MXPA98008187A - Catalyst and a procedure to prepare vin acetate - Google Patents

Abstract

The invention relates to a process for the preparation of vinyl acetate in a gas phase from ethylene, acetic acid and oxygen or oxygen-containing gases while simultaneously forming low levels of high-boiling agents on a catalyst containing palladium and / or compounds thereof, gold and / or compounds thereof and alkali metal compounds on a carrier. The procedures are characterized because the catalyst also contains boron or bovine compounds.

Description

CATALYST AND A PROCEDURE TO PREPARE ACETATE AND VINYL DESCRIPTIVE MEMORY The present invention relates to a catalyst consisting of palladium »gold and an alkali metal compound» a process for its preparation and its use to prepare vinyl acetate from acetic acid »ethylene and oxygen or oxygen-containing gases. It is known from the prior art, that vi acetate can not be prepared from ethane, oxygen and acetic acid by reaction on catalysts consisting of palladium, gold and an alkali metal compound, not on an support material (for example »silicon dioxide). Such catalysts show good activity and generally form little carbon dioxide and ethyl acetate. Although high-boiling compounds are formed as by-products in amounts that appear to be small, they nevertheless present a problem from the procedural and ecological standpoints. The term "high-boiling compounds" refers here, in particular, to the ethyl acetate compounds of ethylene glycol and d-acetoxyethyl logs. References in the literature describing the preparation of such catalysts for the commercial production of vinyl acetate generally describe methods for depositing the noble metals in a shell on the catalyst support. E.U. A.-A-4 »0-4B» 09S describes the preparation of a catalyst for the production of vinyl acetate in which the dissolved salts of noble metal are absorbed by the support from a solution having the same volume as the pores of the support material »with the support particles being agitated in a rotating container. The salts are then fixed using alkalis without drying the support beforehand. US-A-5,332,710 describes the preparation of a catalyst for the production of vinyl acetate in which the noble metal insoluble salts are precipitated on the support particles by stirring them in a rotating drum for at least half an hour during the precipitation by means of alkalis. It has now been surprisingly discovered that the addition of boron or boron compounds improves the selectivity of the catalyst »in particular» with the formation of high-boiling compounds being signifi- cantly reduced. The high-boiling compounds »in particular» are the compounds mentioned in the introduction. The invention provides a catalyst for preparing vinyl acetate in the gas phase from ethylene »acetic acid and oxygen or oxygen-containing gases» at the same time, with low formation of high-boiling compounds, whose catalysts consist of palladium and / or its compounds »gold and / or its compounds and alkali metal compounds on a particulate support» in which the catalyst also comprises boron or boron compounds. In a preferred embodiment, the catalyst is prepared by: a) impregnating the support with soluble palladium and gold compounds; b > converting the soluble palladium and gold compounds on the support into insoluble compounds by means of an alkaline solution; c > reducing the insolubles of palladium and gold on the support by means of a reducing agent in the liquid phase; d) washing and subsequently drying the support; e) impregnating the support with a soluble compound of alkali metal no; and f) finally drying the support at a maximum of 150 ° C. The invention also provides a process for preparing the catalyst by means of the characteristics specified in claim 6 and also its use in a process for preparing vinyl acetate by means of the characteristics specified in the re ind cation 14. The invention therefore provides on the one hand, a process for preparing a catalyst for the production of vinylacetate in the gas phase from ethylene »acetic acid and oxygen or oxygen-containing gases, at the same time» with a low formation of high-boiling compounds, whose catalysts consist of palladium and / or its compounds, gold and / or its compounds and alkali metal compounds on a particulate support, wherein the catalyst is prepared by: a) impregnating the support with soluble palladium and gold compounds; b) converting the soluble palladium and gold compounds on the support into insoluble compounds by means of an alkaline solution; Or by reducing the insolubles of palladium and gold on the support by means of a reducing agent in the liquid phase; d) washing and subsequently drying the support; e) impregnating the support with a soluble alkali metal compound; and f) finally drying the support at a maximum of 150 ° C. On the other hand, the invention provides a process for preparing vinyl acetate in the gas phase from ethylene »acetic acid and oxygen or oxygen-containing gases» at the same time »with low formation of high-boiling point compounds on the catalyst of the invention. The boron content of the catalyst is preferably from 0.1 to 154 by weight, in particular from 0.01 to 0.2% by weight. Boron is applied to the support in the form of its compounds, preferably in the form of borates. The application can be carried out in the step mentioned above a) together with the soluble palladium and gold compounds. in step b) together with the alkaline solution or by using a solution of borate as the alkaline solution "in step e) together with the soluble alkali metal compound or in a separate step before the final drying of the support. Preference is given to the application in step b). The catalyst support particles of the invention can have any geometric shape, for example the shape of spheres, tablets or cylinders of the regular or irregular type. The dimensions of the support particles are generally between 1 and a m. The preference is given to spherical shapes, for example spheres having a diameter of 4 to? mm. The support particles are generally called pellets. Suitable supports are the known inert support materials such as silica »aluminum oxide, aluminosil cates, silicates, titanium oxide, zirconium oxide, titanates. silicon carbide and carbon. Particularly suitable supports are supports of this type having a specific surface area of from 50 to 300 m2 / g (measured by the BET method) and a main pore radius of from 50 to 200 A (measured using mercury porosimetry), especially silica (SiOa) and mixtures of SiOa / Ala03.

