WO2016075200A1 - Process for preparing vinyl acetate - Google Patents

Abstract

The invention provides a process for preparing vinyl acetate, wherein, in a heterogeneously catalysed, continuous gas phase process, a gaseous mixture comprising ethylene, acetic acid and oxygen is reacted in the presence of a supported catalyst in a reactor, the gas mixture (product gas stream) leaving the reactor is worked up, and the remaining gas mixture (cycle gas), after being recharged with ethylene, acetic acid and oxygen, is recycled into the reactor, characterized in that the selectivity is improved by using caesium acetate as promoter. The application likewise relates to supported catalysts comprising palladium, gold and caesium acetate.

Description

 Process for the preparation of vinyl acetate

The invention relates to a process for the preparation of vinyl acetate, wherein in a heterogeneously catalyzed, continuous gas phase process, a gaseous mixture containing ethylene, acetic acid and

Oxygen is reacted in the presence of a supported catalyst in a reactor, the gas mixture leaving the reactor (product gas stream) is worked up, and the remaining gas mixture (circulating gas) is recycled after reloading with ethylene, acetic acid and oxygen in the reactor, wherein the Improvement of the selectivity cesium acetate is used as a promoter.

Vinyl acetate monomer (VAM) can be prepared in a continuous process with recycling of the purified product stream (cycle gas process). In a heterogeneously catalyzed gas phase reaction, ethylene reacts with acetic acid and oxygen on catalysts which generally contain palladium and alkali metal salts on a support material and may additionally be doped with gold. In general, a Pd / Au catalyst mixture is used with calcium acetate as a promoter.

The educts ethylene, oxygen and acetic acid are in an exothermic reaction (VAM: Δ _Β Η ° ₂₉₉ = - 176 kJ / mol), generally at an overpressure of 7 to 15 bar and, depending on the duration of the catalyst, at a Temperature of generally from 130 ° C to 200 ° C, in a fixed bed tubular reactor, but also fluidized bed reactors, converted to vinyl acetate: C ₂ H ₄ + CH ₃ COOH +% 0 ₂ -> CH ₃ COOCH = CH ₂ + H ₂ 0

The major secondary reaction is the total ethylene oxidation to C0 ₂ :

C ₂ H ₄ + 3 0 ₂ -> 2 C0 ₂ + 2 H ₂ 0 (C0 ₂ : Δ _Β Η ° ₂₉₉ = - 1322 kJ / mol)

The ethylene conversion is generally from 5 to 20%, the acetic acid conversion from 20 to 60% and the oxygen conversion up to 90%. Because of the incomplete conversion of ethylene in the production of vinyl acetate, a predominantly consisting of ethylene, carbon dioxide, ethane, nitrogen and oxygen gas mixture (= recycle gas) led in a circle. The cycle gas is mixed with the educts of acetic acid, ethylene and oxygen before the fixed bed tube reactor and brought to reaction temperature by means of heating steam operated heat exchangers.

After the reaction, the reaction products vinyl acetate and water and unreacted acetic acid are condensed out and fed to the further work-up. For further working up of the condensate, the condensed products vinyl acetate and water as well as unreacted acetic acid can be separated from one another in a multistage distillation process which is usually operated with heating steam. The cycle gas is optionally compressed, re-added with the educts, and passed into the reactor for gas phase oxidation. Due to the high expense of working up the mixture formed during the gas-phase oxidation, the highest possible selectivity is desired. The higher the content of vinyl acetate monomer and the lower the by-product formation, the easier and cheaper the work-up process becomes.

From WO 2008/145389 A2, for example, it is known to improve the selectivity by using a catalyst support which has a surface area of less than 130 m ² / g. WO 2013/164458 A1 describes that the application of an alkali metal acetate prior to the application and reduction of the noble metal component in the preparation of a supported catalyst leads to catalysts which show a higher activity and a higher selectivity in the production of vinyl acetate by means of gas phase oxidation. All alkali acetates, namely the acetates of lithium, sodium, potassium, cesium and rubidium are listed as suitable. Potassium acetate is mentioned as preferred and used as the only alkali metal acetate in the examples. A disadvantage of the use of potassium acetate as a component of the supported catalyst is its continuous discharge from the catalyst bed and its tendency to crystallization and accumulation in the At the rear of the catalyst bed, in the course of Gasphasenoxi- dation, which can lead to loss of selectivity and destruction of the supported catalyst. It was therefore an object to provide a process which results in high selectivity to vinyl acetate monomer without having the disadvantages mentioned.

