CA2338961A1

CA2338961A1 - Process for producing supported catalysts and their use for preparing vinyl acetate monomer

Info

Publication number: CA2338961A1
Application number: CA002338961A
Authority: CA
Inventors: Alfred Hagemeyer; Harald Werner; Uwe Dingerdissen
Original assignee: Individual
Current assignee: Celanese Sales Germany GmbH
Priority date: 1998-07-31
Filing date: 1999-07-22
Publication date: 2000-02-17
Also published as: WO2000007727A1; DE59903042D1; ATE225683T1; ES2185391T3; JP2002522202A; EP1102635A1; DE19834569A1; EP1102635B1

Abstract

The invention relates to a supported catalyst containing palladium and doped with hafnium for the heterogeneously catalyzed gas-phase oxidation of ethylene and acetic acid into vinyl acetate. To this end the porous support, for example on a SiO2 basis, during or after coating with the active metals and promoters is exposed to suitable hafnium compounds, for example by saturation with soluble hafnium compounds.
The advantage of this hafnium-doped catalyst system is its improved activity and long-term stability in comparison with undoped catalysts.

Description

Description Process for producing supported catalysts and their use for preparing vinyl acetate monomer The present invention describes an Hf-doped palladium-containing supported catalyst, a process for producing it and its use for the synthesis of vinyl acetate.
It is known that vinyl acetate (VAM) can be prepared in the gas phase from ethylene, acetic acid and oxygen;
the supported catalysts used for this synthesis comprise Pd as active metal and an alkali element as promoter, preferably K. Further additives used are Cd, Au or Ba. The metal salts can be applied to the support by steeping, impregnation, spraying, vapor deposition, dipping or precipitation.
US-A-3 743 607 and GB-1 333 449 describe the production of supported Pd/Au catalysts for the synthesis of VAM
by impregnation with Pd/Au salts and subsequent reduction. However, this method does not give surface impregnated catalysts, but instead the noble metals are uniformly distributed over the entire cross section of the pellet.
GB-1 283 737 discloses the production of a surface-impregnated noble metal catalyst by preimpregnation of the support with an alkaline solution and saturation with 25-90o of water or alcohol. Subsequent impregnation with Pd salts and reduction of the precipitated salts to the metal gives surface impregnated catalysts in which the penetration depth of the noble metals is said to be up to 500 of the pellet radius.
In the case of Pd/Au/K catalysts, it has been found to be advantageous to apply the two noble metals to the support in the form of a shell, i.e. the noble metals are distributed only over a zone close to the surface while the regions of the shaped support body lying further inside it are virtually free of noble metal.
The thickness of these catalytically active shells is about 0.1-2 mm.
According to US-A-3 775 342 and US-A-3 822 308, surface-impregnated catalysts are produced by impregnating the support with a solution of Pd/Au salts and with an aqueous base, preferably NaOH, with insoluble palladium and gold hydroxides being precipitated on the pellets in a shell-like surface zone. The hydroxides which have been fixed in the shell in this manner are then reduced to the metals.
GB 1 521 652 uses the same procedure (preimpregnation with Pd/Au salts, drying, precipitation with base, reduction) to obtain surface-impregnated catalysts of the eggwhite type, i.e. only an inner ring of the spherical Si02 support comprises the noble metals, while the inner core and a thin outer shell remain virtually free of noble metals.
According to EP-A-0 723 810, a support preferably doped with Al, Zr, Ti is produced by pretreatment (impregnation) of the support with metal salt solutions and is subsequently used for the above-described base precipitation to form a surface-impregnated Pd/Au/K
catalyst. Starting materials used for doping are, in particular, expensive alkoxides, e.g. zirconium alkoxides. Finally, the catalysts obtained have to be dried at elevated temperatures and calcined at temperatures up to 800°C.
It is therefore an object of the present invention to provide a simple and inexpensive process for producing active and selective VAM catalysts, in particular surface-impregnated catalysts, based on Pd and having a long operating life.
The present invention accordingly provides a process for producing a supported catalyst for the production of vinyl acetate monomer by impregnating the support with a basic solution and a solution comprising palladium salts, where the impregnation is carried out simultaneously or successively, with or without intermediate drying, washing the support to remove any chloride present and reducing the insoluble compounds precipitated on the support before or after washing, drying the catalyst precursor obtained in this way and impregnating it with alkali metal compounds which are converted completely or partially into alkali metal acetates under the reaction conditions in the production of vinyl acetate monomer, where the support is impregnated with one or more hafnium compounds either simultaneously with the noble metal impregnation or in an after-treatment and is dried at a temperature of S 160°C and is not subsequently calcined.
It has now been found that Hf doping is advantageous for the activity and the selectivity and guarantees better adhesion of noble metal to the support. In addition, it has been found that when the catalysts of the invention are produced using hafnium as activator, the additional calcination step can be dispensed with.
The hafnium-doped catalysts of the invention also have a more uniform Pd or Pd/Au active metal distribution and better dispersion of the noble metals than undoped VAM catalysts. The high degree of dispersion is also largely retained in long-term operation owing to reduced agglomeration of the noble metal particles, as a result of which the deactivation of the catalysts of the invention is slowed and long operating lives are obtained.