The total volume of the support is preferred from 0.4 to 1.2 ml / g Less than 10% of this volume would be made by "micropores" having a pore diameter of less than 30A (Angstrom). prepared from aerogenic S Ojg or an aerobic mixture Si0a! / Al! -0: B in the form of vitreous microspheres which can be prepared, for example by flame hydrolysis of silicon tetrachloride or a mixture of tetrachloride si 1 / aluminum trichloride in an oxygen-hydrogen flame (EUA-A-3939 199) These microspheres are commercially available under the name • '• Aerosil or "Carbos 1. The dissolved salts of gold and palladium are adsorbed on the pores of the support material, which is referred to in the prior art as the pore volume impregnation method. Substrates impregnated in this way are treated with an alkaline solution "with the use of a borate solution being preferred" in order to deposit the noble metals as insoluble compounds. These compounds are subsequently subject to a reductive treatment, with the reductive agent being present in the liquid phase. The solids suitable for the catalytically active substances are. in particular »water or unsubstituted carboxylic acids having from 2 to 10 carbon atoms» for example acetic acid, propionic acid. n- and iso-butyric acid and the various acids are valid. Due to its physical properties and also for economic reasons, acetic acid is preferably used as the carboxylic acid. The additional use of an inert solvent is advantageous when the carboxylic acid used is one in which the substances are not sufficiently soluble. Thus, for example, palladium chloride dissolves significantly better in aqueous acetic acid than in glacial acetic acid. Suitable additional solvents are those which are inert and are miscible with the carboxylic acid, for example water or ethers such as tetrahydrofuran or dioxane, but also hydrocarbons such as benzene. Two suitable methods for preparing the catalyst of the invention, designated by I and II 'are described below. A particular step in method II can be carried out in two variants A) and B). In the method I »soluble boron and palladium salts are dissolved in such an amount of solvent that the volume of the solution corresponds to almost 90-110% of the pore volume of the support material. The palladium chloride (II). Sodium chloride palladium (II) and palladium nitrate (II) are suitable examples of soluble palladium compounds, and gold chloride (III), tetrachloroauric acid (III) and its alkali metal salts can be used as soluble gold compounds. In general, the amounts of these compounds used are such that the finished catalyst contains between almost 2 to almost 14 g / l, preferably between 4 and 8 g / l of palladium and between almost 1 to 8 g / l. preferably between 2 and 5 g / 1. of gold. According to with this »the gold content of the catalyst is generally from about 10 to 70% of the mass of the palladium present therein. The solution is adsorbed on the support and the metals are fixed by placing the support for a sufficiently long period in an alkaline solution containing a sufficiently high concentration to precipitate the insoluble metal salts. The fixing step b) can be carried out by immersing the impregnated supports in sufficient alkaline fixing solution so that they are completely covered, with the support particles preferably being agitated »for example as described in EUA-A-5» 332,710 . This method is incorporated here by reference. According to E.U.A.-A-5 »332,710, the impregnated support materials are immersed in an alkaline solution and agitated by rotation from the beginning of the deposition of insoluble noble metal compounds. This rotation of the support particles in the alkaline solution should be continued for at least half an hour from the beginning of the treatment »preferably one hour. The rotation and immersion can be up to 4 hours. The treated support particles are then allowed to stand in the fixing solution in order to ensure that complete precipitation of the noble metal compounds occurs. The alkaline fixing solution can be any solution that is capable of precipitating gold and palladium; Preference is given to the use of borate solutions.