The invention relates to a process for the preparation of vinyl acetate, in which a gaseous mixture comprising ethylene, acetic acid and oxygen is reacted in the presence of a supported catalyst in a reactor in a heterogeneously catalyzed continuous gas phase process, the gas mixture leaving the reactor (product gas stream). is worked up, and the remaining gas mixture (circulating gas) is recycled after re-loading with ethylene, acetic acid and oxygen in the reactor, characterized in that is used to improve the selectivity cesium acetate as a promoter. The use of cesium acetate as a promoter can be carried out so that the supported catalyst is doped with cesium acetate, or the cesium acetate is added to the cycle gas, or both the supported catalyst is doped with cesium acetate and the cesium acetate is added to the cycle gas. Another object of the invention is therefore a supported catalyst which is doped with cesium acetate, for use in processes for the preparation of vinyl acetate.

The heterogeneously catalyzed, continuous gas phase reaction is preferably carried out in a tubular reactor, preferably made of stainless steel, which is charged with a fixed bed catalyst. In general, this is a fixed bed tube bundle reactor with several thousand, usually 2000 to 20,000, densely packed and generally vertically disposed cylindrical tubes. For large-scale use, pipes with a length of 4 m to 10 m and an inner diameter of generally 20 mm to 50 mm are generally used. The spaces between the tubes themselves, and the tubes and the container, are used for cooling. For example, flows through a water / steam mixture (boiling water cooling).

The tubes of the tubular reactors are filled with commercially available supported catalysts as fixed bed catalysts. Commercially available are supported catalysts based on an inorganic support material such as titanium oxide, silica or alumina, which are generally coated with palladium and gold in combination with an activator component such as potassium acetate. These supported catalysts can be in different shape, for example, as balls, cylinders or rings, the dimensions of which are adapted to the tubes used and generally lengths of 1 mm to 10 mm or

Diameter from 1 mm to 20 mm. In the process according to the invention, the reactor is charged with ethylene, oxygen and acetic acid for starting up or charged with the cycle gas laden with ethylene, oxygen and acetic acid in continuous operation. The amounts of educts to be used are known per se to those skilled in the art. Ethylene is generally used in excess of the stoichiometric ratio to acetic acid. The amount of oxygen is limited upwards by the ignition limit in the recycle gas. The gas phase reaction is abs at a pressure of preferably 7 to 15 bar. and at a temperature of preferably 130 ° C to 200 ° C performed. The reaction temperature is adjusted, for example, by means of boiling water cooling at a pressure of from 1 to 30 bar abs., Preferably from 8 to 14 bar abs. In this case, water vapor at a temperature of 120 ° C to 185 ° C at a pressure of 1 to 10 bar abs., Preferably 2.5 to 5 bar abs. Formed. The product gas stream leaving the reactor contains essentially vinyl acetate, ethylene, acetic acid, water, and C0 ₂ . The gas phase oxidation is in fact incomplete: ethylene conversion is generally about 5 to 20%, acetic acid conversion generally 20 to 60% and oxygen conversion generally up to 90%. After the reaction, the reaction products vinyl acetate and water and unreacted acetic acid are preferably removed from the cycle gas in a so-called pre-dewatering column. Siert and fed to further workup. Uncondensed product, essentially ethylene, C0 ₂ and vinyl acetate, can be removed at the top of the pre-dewatering column and washed out in a scrubber operated with acetic acid (circulating gas scrubber). The overhead product of the pre-dewatering column (cycle gas), or at least a subset thereof, can be purified in a C0 ₂ scrubber from the carbon dioxide formed. The cycle gas is optionally compressed, re-added with the educts, and passed into the reactor for gas phase oxidation.

