CH630815A5 - Catalyst comprising noble metal, a support material and optionally additives - Google Patents

Description

The present invention relates to a catalyst which consists of noble metal, a support material and optionally additives, and its production and use.

DE-GS 1 944 933 discloses a process for producing a noble metal on a catalyst comprising an inert support, which is characterized in that the noble metal is brought together by bringing together a solution of a salt thereof with a base-treated, inert support with a moisture content deposits in the range of about 10-90% of the saturation on the resulting carrier. In this process, catalysts are obtained in which the zone of metal penetration is up to approximately 50% of the carrier pellet radius.

US Pat. Nos. 3,775,342 and 3,822,308 describe noble metal supported catalysts on silica supports which are prepared by impregnating the support with noble metal salts, treatment with alkali after the impregnation to give the noble metals as oxides and / or Precipitate hydroxides, which are then reduced to the precious metals. These catalysts can give satisfactory results, especially if they contain palladium and gold and are used for the production of unsaturated acetates from olefins, oxygen and acetic acid. However, it has been shown that the activity and other properties of the catalysts vary from time to time and from batch to batch.

The present invention therefore relates to a noble metal carrier catalyst, in particular a palladium- and gold-containing catalyst with silica as the carrier material, which has a special activity which is obtained reproducibly and lasts for a long time.

The present invention further provides a process for the preparation of such catalysts and their use in the production of vinyl acetate in the gas phase from ethylene, oxygen and acetic acid.

According to the present invention, catalysts have been found which contain reproducible noble metal in a substantially uniform distribution. If the catalysts according to the invention contain 2 metals, they can contain both metals in essentially the same distribution.

A catalyst has now been found which consists of noble metal, a support material and, if appropriate, additives, which is characterized in that it a) a spatial element which is low in noble metal or free of noble metal

Edge zone,

b a precious ring zone rich in precious metals,

c) has a core that is low in precious metals or free of precious metals.

In a special embodiment, these catalysts contain noble metal on a support and consist of

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a) a non-precious or non-precious spatial peripheral zone with a thickness of 5-20% of the radius of the catalyst particles,

b) a noble metal-rich spatial ring zone with a thickness of 5-45% of the radius of the catalyst particles,

c) a noble metal-poor or noble metal-free core with a thickness of 50-75% of the radius of the catalyst particles.

The catalyst according to the invention can contain one or more noble metals. The amount of the noble metal or the noble metals can vary within wide limits. For example, noble metal amounts of 1-10 g per liter of catalyst are suitable. The catalyst according to the invention preferably contains palladium or palladium and gold in the stated quantity range. One liter of catalyst preferably contains 2-5 g of palladium and 1-2 g of gold. Palladium and gold normally form a complete row of mixed crystals (see Gmelin, Handbuch der inorganic chemie, volume 68, page 691). Within the stated quantity ranges, those quantities of palladium and gold are preferred which correspond to mixtures which have a maximum adsorption capacity for hydrogen at temperatures around 200 ° C. According to Gmelin, op. Cit. Page 702, palladium-gold mixed crystals containing 20-45% by weight gold have a maximum adsorption capacity for hydrogen at a temperature of 223 ° C. In particular, the catalysts according to the invention contain 30-35% by weight of gold, based on the total amount of palladium and gold.

The carrier material of the catalyst according to the invention can be of various geometric shapes, for example the carrier material can be designed as balls, tablets or sausages. The geometrical dimensions of the carrier material can be, for example, in the range of 1-8 mm. They can also be in the ranges 4-6 mm or 2-4 mm or 3-6 mm. A suitable geometric shape is in particular the spherical shape, e.g. B. balls with diameters in the range of 4-6 mm.

The specific surface of the carrier material can vary within wide limits. Support materials with an inner surface of 50-300 m2 / g, in particular 100-200 m2 / g (measured according to BET) are suitable, for example.

Suitable carrier materials are, for example, silica, aluminum oxide, aluminum silicates or spinels. A preferred carrier material is silica.

A preferred embodiment of the catalyst according to the invention is a spherical catalyst in which the spatial ring zone rich in noble metal lies within a spherical shell, the inner diameter of which is 70% and the outer diameter of which is 85% of the outer diameter of the spherical catalyst. It is generally advantageous if the mean thickness of the spatial ring zone rich in precious metals is less than 10% of the radius of the spherical catalyst.

Another preferred embodiment of the catalyst according to the invention is that 70-100% by weight of the noble metal present in the catalyst is present in the noble metal-rich spatial ring zone.