Any type of rotation or similar treatment that keeps the support particles in motion can be used, because the precise method is not critical. However, the intensity of the attack is important. This must be sufficiently uniform to moisten the entire surface of the supports impregnated with the alkaline fixing solution. The agitation of the support particles must be not so vigorous that the insoluble noble metal compounds are lost as a result, ie they are removed from the support surface. The percentage of rotation must be preferably from 1 to 20 revolutions per minute "but it can also be greater depending on the type of support material and the amount of noble metal to be deposited. The different percentages of rotation can be selected and the percentage of rotation also depends on the apparatus used: the size and shape of the supports, the type of support, the impregnation of metal, etc., but it must correspond approximately to the percentages of rotation previously. mentioned. The fixing solution is an alkaline solution, preferably an aqueous solution containing boron compounds. Particular preference is given to using aqueous solutions of borax »potassium tetraborate or mixtures of alkali metal hydroxide solution and boric acid. The alkaline solution may have regulatory properties. Steps c) to f) are then carried out.

In method II »a suitable catalyst support is first impregnated with a solution containing soluble palladium and gold compounds. Separate solutions of palladium and gold compounds can also be used successively »although a drying step must then be carried out between them. For effective impregnation »the volume of the impregnation solution should be from 95 to 100% pore volume of the catalyst support, preferably from 98 to 99%. After impregnation of the support with the soluble palladium and gold compounds, the impregnated support is dried before the palladium and gold compounds are fixed as insoluble compounds. The fixing step consists of at least two separate treatment steps with the alkaline fixing solution. In each of these steps »the amount of alkaline reagent used is at least equal to the amount required to react with the total amount of the soluble noble metal salts present on the support. This amount may be greater than the amount required stoichiometrically for the reaction. The amount of alkaline reagent used in each setting step is preferably less than the amount required for the complete reaction with the soluble noble metal salts. The first fixing step is carried out by bringing the impregnated support and then drying in contact with the alkaline fixing solution. The volume of the fixing solution corresponds to the pore volume of the dried support material.