For further workup of the condensate from the pre-dewatering column, the condensed products vinyl acetate and water and unreacted acetic acid in a multi-stage, usually operated with heating steam, distillation process can be separated. The usual distillation steps for obtaining the vinyl acetate and the acetic acid are pre-dewatering column, azeotrope column, dewatering column, pure VAM column, as well as columns for residue workup and for low boiler and high boiler separation. Such a work-up process is described, for example, in WO 2011/089070 Al, the details of which are part of the application (incorporated by reference). The recycle gas is then charged again with ethylene and acetic acid and then returned to the fixed bed tubular reactor. The enrichment of the circulating gas with acetic acid is usually carried out by means of an acetic acid saturated with Heizdampf.

In the method according to the invention, the reactant gas mixture or the circulating gas prior to its supply to the fixed bed tubular reactor, preferably after saturation with acetic acid, 1 to 50 g (1 to 50 ppm by weight), preferably 5 to 20 g (5 to 20 ppm by weight) ) Cesium acetate are added, in each case per tonne of vinyl acetate formed in the reactor (based on vinyl acetate formed in the reactor). Preferably, the cesium acetate is added as acetic acid solution. As an alternative to the addition of cesium acetate to the cycle gas or in addition to this measure, a commercial, potassium acetate-doped supported catalyst can be used in the process according to the invention completely or be partially replaced with a supported catalyst which has been doped with cesium acetate. Preferably, 5 to 100 vol .-%, based on the total volume of the catalyst bed, with supported catalysts with cesium acetate as a promoter and the remaining shares with supported catalysts with potassium acetate as a promoter. The

Preparation of such supported catalysts is known to the person skilled in the art. For the preparation of the supported catalysts, the method can be used from DE 10 2006 058 800 AI, the relevant information is part of the application (incorporated by reference).

The shaped catalyst bodies are produced, for example, from metal oxides. Preferred metal oxides are, for example, silicon oxide (Si _x O _y ), aluminum oxide (Al _x O _y ), titanium oxide (Ti _x O _y ), zirconium oxide (Zr _x O _y ), cerium oxide (Ce _x O _y ) or mixtures of these pyrogenic metal oxides. Particular preference is given to using pyrogenically prepared silicon oxide, most preferably silicon dioxide (SiO ₂ ), for example WACKER HDK ^S T40 from Wacker Chemie AG.

The metal oxide is first suspended in water, for example by means of a dissolver or planetary dissolver. The solids content of the aqueous metal oxide suspension is preferably adjusted to values of 15 to 30 wt .-%. In the next step, the aqueous suspension of the metal oxide is coagulated. In the case of silicon dioxide, this can be done, for example, by shifting the pH of the suspension to a range of pH = 5 to 10.

The mass thus obtained is then shaped into shaped articles, for example into spheres, cylinders, perforated cylinders or rings. In general, the shaped catalyst bodies have a diameter of 1 to 20 mm, preferably 2 to 10 mm. In the case of cylinders, perforated cylinders and rings, their length is preferably 1 to 10 mm. Most preferred are rings with a length of 1 mm to 2 mm, an outer diameter of 3 mm to 5 mm, an inner diameter of 2 mm to 3 mm and a wall thickness of 0.5 mm to 1.5 mm. The shaping is preferably carried out by means of extrusion, the length of the Extrudates is adjusted by cutting the extrudates with a cutter accordingly.

The moldings thus obtained are subsequently dried, preferably at a temperature of from 25.degree. C. to 100.degree. The drying step is followed by the calcination of the moldings. The calcination can be carried out in an oven under an air atmosphere, if appropriate under protective gas. Generally, it is heated to a temperature of 500 ° C to 1000 ° C. The sintering time is generally between 2 and 10 hours.

The conversion of the catalyst molding in an active catalyst is done by applying one or more catalytically active compounds such as palladium and gold or their precursor compounds, as well as cesium acetate and optionally further dopants, which are optionally converted in a subsequent step in an active compounds can.

For loading with palladium and gold, the shaped catalyst bodies can be impregnated with a solution containing palladium salt and gold salt. Simultaneously with the noble metal-containing solution, the support materials used can be impregnated with a basic solution. The latter serves to transfer the palladium compound and gold compound into their hydroxides. Suitable palladium salts are, for example, palladium chloride, sodium or potassium palladium chloride, palladium acetate or palladium nitrate. Suitable gold salts are gold (III) chloride and tetrachloroauric (III) acid. The compounds in the basic solution are preferably potassium hydroxide, sodium hydroxide or sodium metasilicate.