The position of the ring zone rich in precious metals can be determined using the relative measured numbers P20s and P20m, which are determined as follows:

20 catalyst balls are embedded in plastic and a cut is placed through the center of each catalyst ball. In the cut you can see a white or light gray edge zone corresponding to the precious metal-poor or precious metal-free spatial edge zone, a dark gray or black ring zone corresponding to the precious metal-rich spatial ring zone and a white or light gray core corresponding to the low-precious metal or non-precious metal core. On each of the 20 balls, the outer diameter of the catalyst ball (d), the outer diameter of the dark gray or black ring zone (da) and the inner diameter of the dark gray or black ring zone (di) are determined, for example with the aid of a measuring microscope. The relative measured number P20s is obtained from these measured values by taking the value for each of the 20 balls

(di + da) • 100 2 • d and calculates the arithmetic mean of these individual values.

The determination of the relative measurement number P20M is carried out in a similar manner as for the determination of the relative measurement number P20s, but the measurement of the ring zone rich in noble metals is carried out by a line analysis along a diameter on the cut through the center of the catalyst ball. The line analysis can be carried out by means of an electron beam microanalyser, the structure and mode of operation of which is described in “Farbe und Lack”, Volume 76, pages 115-123 (1970).

Another preferred embodiment of the catalyst according to the invention has relative measured numbers P2GS and P20m, which are between 70 and 85.

It is advantageous if when there are several precious metals, e.g. of palladium and gold, the distribution of the noble metals along a radius in the catalyst particle is essentially the same. For example, in the preferred catalysts, gold is essentially where palladium is.

Since the catalysts according to the invention contain practically no noble metal in the core, which can account for up to 70% of the particle radius, substantial amounts of noble metal can be saved and at the same time high efficiencies of the catalysts, based on the amount of noble metal used, can be achieved.

Since the precious metal components close to the surface are particularly effective, but the surface is rubbed off when the catalysts are used, it is generally not desirable for economic reasons that precious metal is on or immediately on the surface.

The catalysts according to the invention have precious metal close to the surface, but generally not so close that significant losses due to abrasion are observed during normal use. The noble metal-free or low-noble metal edge zone of the catalyst particles preferably makes up about 15% of the total radius or in absolute values about 0.25 to 0.5 mm, preferably 0.3 to 0.45 mm.

The majority of the precious metal is located between the core, which is free of or free of precious metals, and the peripheral zone, which is free of or free of precious metals.

If the present invention relates to palladium and gold-containing catalysts, it is usually important that within the zone which contains the majority of noble metal, the palladium and the gold occur at the same or almost the same location. A corresponding measurement is possible, for example, by determining the relative P20M for palladium and gold separately. These new measures are called P20M (Pd) and P20M (Au).

Preferred catalysts according to the invention show P20M (Pd) and P20m (Au) values in the range from 70 to 85. A particularly high activity normally occurs when the values of P20M (Pd) and P20m (Au) are the same or only we5

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are slightly different, for example if P20M (Pd) -P2oM (Au) is in the range from + 5 to - 5.

An important way to change the values P20M (Pd) and P20m (Au) is, for example, to check the pH of an aqueous solution, which is obtained, for example, as follows:

1 liter of support material is impregnated with a solution containing palladium salt and gold salt, the amount of salts being sufficient to obtain the desired amount of these metals on the finished catalyst. After the impregnation, the mass is dried and an aqueous alkaline solution is added in an amount which corresponds to 80-100% of the absorbency of the carrier. The alkaline solution is left to act at room temperature for 24 hours. Then enough distilled or deionized water is added until the carrier material is just covered. After 2 hours, the aqueous phase is filtered off and the pH is determined therein. This pH value is called the pH (K) value below.

In a preferred embodiment for producing the catalysts according to the invention, the amount of the alkaline compounds in the solution for treating the impregnated carrier particles is adjusted so that a pH (K) value of more than 7, for example between 7 and 9, preferably between 7, 5 and 8.5 results.

The support material used to produce the catalysts can have different properties, for example a different acidity, which can lead to a different distribution of the noble metals or the noble metal in the catalyst particle, even if the catalyst particles are produced in the same way.

Even carrier material that is produced by the same method can change its properties if the carrier material is subjected to an aftertreatment, for example drying, calcining or storage. It is therefore generally advantageous to determine the optimum amount of the alkaline compound required for the preparation of the catalyst in a preliminary test in which the pH (K) value is determined using the method described above.