The amount of the alkaline compound present therein should be such that the molar ratio of alkali metal of the alkaline compound to anions of the soluble salts of metal is from 0.7: 1 to 2: 1. and the volume of the solution should correspond to the absorption capacity of the support in a dry state. The fixing alkaline solution is poured onto the agitated support particles in order to be absorbed and the support particles are allowed to stand for up to 24 hours, preferably 2 to 8 hours. The second fixing stage can be carried out in 2 variants A) and B). Variant A) is carried out by treating the non-dried support particles with a second fixing solution. In this solution, the molar ratio of alkali metal of the alkaline compound to anions of the metal salts is from about 0.2: 1 to 2: 1. The volume of the solution should at least cover the supports. The treatment of the support particles with the second fixing solution should be for a time of up to 16 hours, at least 2 hours »preferably at least 4 hours. In variant B), the supports are treated using the rotation-immersion method according to US-A 5,332,710. In this method, the supports previously fixed in the first stage are immersed in the alkaline fixing solution of the second stage and maintained in rotary movement therein during the initial phase of the second stage. This rotation should be for a time of at least half an hour, preferably one hour. The treatment can be for up to 4 hours before the supports are allowed to remain in the fixing solution in order to ensure complete deposition. Here too »any type of apparatus can be used for the rotation of the support particles. The percentage of rotation is important. This has to be sufficient to bring all the surfaces of the support particles uniformly in contact with the alkaline fixing solution. This should not be so large that the insoluble metal compounds are removed from the surface of the support. The rotation percentage should preferably be from 1 to 20 revolutions per minute, if desired, depending on the type of support material and the amount of metal that must be precipitated on the support. The percentage of rotation also depends on the type of apparatus used »of the size and shape of the supports» of the type of supports »of the amount of metal with which they are treated» etc .. but it should correspond approximately to the percentage of rotation mentioned above. The treatment in the second stage can be equivalent to that of the first stage, using a fixing solution of the same concentration. The total molar ratio of alkali metal to anions of the metal salts for both fixing steps together should preferably be from 1.1: 1 to 3.3: 1. Preference is given to the use of a borate solution as a fixing solution in both stages. Following the fixation stage of method I or the last stage of fixation of method II, the supports are treated with a reducing agent in order to convert the precipitated salts of noble metal and the noble metal compounds present in the same to the metallic shape. This reduction is carried out in the liquid phase »for example using aqueous hydrazine hydrates or an alkali metal borhydride, preferably sodium borohydride. The reduction is carried out preferentially at room temperature. The reducing agent is added in excess to ensure that all metal salts and all metal compounds are converted to the metallic form. Depending on the type of reducing agent, the boron compounds can also be converted to the boron element. The support particles are washed, preferably with distilled water, "in order to remove any chlorine still present in the support material that originates from the impregnation step and has been released by precipitation of the noble metals. The washing is continued until the chlorides have been removed from the support. No more than 1000 ppm of chloride should remain on the catalyst. To ensure the success of the washing process, the washes can be tested with silver nitrate solution. The washing process also serves to remove residues of reducing agent from step c). The catalyst is then dried at temperatures of up to almost 150 ° C, preferably in a stream of nitrogen or air. Finally »the addition of at least one alkali metal compound is necessary. The catalyst is preferably impregnated with an aqueous solution of potassium acetate and then dried. The potassium content of the finished catalyst is between 1 and 4% by weight, preferably between 1.5 and 3% by weight. The preparation of the vinyl acetate is carried out by passing acetic acid »ethylene and oxygen or oxygen containing gases on the finished catalyst at temperatures of 100 to 220 ° C »preferably 120 200 ° C» and pressures of 1 to 25 bar »preferably 1 to 20 bar» with unreacted components being able to be recirculated. The oxygen concentration is advantageously maintained below 10% by volume (based on the mixture of acetic acid gas). However, dissolution with inert gases such as nitrogen or carbon dioxide is sometimes advantageous. Carbon dioxide is particularly suitable for dissolution in a recirculation process because it is formed in small amounts during the reaction.