The reaction of the noble metal salt solution with the basic solution to form insoluble noble metal compounds can be slow and, depending on the preparation method, is generally completed after 1 to 24 hours. Thereafter, the water-insoluble noble metal compounds are treated with reducing agents. It may be a gas phase reductant tion with hydrogen, ethene or forming gas, for example.

Before and / or after the reduction of the noble metal compounds, the chloride which may be present on the support can be removed by a thorough washing with water. After the wash, the catalyst preferably contains less than 500 ppm of chloride.

The catalyst precursor obtained after the reduction can be dried and finally impregnated with cesium acetate.

The finished catalyst can then be dried to a residual moisture of less than 5%. The drying can be carried out in air, optionally under nitrogen, as an inert gas.

The palladium content of the catalysts is 0.2 to 5.0 wt .-%, preferably 0.3 to 3.0 wt .-%, each based on the total weight of the shaped catalyst body.

 The gold content of the catalysts is 0.2 to 5.0 wt .-%, preferably 0.3 to 3.0 wt .-%, each based on the total weight of the shaped catalyst body.

 The cesium content of the catalysts is 0.5 to 20 wt .-%, preferably 1.0 to 15 wt .-%, each based on the total weight of the shaped catalyst body.

With the method according to the invention in which cesium acetate is used as a promoter in the gas phase oxidation of ethylene and acetic acid with oxygen to vinyl acetate, one obtains an improvement in the selectivity of the reaction in a range of 1 to 2%. For large-scale plants, with a capacity of about 200,000 to vinyl acetate per year, this means an increase of 2,000 to 4,000 tons of additional product. Surprisingly, it has also been found that the catalyst shaped bodies, which are coated with cesium acetate instead of potassium acetate, are characterized by higher mechanical strength. This considerably simplifies the filling of the plants and minimizes catalyst breakage. In the following examples, the selectivity obtained with cesium acetate as promoter is compared to that obtained using conventional catalysts containing the same amount of potassium acetate.

Examples:

Comparative Example 1: 4 kg of pyrogenic silica (HDK ^® WACKER T40) of deionized water were stirred in 35 kilograms. By addition of hydrochloric acid, a pH of 2.8 was set and kept constant. With constant stirring, an additional 4.5 kilograms of fumed silica (WACKER ^HDK® T40) were stirred in. After complete addition of the metal oxide powder, the mixture was homogenized for a further 10 minutes before the suspension was stirred for 45 minutes in a stirred ball mill with grinding beads of silicon nitride (diameter of the grinding beads 2.0 mm, filling level 70% by volume). ) was ground under pH-constant at a pH of 2.8 by addition of further hydrochloric acid. The angular velocity during the milling step was 11 meters per second. After completion of the grinding, an aqueous ammonia solution was added to the suspension with constant stirring until a pH of 6.2 was obtained and at this point gelation of the mass took place. The resulting mass was extruded and cut in a ram extruder into 1 mm long, 4 mm outside diameter and 2.5 mm bore rings. The resulting rings were dried for 24 hours at a temperature of 85 ° C and a humidity of 70% and then calcined at 900 ° C for a period of 2 hours.

500 grams of the carrier material was impregnated with 375 milliliters of an aqueous solution containing 27.60 grams of a 41.8% (wt%) solution of tetrachloroauric acid and 42.20 grams of a 20.8% (w / w) weight of the solution. %) Solution of tetrachloropalladic acid. After a period of 2 hours, in a next step, the catalyst was dried in vacuo at a temperature of 80 ° C for a period of 5 hours. Subsequently, 236 milliliters of a 1 molar sodium umcarbonat solution applied together with 139 milliliters of distilled water. After a period of 2 hours, the catalyst was dried at a temperature of 80 ° C for a period of 5 hours in vacuo. Subsequently, the catalyst was washed with an aqueous ammonia solution containing 0.25% by weight of ammonia for a period of 45 hours. The catalyst was reduced at a temperature of 200 ° C for 5 hours with forming gas (95% N ₂ /5% H ₂ ). Subsequently, the catalyst was impregnated with acetic acid-containing Kaiiumacetat solution (71.65 grams of potassium acetate in 375 milliliters of acetic acid) and finally dried at a temperature of 80 ° C for a period of 5 hours in vacuo. The final catalyst had a concentration of 2.0 wt% palladium (7.4 g / l), 2.0 wt% gold (7.4 g / l) and 6.5 wt% potassium ( 24.1 g / l). Example 2:

The procedure was analogous to Comparative Example 1, with the difference that, instead of potassium acetate, the same molar amount of cesium macacetate was used for the impregnation. The final catalyst also had a concentration of 2.0 wt% palladium (7.4 g / l), 2.0 wt% gold (7.4 g / l), and 15 wt% cesium (45 th) , 8 g / 1).

Testing of the catalysts of Comparative Example 1 and Example 2:

Activity and selectivity of the catalysts of Comparative Example 1 and Example 2 were measured over a period of 200 hours. The catalysts were placed in a flow-controlled with oil flow reactor (reactor length 1200 mm, inner diameter 19 mm) and tested at an absolute pressure of 9.5 bar and a space velocity (GHSV) of 3500 Nm ³ / (m ³ * h) with the following gas composition : 60% by volume of ethene, 19.5% by volume of carbon dioxide, 13% by volume of acetic acid and 7.5% by volume of oxygen. The catalysts were investigated in the temperature range from 130 ° C to 180 ° C, measured in the catalyst bed.

The reaction products were analyzed at the outlet of the reactor by means of online gas chromatography. As a measure of the catalyst activity the space-time yield of the catalyst in grams of vinyl acetate monomer per hour and liter of catalyst (g (VAM) / 1Kat * h) was determined. Carbon dioxide, which is formed in particular by the combustion of ethene, was also determined and used to assess the catalyst selectivity.

Table 1:

Designation V.Bsp. 1 example 2

 Pd [wt%] 2, 0 2, 0

 Au [wt%] 2.0 2, 0

 K / Cs [wt%] 6.5 15

 RZA

g (VAM) / l (cat) * h 1035 1020

 Selectivities

 Ethene [%] 93, 6 94, 9

Claims

Claims: 1. A process for the preparation of vinyl acetate, wherein in a heterogeneously catalyzed continuous gas phase process, a gaseous mixture containing ethylene, acetic acid and oxygen in the presence of a supported catalyst is reacted in a reactor, the reactor leaving the gas mixture (product gas stream) is worked up, and the remaining gas mixture (recycle gas) is returned to the reactor after renewed loading with ethylene, acetic acid and oxygen, characterized in that cesium acetate is used as a promoter to improve the selectivity. 2. A process for producing vinyl acetate according to claim 1,  characterized in that the cesium acetate is added to the cycle gas. 3. A process for the preparation of vinyl acetate according to claim 1 or 2, characterized in that the supported catalyst is doped with caesium umacetat. 4. A process for the preparation of vinyl acetate according to claim 1 to 3, characterized in that after saturation with acetic acid the cycle gas 1 to 50 ppm by weight of cesium acetate, based on vinyl acetate formed in the reactor, are added. 5. A process for the preparation of vinyl acetate according to claim 1 to 4, characterized in that 5 to 100 vol .-%, based on the total volume of the Katalysatorbettess, are loaded with supported catalysts with cesium acetate as a promoter and the remaining Particles with supported catalysts with potassium acetate as a promoter. 6. A process for the preparation of vinyl acetate according to claim 1 to 5, characterized in that as a supported catalyst rings having a length of 1 mm to 2 mm, an outer diameter of 3 mm to 5 mm, an inner diameter of 2 mm to 3 mm and a wall thickness from 0.5 mm to 1.5 mm. 7. Supported catalysts of metal oxide, which are equipped with palladium and gold and cesium acetate, for use in a process for the preparation of vinyl acetate according to claim 1 to 6. 8. Supported catalysts of metal oxide according to claim 7, characterized in that as a supported catalyst rings having a length of 1 mm to 2 mm, an outer diameter of 3 mm to 5 mm, an inner diameter of 2 mm to 3 mm and a wall thickness of 0.5 mm to 1.5 mm are used.