Customary additives can be added to the catalyst according to the invention before use. For example, when using the catalyst according to the invention in the production of unsaturated esters from olefins, oxygen and organic acids, the known additions of alkali metal acetates or of compounds which give alkali metal acetates under reaction conditions are advantageous. For example, between 1 and 20% alkali acetate based on the carrier material can be added.

The catalysts according to the invention can be produced as follows:

A solution of one or more noble metal salts is impregnated onto the carrier material described above, for example on silica in the form of spheres with diameters in the range of 4-6 mm and an inner surface in the range of 50-300 m 2 / g. An aqueous solution of the noble metal salts is preferably used here. If palladium is to be applied to the carrier material, the solution to be impregnated can contain, for example, palladium chloride, sodium palladium chloride, palladium nitrate and / or palladium sulfate. If gold is also to be applied to the carrier material, the solution to be impregnated may, for example, additionally contain gold (III) chloride and / or tetrachlorogold (III) acid. In the preparation of the particularly preferred catalysts containing palladium and gold, an aqueous solution of sodium palladium chloride and tetrachlorogold (III) acid is preferably used. The amount of the noble metal salt solution to be impregnated is expediently such that the entire noble metal salt solution is sucked up by the carrier material. For example, you can use as much noble metal salt solution as 80-100% corresponds to the absorbency of the carrier material, preferably as much as 90-95% corresponds to the absorbency of the carrier material. The content of noble metal salts in the solution is usually chosen such that the desired amount of noble metal in salt form is present in the carrier material after the impregnation. After impregnation, drying is usually carried out, preferably at temperatures below 120 ° C., for example drying in an air stream at 100-120 ° C. Subsequently, an alkaline solution is usually added, for example an aqueous solution which contains alkali metal hydroxides, alkali metal carbonates and / or alkali metal carbonates. The alkaline solution is left to act for a period of time, for example 1-50 hours or 10-30 hours. The alkaline solution is generally used in an amount such that an aqueous extract obtained after the exposure time has a pH of more than 7, preferably in the range of 7-9.

It is particularly preferred to use aqueous solutions of sodium or potassium hydroxide. Whether the correct amount of alkali has been used to obtain the desired noble metal distribution in the catalyst can be determined by measuring the pH (K) value using the method described above. If the pH (K) value is less than 7, the distribution of the noble metal is not as desired, as can be determined by more complex measurement methods, for example by determining the values of P20M, P20s, P2oM (Pd) and / or P20M ( Au). If the pH (K) value is greater than 9, there are generally no adverse effects with regard to the noble metal distribution, but in individual cases it can happen that the carrier material is attacked or the noble metal compounds are partially dissolved.

It is generally not possible to change the distribution of the noble metal or metals at this point in the manufacturing process.

Expediently, several samples are taken before the catalyst is prepared and treated with slightly different amounts of alkali, and it is thus determined which amount of alkali gives the desired pH (K) value.

In cases where two precious metals are used, e.g. Palladium and gold, the distribution of the metals over the catalyst cross section is usually very similar if the specified pH (K) value range is maintained. Outside of this pH (K) value range occur zones which contain relatively much of one metal and relatively little of the other metal.

Treatment with the alkaline solution normally converts the precious metal salts into water-insoluble compounds. The composition of these insoluble compounds is generally not known; they may be hydroxides and / or oxides, at least in the cases where the alkaline solution is a solution of sodium or potassium hydroxide. If necessary, washing, for example with water and / or drying, can then be carried out. Then it is usually treated with a reducing agent to convert the present noble metal salts or compounds into the metal form. The reduction can be in the liquid phase, e.g. with aqueous hydrazine hydrate, or in the gas phase, e.g. with hydrogen or hydrocarbons e.g. Ethylene. If the hydrazine hydrate solution is used for the reduction, work is preferably carried out at normal temperature when carrying out the

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Reduction in the gas phase it can be advantageous to work at elevated temperature, for example when reducing with ethylene at 100-200 3C. The reducing agent is expediently used in excess, so that the entire noble metal salts or compounds are certainly converted into the metal form. The catalyst thus prepared is then washed with water to remove salts, for example when using sodium palladium chloride solution, washing is carried out until no more chloride ions can be found. After drying, a catalyst is obtained which a) has a low-precious or non-precious spatial

Edge zone,

b) a noble metal-rich spatial ring zone,

c) has a core that is low in precious metals or free of precious metals. The washing after the reduction can optionally be omitted if all the salts have already been removed by washing carried out after the treatment with alkalis.

