, in particular, the direct oxidation of propene on a technical and industrial scale. The invention is illustrated by the following examples, which represent preferred embodiments of the invention.

example 1

100 g of water were mixed with 101.7 g of ethylenediamine and 103.8 g of oxalic acid dihydrate were dissolved therein with stirring. 181.6 g of silver (I) oxide and 38.3 g of ethanolamine were added to the solution. After the silver (I) oxide had completely dissolved, it was made up to a total volume of 500 ml with water. 269.2 g of this solution were removed and mixed with a solution of 2.5 g of ammonium heptamolybdate tetrahydrate in 80 ml of water. 64.8 g of calcium carbonate were added and the suspension was mixed intensively with an Ultra-Turrax. 112.8 g of an α-aluminum oxide support (Norton, SA 5262) in the form of a grit fraction (0.6 to 1.6 mm) were stirred into this mixture. The moist support was then dried in a vacuum drying cabinet at 60 ° C for 20 hours and then calcined in a rotary tube oven in air at 300 ° C for 3 hours.

25 g of the catalyst precursor obtained were impregnated with a solution of 0.06 g of sodium nitrate and 1.5 g of potassium nitrate in 10 ml of water and then dried at 100.degree.

Examples 2 to 18 and Comparative Examples 1 and 2

Example 1 was repeated with the difference that instead of 0.06 g of sodium nitrate and 1.5 g of potassium nitrate, the promoters or promoter mixtures given in Table 1 below were used: Table 1

* taken from J. Phys. Chem. Ref. Data, Vol. 1, No. 3, 1972 propene epoxidation - carrying out the reaction

An apparatus consisting of a jacket-heated stainless steel tube of 400 mm in length and 8 mm in inside diameter was charged with 20 ml of the catalysts described in Examples 1 to 18 and Comparative Examples 1 and 2. There were 86.6 ml / min. a gas mixture consisting of 5.3% by volume of propene, 17.6% by volume of oxygen, 45 ppm of nitrogen oxide (NO and / or NO ₂ ), 220 ppm of ethyl chloride and the remainder of nitrogen were passed through the catalyst bed at 250 ° C. and 3 bar gauge pressure. The exhaust gas was analyzed using an on-line gas chromatograph. The results after 50 to 150 hours of operation are shown in Table 2 below, with a steady state generally being achieved after 20 to 40 hours of operation.

Table 2

^ Propene oxide selectivity S o = Q _P O / (C _PO + 1/3 Cco ₂ ), where CP _O and Cco2 the concentration of propene oxide or carbon dioxide in vol.% In the exhaust gas, based on 100 vol.% Exhaust gas, specify;

²⁾ Propene oxide concentration Cpo in vol .-% in the exhaust gas of the reactor, based on 100 vol .-% exhaust gas.

As can be seen from the results in Table 2, the selectivity and / or the activity of the catalyst in the propene oxidation increase if, according to the invention, a further component is added to the potassium nitrate, so that the melting point of the promoter mixture is below 340 ° C.

Claims

claims 1. Catalyst containing (a) silver, (b) at least one alkaline earth metal compound and (c) a promoter, which is a mixture of at least two compounds, the mixture being at least one Contains potassium compound and has a melting point below 340 ° C. 2. Catalyst according to claim 1, characterized in that the at least one alkaline earth metal compound (b) is an alkaline earth metal carbonate, an alkaline earth metal titanate, calcium phosphate, magnesium aluminate, calcium molybdate, calcium fluoride and / or a mixture thereof, the at least one alkaline earth metal compound (b) is optionally applied to a carrier which is selected from aluminum oxide, silicon dioxide, silicon carbide, titanium dioxide and / or zirconium dioxide. 3. Catalyst according to claim 1 or 2, characterized in that the mixture of the promoter (c) contains, in addition to the potassium compound, a compound which contains a metal other than potassium, selected from alkali metals, alkaline earth metals, lead, silver and / or thallium , 4. Catalyst according to one of claims 1 to 3, characterized in that the promoter contains a mixture of potassium nitrate and a compound selected from sodium nitrate, sodium nitrite, lithium nitrate, lead nitrate, barium nitrate, silver nitrate, cesium nitrate, rubidium nitrate and / or thallium nitrate. 5. Catalyst according to one of the preceding claims, characterized in that the mixture of the promoter (c) has a melting point below 320 ° C. 6. Catalyst according to one of claims 2 to 5, characterized by the following contents, in each case based on 100% by weight of catalyst: 1-50% by weight of silver, 1-50% by weight of alkaline earth metal compound, 0-95% by weight. % Carrier material, 0.1-20% by weight of potassium compound and 0.01-20% by weight of further compound of the promoter (c). 7. A process for the preparation of a catalyst according to any one of the preceding claims, characterized in that silver (a) and the promoter (c) are applied to the alkaline earth metal compound (b), which has optionally been applied to a support. 8. Use of a catalyst according to any one of claims 1 to 6 or a catalyst obtainable by a process according to claim 7 for the epoxidation of alkenes. 9. Use according to claim 8, characterized in that the catalyst is used for the epoxidation of propene. 10. A process for the epoxidation of propene in the presence of a catalyst and a starting mixture which in addition to propene and an oxygen-containing Gas contains a halogen or a halogen compound and / or a nitrogen compound, characterized in that the catalyst used is a catalyst according to one of Claims 1 to 6 or a catalyst obtainable by a process according to Claim 7.