EXAMPLE X 250 ml of silicon dioxide catalyst support (manufactured by Südchemie) in the form of spheres having a diameter of 7.3 mm were impregnated with 85 ml of an aqueous solution containing 4.6 g of Na.?PdCl.sub.2 and 1.4 g of NaAuCl.sub.4 . The precipitation of the insoluble metal compounds was achieved by the addition of 283 ml of an aqueous solution of 17 g of borax. The vessel was then rotated by means of a rotary evaporator (without application of vacuum) for 2.5 hours at 5 revolutions per minute (rpm). The reduction was subsequently achieved by the addition of 7 ml of hydrazine hydrate in 20 ml of water and the immediate rotation of the vessel at 5 rpm per 1 hour. The supports thus treated were left to rest subsequently for 16 hours. The liquid was then spilled out and the treated supports were washed with deionized water in order to remove the chloride ions. This required a water flow in percentage of 200 ml per minute for almost 5 hours. The pellets thus obtained were dried for one hour at 100 ° C. The reduced catalyst was impregnated with an aqueous solution containing 10 g of potassium acetate and having a volume corresponding to the absorption capacity of the dry support material. The catalyst was then dried again. 200 ml of the finished catalyst were placed in a reaction tube having an internal diameter of 20 mm and a length of 1.5 m. The gas mixture to be reacted was then passed over the catalyst at a pressure of 8 bar to the reactor outlet and at a wall temperature of 150 ° C. This gas mixture consisted of 50% by volume of ethylene »12% by volume of acetic acid» 6% by volume of oxygen and 32% by volume of nitrogen. The results are shown in Table I.

COMPARATIVE EXAMPLE I The procedure of Example I was repeated except that the precipitation of the insoluble metal compounds was effected by the addition of 283 ml of an aqueous solution of 2 g of NaOH. The results are shown in Table I.

CUADflQ 1 Units: Rendim ent: Grams of vinyl acetate per liter of catalyst per hour. Selectivities: Percentage mol based on ethylene reacted.

Claims (14)

NOVELTY PE L? INVENTION CLAIMS

1. - A catalyst for preparing vinyl acetate in the gas phase from ethylene »acetic acid and oxygen or oxygen-containing gases» at the same time »with low formation of high-boiling compounds, whose catalysts consist of palladium and / or its compounds »gold and / or its compounds, and alkali metal compounds on a particulate support, in which the catalyst further comprises boron or boron compounds.

2. A catalyst according to claim 1. further characterized in that the boron content is 0.01 to 1% by weight.

3. A catalyst according to claim 1 »further characterized in that the boron content is 0.01 to 0.2% by weight.

4. A catalyst according to any of claims 1 to 3 »further characterized in that it consists of borates.

5. A catalyst according to any of claims 1 to 4 »further characterized in that the alkali metal compound is potassium acetate.

6. A process for preparing a catalyst for producing vinyl acetate in the gas phase from ethylene. acetic acid and oxygen or gases containing oxygen at the same time, with low formation of high-boiling compounds, whose catalysts consist of palladium and / or its compounds »gold and / or its compounds and alkali metal compounds on a particulate support »In which the catalyst is prepared: a) by impregnating the support with soluble palladium and gold compounds; b) converting the soluble palladium and gold compounds on the support into insoluble compounds by means of an alkaline solution; c) reducing the insolubles of palladium and gold on the support by means of a reducing agent in the liquid phase; d) washing and subsequently drying the support; e) impregnating the support with a soluble alkali metal compound; and f) finally drying the support at a maximum of 150 ° C »in which the boron or boron compounds are applied to the catalyst before final drying.

7. The process according to the rei indication 6 »further characterized in that the content of boron on the catalyst is 0.01 to 1% by weight.

8. The process according to the rei indication 6 »further characterized in that the content of boron on the catalyst is 0.01 to 0.2% by weight.

9. The process according to any of the rei indications 6 to 8. characterized further because the alkaline solution used in step b) consists of boron compounds.

10. - The process according to any of claims 6 to 9 »further characterized in that the alkaline solution used in step b) consists of borates as boron compounds.

11. The process according to any of claims 6 to 10 »further characterized in that the reducing agent in the liquid phase used in step c) is an aqueous solution consisting of hydrazine or an alkali metal borhydride.

12. The process according to any of claims 6 to 11 »further characterized in that the reducing agent used in step c) is sodium borohydride.

13. The process according to any of the rei indications 6 to 12 »further characterized in that the soluble alkali metal compound used in step e) is potassium acetate.

14. The process for preparing vinyl acetate in the gas phase from ethylene »acetic acid and oxygen or oxygen-containing gases» at the same time with low formation of high-boiling compounds on a catalyst in accordance with any of Claims 1 to 5.