Depending on the intended use of the catalyst produced in this way, it can also be provided with conventional additives. For example, additions of alkali metal acetates are advantageous if the catalyst is to be used for the production of unsaturated esters from olefins, oxygen and organic acids. In this case, for example, an aqueous solution of potassium acetate can be impregnated onto the catalyst and then dried.

The catalysts according to the invention have the advantage that they contain no precious metal or only very small amounts of precious metal in the vicinity of the surface and therefore can practically lose no precious metal due to abrasion. On the other hand, it is almost or completely avoided that there is practically no precious metal involved in the chemical reaction in the core of the carrier, so that overall an optimal distribution of the active precious metal is generally achieved.

The catalysts according to the invention can advantageously be used in all reactions for which supported catalysts usually containing noble metals are used.

The catalysts according to the invention can be used with particular advantage in the production of vinyl acetate from ethylene, oxygen and acetic acid in the gas phase. For this purpose, catalysts according to the invention are particularly suitable which contain palladium and gold as the noble metal, silica as the support material and additives of alkali metal acetates. Such catalysts are characterized in the above. Vinyl acetate production, for example, is also characterized by high activity, selectivity and lifespan.

In the production of vinyl acetate using the catalysts according to the invention, a gas stream which contains ethylene, oxygen or air and acetic acid and advantageously small amounts of gaseous alkali metal acetate is normally passed over the catalyst. The composition of the gas flow can be varied within wide limits, taking into account the explosion limits. For example, the molar ratio of ethylene to oxygen can be 80:20 to 98: 2 and the molar ratio of acetic acid to ethylene can be 100: 1 to 1: 100, and the content of gaseous alkali acetate can be 2-200 ppm based on the acetic acid used. Other inert gases such as nitrogen, carbon dioxide and / or saturated hydrocarbons can also be contained in the gas stream. Elevated temperatures are suitable as reaction temperatures, preferably those in the range of 100-250 ° C. Somewhat reduced pressure, normal pressure or increased pressure, preferably a pressure up to 20 atm, can be used as the pressure.

Example 1 (catalyst preparation)

1 liter of a silica carrier in the form of spheres with diameters in the range 4-6 mm, an inner surface of 150 m2 / g (according to BET) and an absorbency of 350 ml water per liter was mixed with 340 ml of an aqueous solution of sodium palladium chloride and tetrachlorogold (IH) acid impregnated. The amount of sodium palladium chloride and tetrachlorogold (III) acid in the solution corresponded to 3.3 g palladium and 1.5 g gold. The impregnated carrier was then dried with hot air at not more than 120 ° C. until the moisture content had dropped below 4%. This was followed by treating the dried material with a solution of 6 g sodium hydroxide in 340 ml water. The mixture was then left to stand at room temperature for 16 hours. Then enough distilled water was added until the catalyst particles were just covered. After 2 hours the water was filtered off and the pH (K) was determined. This was 7.5. Then the catalyst was washed with distilled water for 24 hours. No chloride ions could be detected in the last wash water fractions. The material obtained in this way was dried again and reduced for 8 hours in an unpressurized ethylene stream at 150 ° C. Finally, 30 g of potassium acetate were soaked in an aqueous solution and dried again.

The catalyst produced in this way has a) a space which is low in precious metals or free of precious metals

Edge zone,

b) a noble metal-rich spatial ring zone,

c) a core that is low in precious metals or free of precious metals.

When embedding the catalyst balls in plastic and placing a cut through the center of the ball, you can see a light gray border zone, a black ring zone and a white core. Measuring the ring zone shows

that the ring zone is in an area that makes up 70-85% of the diameter of a catalyst ball.

The determination of the measured numbers P20s and P20M using the methods described above gives the following values: P2oS = 77, P20M = 78. The mean thickness of the spatial ring zone is approximately 5% of the radius of the catalyst balls.

Approx. 90% of the precious metal present in the catalyst is in the spatial ring zone. This catalyst is designated A below.

Example 2 (Catalyst Use)

450 ml of the catalyst prepared according to Example 1 were introduced into a reaction tube with an inside diameter of 25 mm. Abs were over the catalyst at a temperature of 160 ° C and a pressure of 8 bar. 14.4 moles of acetic acid, 58 moles of ethylene and 4.4 moles of oxygen passed in gaseous form per hour. 100 ml of a 0.05 percent by weight solution of potassium acetate in acetic acid were sprayed into this gas stream in front of the reaction tube. 615 g of vinyl acetate were formed per liter of catalyst. 92% of the converted ethylene had converted to vinyl acetate and 8% to carbon dioxide. After 1000 hours of running, no deterioration of these catalyst performances could be determined.