According to the invention, a porous support, preferably a shaped Si02 support, is doped with Hf by treating the porous support with a suitable Hf precursor during or after loading with a Pd precursor or the fixing/reduction of the Pd precursor.
The support is preferably impregnated with a solution comprising palladium salts or palladium and gold salts and one or more hafnium compounds.
In a preferred embodiment of the invention, Hf doping is carried out by impregnation with a soluble Hf precursor from aqueous or acetic acid or alcoholic solution. Preference is given to a water-soluble precursor. Between the treatment with Hf and loading with the other elements, it is possible to insert further pretreatment or after-treatment steps such as washing, drying, oxidation or reduction.
As support materials, it is possible to use all support materials which are customarily used, e.g. SiO~ or A1203, their oxide mixtures and mixed oxides (aluminosilicates) or C. Preference is given to Si02.
The Si02 support is preferably used as a shaped body and is in the form of spheres, pellets, rings, stars or other industrial shaped bodies. The diameter or the length and thickness of the support particles is generally from 3 to 9 mm. The surface area of the supports, measured by the BET (Brunnauer, Emmet and Teller) method, is generally 10-500 m2/g, preferably 20-250 m2/g. The pore volume is generally from 0.3 to 1.2 ml/g.
The hafnium content of the catalysts of the invention is in the range from 0.01 to 50o by weight. Preference is given to an Hf content in the range from 0.05 to 250 by weight.
Suitable Hf precursors for doping the Si02 are all inorganic Hf salts and organometallic Hf compounds _ 5 which are soluble and contain no catalyst poisons such as sulfur. Preferred precursors are hafnium oxychloride - HfOClz, hafnium chloride - HfCl9, substituted hafnocene dichlorides, substituted cyclopentadienyl-hafnium trichlorides, hafnium acetylacetonate, hafnium alkoxides such as the ethoxide or butoxide or propoxide, hafnium oxide, hafnium hydroxide, hafnium nitrate, Hf-thd (= Hf (C11H190z)4) .
By means of further after-treatments of the catalyst loaded with the Hf precursor or precursors, e.g.
washing and drying steps, calcination steps, fixing and reduction of the active Pd component, after impregnation with alkali metal acetate promoters, etc., the Hf precursor can be converted into the final compound, e.g. the oxide or oxychloride, which is then present in the ready-to-use catalyst.
Loading with the active component Pd and the further promoters such as Au, Cd, Ba, K can be carried out by the known methods of the prior art. In general, the catalyst systems based on Pd/Cd/K, Pd/Ba/K or Pd/Au/K
have become established for the synthesis of vinyl acetate.
The catalysts produced using the process of the invention generally have the following metal contents:
The Pd content of the Pd/K/Cd and Pd/K/Ba catalysts is generally from 0.6 to 3.5o by weight, preferably from 0 . 8 to 3 . 0 o by weight, in particular from 1 . 0 to 2 . 5 0 by weight. The Pd content of the Pd/Au/K catalysts is generally from 0.5 to 2.Oo by weight, preferably from 0.6 to 1.5o by weight.
The K content is generally from 0.5 to 4.Oo by weight, preferably from 1.5 to 3.Oo by weight.
The Cd content of the Pd/K/Cd catalysts is generally from 0.1 to 2.5o by weight, preferably from 0.4 to 2.Oo by weight.

The Ba content of the Pd/K/Ba catalysts is generally from 0.1 to 2.Oo by weight, preferably from 0.2 to l.Oo by weight.
The Au content of the Pd/K/Au catalysts is generally from 0.2 to l.Oo by weight, preferably from 0.3 to 0.80 by weight.
In a preferred embodiment of the invention, the catalyst comprises the following composition:
PdXAuyHfZ
where x = 0.7-1.3o by weight, in particular from 0.9 to 1.1% by weight, y - 0-1o by weight, preferably 0.5-1o by weight, in particular from 0.6 to 0.9o by weight, z - 0 . 05-5 o by weight, in particular from 0 . 1 to 2 o by weight.
Suitable salts are all salts of palladium, cadmium, barium, gold and potassium which are soluble and contain no catalyst poisons such as sulfur. Preference is given to the acetates and the chlorides. In the case of the chlorides, PdCl2, NaPdCl4 and HAuCl4 are particularly preferred precursors. However, in the case of the chlorides it has to be ensured that the chloride ions are removed before the catalyst is used. This is achieved by washing the doped support, e.g. with water, after Pd and, if used, Au have been fixed on the support by reduction to the metals.
Suitable solvents are all compounds in which the salts selected are soluble and which can be easily removed again by drying after the impregnation. Solvents which are especially suitable for the acetates are unsubstituted carboxylic acids, in particular acetic acid. Water is especially suitable for the chlorides.
The additional use of a further solvent is advantageous when the salts are not sufficiently soluble in the acetic acid or in the water. Suitable additional solvents are those which are inert and miscible with acetic acid or water. Examples of additional solvents which can be added to acetic acid are ketones such as acetone and acetylacetone, also ethers such as tetrahydrofuran or dioxane, acetonitrile, dimethylformamide and also hydrocarbons such as benzene.
At least one salt of each of the elements (Pd/K/Au/Hf, Pd/K/Cd/Hf, Pd/K/Ba/Hf) to be applied to the Si02 support particles has to be applied. It is possible to apply a plurality of salts of one element, but in general exactly one salt of each of the three elements is applied. The required amounts of the salts can be applied in one step or by multiple impregnation. The salts can be applied to the support by known methods such as steeping, impregnation, spraying, vapor deposition, dipping or precipitation.
Suitable reducing agents are all compounds which are able to reduce the Pd and Au salts used to the metals.
Possible reducing agents are therefore, for example, citrates, formates, hydrazine, hydroxylamine and alkali metal borohydrides. Gaseous reducing agents such as H
or CO or ethylene can likewise be used, but in the case of surface-impregnated catalysts only when a shell structure has already been produced in the impregnation with the metal salts.
The preparation of vinyl acetate is generally carried out by passing acetic acid, ethylene and oxygen or oxygen-containing gases over the finished catalyst at temperatures of from 100 to 220°C, preferably from 12G
to 200°C, and pressures of from 1 to 25 bar, preferably from 1 to 20 bar, with unreacted components being able to be circulated. It is advantageous to keep the oxygen concentration below 10o by volume (based on the gas mixture without acetic acid). However, dilution with _ g _ inert gases such as nitrogen or carbon dioxide may also be advantageous in some cases. Carbon dioxide is particularly suitable as diluent, since it is formed in small amounts during the reaction.
The following examples illustrate the invention.
Example 1 (1o Hf, not calcined):
1.67 g of palladium dichloride PdCl2, 1.40 g of tetrachloroauric acid trihydrate HAuC14y3H20 and 2.29 g of hafnium oxide dichloride octahydrate HfOCl2*8H20 are dissolved in 75 ml of water and added to 100 g of Si02 support pellets of the type KA-160 (Sudchemie) (pore filling method). After the solution has been absorbed, the impregnation mixture is uniformly dried on a rotary evaporator while passing air over the pellets at 70°C
and the impregnated pellets are dried further overnight at 110°C in a drying oven. 2.70 g of KOH are then dissolved in 75 ml of water and added to the dried pellets, mixed thoroughly for 30 minutes on a rotary evaporator, allowed to stand overnight and then washed free of chloride in a Soxhlet using about 10 1 of water. After standing overnight, the pellets are dried at 110°C in a drying oven.
The catalyst is reduced in 10o ethylene/90o N2 at 150°C
for 2 hours.
Subsequently, 2.6 g of potassium acetate are dissolved in 25 ml of water, added to the pellets and mixed well.
The pellets are dried overnight at 110°C.
The finished catalyst comprises 10 of Pd, 0.70 of Au and 1 0 of Hf .