Example 3

A catalyst B was produced, following the procedure of Example 1, but the support material was heated to 650 ° C. for 6 hours before being mixed with the aqueous solution of sodium palladium chloride and tetrachlorogold.

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was impregnated with acid. The determination of the pH (K) value gave 6.5.

Example 4

A catalyst C was prepared in accordance with the procedure of Example 3, but the support material was additionally stored at room temperature for 3 months before it was impregnated with the aqueous solution of sodium palladium chloride and tetrachloroauric acid. The pH (K) value was found to be 7.0.

Example 5

A catalyst D was prepared according to the procedure of Example 3, but the amount of sodium hydroxide was increased so that the determination of the pH (K) gave 8.5.

Example 6

For the catalysts A, B, C and D, the difference between the values P20M (Pd) and P2oM (Au) was determined, which resulted in the following:

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Catalyst pH (K) value P2oM (Pd) - P20M (Au)

(in absolute numbers)

A 7.5 1

5 B 6.5 12

C 7.0 5

D 8.5 1

Example 7

10 Example 2 was repeated, but the catalysts B, C and D were used. The following results were obtained:

Catalyst space-time yield selectivity

(g per liter catalyst% per hour)

• B 350 91

C 550 92

20 D 605 92

Claims (9)

630815 1. Catalyst, which consists of noble metal, a support material and optionally additives, characterized in that it a) a spatial edge zone low in noble metal or free of noble metal, b) a noble metal-rich spatial ring zone, c) has a core that is low in precious metals or free of precious metals. 2. Catalyst according to claim 1, characterized in that zone a) has a thickness of 5-20% of a catalyst radius, zone b) a thickness of 5-45% of a catalyst radius and the core c) a thickness of 50-75% has a catalyst radius. 2nd PATENT CLAIMS 3. Catalyst according to claim 2, characterized in that 70-100 wt .-% of the noble metal present in the catalyst is present in the noble metal-rich spatial ring zone. 4. Catalyst according to claims 1 to 3, characterized in that the catalyst is spherical and the noble metal-rich spatial ring zone lies within a spherical shell, the inner diameter of which is 70% and the outer diameter of which is 85% of the outer diameter of the spherical catalyst. 5. Catalyst according to claim 1, characterized in that it has silica as support material in the form of spheres with a diameter in the range 4-6 mm and an inner surface in the range 100-200 m2 / g, as noble metals gold in an amount of 30 Contains -35% and additionally 100% palladium, the amount of palladium being 2-5 g per liter of catalyst, the spatial ring zone containing at least 70% of the total noble metal and being in a range which has an inside diameter of 70% and an outside diameter of 85% of the total diameter of the catalyst particle, the mean thickness of the ring zone being less than 10% of the radius of the catalyst particle, the catalyst particles P20s-> P20m, P20M (Pd) and P20M (Au) values between 70 and 85 and the difference of P20M (Pd) -P2oM (Au) is between +5 and -5. 6. A process for the preparation of a catalyst as claimed in claim 1, characterized in that a) the support material is impregnated with a solution of at least one noble metal salt, so that the support material absorbs the solution, b) drying the carrier and the absorbed solution, c) adding an alkaline solution to the dried carrier in such an amount that it is absorbed, the noble metal salts being converted into insoluble compounds and an aqueous extract obtained thereafter having a pH of 7-9 and d) treating the carrier with a reducing agent to convert the insoluble precious metal compounds into the corresponding precious metal. 7. The method according to claim 6, characterized in that after step d) an aqueous solution of alkali acetate is applied to the particles and then dried. 8. The method according to claim 6, characterized in that a solution of noble metal salts is used which contains at least one of the salts palladium chloride, sodium palladium chloride and palladium nitrate and at least one of the salts gold (III) chloride and tetrachloroauric acid and chloride ions, the amount of the salt solution Corresponds to 80 to 100% of the absorbency of the carrier material, the carrier is then dried at temperatures below 120 ° C, the alkaline solution in step c) contains sodium hydroxide and is used in an amount which has a pH of 7- in an aqueous extract 9 results, the reduction in step d) is carried out at normal temperature with aqueous hydrazine hydrate or at elevated temperature with gaseous hydrocarbon or hydrogen, the particles are then washed with water until chloride ions are no longer detectable, an aqueous solution of alkali acetate on the particles applied and the particles are then dried . 9. Use of catalysts according to claims 1-5 for the production of vinyl acetate from ethylene, oxygen and acetic acid in the gas phase.