Example 2 (Comparative example) (1o Hf, calcined in air for 4 hours at 300°C):
The catalyst was produced by a method similar to Example 1, with the sole difference being that after the KOH precipitation and before the removal of chloride by washing, the pellets were calcined in air for 4 hours at 300°C.
Example 3 (Comparative example) (1o Hf, calcined in air for 4 hours at 400°C):
The catalyst was produced by a method similar to Example 1, with the sole difference being that after the KOH precipitation and before the removal of chloride by washing, the pellets were calcined in air for 4 hours at 400°C.
Example 4 (5o Hf, not calcined):
The catalyst was produced using a method similar to Example 1 (HAWE65), except that 11.47 g of HfOCl~*8H20 were introduced in place of 2.29 g of HfOC12Y8H20. The catalyst was not calcined.
The finished catalyst comprises 10 of Pd, 0.70 of Au and 50 of Hf.
Example 5 (Comparative example) (5o Hf, calcined in air for 4 hours at 300°C):
The catalyst was produced by a method similar to Example 4, with the sole difference being that after the KOH precipitation and before the removal of chloride by washing, the pellets were calcined in air for 4 hours at 300°C.
Example 6 (Comparative example) (5o Hf, calcined in air for 4 hours at 400°C):
The catalyst was produced by a method similar to Example 4, with the sole difference being that after the KOH precipitation and before the removal of chloride by washing, the pellets were calcined in air for 4 hours at 400°C.
Example 7 (Comparative example) Example 7 corresponds to Example 1 except that Hf doping was omitted.
Example 8 (2o Hf, C1-containing):
0.84 g of palladium chloride, 0.70 g of tetra-chloroauric acid and 2.29 g of hafnium oxide dichloride are together dissolved in 35 ml of water and added to the pellets; the impregnation mixture is dried at 60°C
for 3 hours in an oil pump vacuum.
1.35 g of KOH are dissolved in 35 ml of water and added to the dried pellets. The mixture is allowed to stand overnight. The pellets are washed with about 8 1 of water, dried overnight at 110°C and reduced by means of C2H4 at 170°C. 4 g of potassium acetate are dissolved in 40 ml of water and added to the pellets and the mixture is then dried overnight at 110°C.
Example 9 (2o Hf) 2.29 g of hafnium oxide dichloride are first dissolved in 35 ml of water and added to the pellets. The impregnation mixture is dried in an oil pump vacuum.
1.06 g of palladium acetate and 0.81 g of gold acetate are dissolved in 35 ml of glacial acetic acid and added to the pellets. The impregnation mixture is allowed to dry as described above. 1.35 g of KOH are dissolved in ml of water and added to the pellets and the mixture is allowed to stand overnight. The pellets are washed in a Soxhlet using about 8 1 of water and after washing 35 are dried overnight at 110°C and reduced with C2H4 at 170°C for 3 hours. 4 g of potassium acetate are dissolved in 40 ml of water and added to the pellets.
The pellets are dried overnight at 110°C in a drying oven.

Example 10 (0.5o Hf) 1.06 g of palladium acetate and 0.81 g of gold acetate are dissolved in 40 ml of glacial acetic acid, 0.58 g of hafnium oxide dichloride is added and after 2 minutes the solution is added to the pellets. As much glacial acetic acid as possible is taken off at 60°C in an oil pump vacuum and the pellets are dried overnight at 60°C in a vacuum drying oven. 1.40 g of KOH are dissolved in 40 ml of water and added to the pellets and the mixture is allowed to stand overnight. The pellets are washed in a Soxhlet using about 6 1 of water, dried overnight at 110°C in a vacuum drying oven and reduced in 10o C2H4 at 170°C for 2 hours. 4 g of potassium acetate are dissolved in 40 ml of water and added to the pellets. The pellets are dried overnight at 110°C.
Example 11 (0.250 Hf) 1.06 g of palladium acetate and 0.81 g of gold acetate are dissolved in 40 ml of hot glacial acetic acid, 0.29 g of hafnium oxide dichloride is added and after 2 minutes the solution is added to the pellets. As much glacial acetic acid as possible is taken off at 60°C in an oil pump vacuum. 1.40 g of KOH are dissolved in 40 ml of water and added to the pellets and the mixture is allowed to stand overnight. The pellets are washed in a Soxhlet using about 8 1 of water, dried overnight at 110°C in a vacuum drying oven and reduced in 10o CzH4 at 170°C for 2 hours. 4 g of potassium acetate are dissolved in 40 ml of water and added to the pellets.
The pellets are dried overnight at 110°C.
An overview of the Hf-doped catalysts produced is given in the following table:

Example Hf loading Calcination 5~ 4 hours at 300C in air 6 5g 4 hours at 400C in air 4 5~ not calcined 2 1~ 4 hours at 300C in air 3 1~ 4 hours at 400C in air 1 1~ not calcined 7 0 not calcined 8 2~ not calcined 2~ not calcined 0.5o not calcined 11 0.250 not calcined Reactor tests on the gas-phase oxidation of ethylene and acetic acid to give vinyl acetate:
The catalysts are tested in a fixed-bed tube reactor 5 having a tube diameter of 2 cm. The reactor is heated externally using oil jacket heating. 15 ml of the catalyst pellets are placed in the reactor. The reactor volume upstream and downstream of the catalyst bed is filled with glass spheres. The test apparatus is 10 controlled by a process control system and is operated continuously. The catalyst is first activated and then tested under constant reaction conditions.
The activation comprises a plurality of steps: heating under N2, addition of ethylene, increasing the pressure, addition of acetic acid, maintenance of the conditions, addition of oxygen.
The reaction conditions in the tests are: reaction temperature - 160-170°C, pressure - 8-9 bar gauge pressure. The feed is composed of 64.50 by volume of ethylene, 16.1 by volume of N~, 14.3o by volume of acetic acid and 5.1o by volume of 02. A full analysis of the reactor output is carried out directly at the outlet from the reactor by means of on-line GC (2 column arrangement).

The GC data were used to determine the VAM
selectivities S (= moles of VAM/(moles of VAM + 0.5 moles of COX) and the STY (space-time yield - g of VAM/1 of cat. * h):
Example Fif loading T p S STY

No. o (C) (bar) (o) g/1*h 0.5 170 9 89 1100 11 0.25 170 9 90 820 As can be seen from the table, Hf-doped catalysts which have not been calcined have better performance than the undoped catalyst. Calcined catalysts are all poor.

10 Calcination leads to sintering and thus to deactivation of the noble metal particles on the support.

Claims

1. A process for producing a supported catalyst by impregnating the support with a basic solution and a solution comprising palladium salts, where the impregnation is carried out simultaneously or successively, with or without intermediate drying, washing the support to remove any chloride present and reducing the insoluble compounds precipitated on the support before or after washing, drying the catalyst precursor obtained in this way and impregnating it with alkali metal compounds which are converted completely or partially into alkali metal acetates under the reaction conditions in the production of vinyl acetate monomer, where the support is impregnated with one or more hafnium compounds either simultaneously with the noble metal impregnation or in an after-treatment and is dried at a temperature of ~ 160°C and is not subsequently calcined.

2. The process as claimed in claim 1, wherein the support is impregnated with a solution which further comprises Au, Cd and/or Ba compounds as activators in addition to the palladium salts and the hafnium compound or compounds.

3. The process as claimed in claim 1 or 2, wherein hafnium compounds which are soluble in water, acetic acid or alcohol are used.

4. The process as claimed in claim 3, wherein the hafnium compounds are selected from the following group: HfOCl2, HfCl4, substituted hafnocene dichlorides, substituted cyclopentadienylhafnium trichlorides, hafnium acetylacetonate, hafnium alkoxides, HfO2, Hf(OH)4, Hf(NO3)4, Hf(C11H18aO2)4).

5. The process as claimed in any one of claims 1 to 4, wherein the catalyst has a hafnium content in the range from 0.01 to 50% by weight.

6. A supported catalyst comprising palladium and one or more alkali metal compounds and one or more hafnium compounds as catalytically active components on a porous support material, and obtainable by a process as claimed in claim 1.

7. A supported catalyst as claimed in claim 6, wherein the catalyst has a hafnium content in the range from 0.01 to 50% by weight.

8. A supported catalyst as claimed in claim 6 having the following composition:
Pd x Au y Hf z, where x = 0.7-1.3% by weight, y = 0.5-1% by weight and z = 0.05-5% by weight.

9. A supported catalyst as claimed in any one of claims 6 to 8 for preparing vinyl acetate.