Catalyst and method for producing vinyl acetate

Details Catalyst and method for producing vinyl acetate Catalyst and method for producing vinyl acetate

2001-10-24

2004-02-24

Nazario-Gonzalez, Porfirio (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acid esters

C560S243000, C560S261000, C502S330000

Reexamination Certificate

active

06696596

ABSTRACT:

The present invention relates to a catalyst comprising palladium and/or its compounds, gold and/or its compounds, alkali metal compounds and vanadium and/or its compounds, and to its use for preparing vinyl acetate from acetic acid, ethylene and oxygen or oxygen-containing gases.
It is known that ethylene can be reacted in the gas phase with acetic acid and oxygen or oxygen-containing gases over fixed-bed catalysts comprising palladium/gold/alkali metal to give vinyl acetate.
The catalysts comprising palladium/gold/alkali metal generally have a particular noble metal distribution in which the noble metals are present in a shell on the support particles, while the core of the support particles is largely free of noble metals. The noble metal distribution in shell form is achieved by impregnation and subsequent precipitation of the noble metals using alkaline compounds. Catalysts having this noble metal distribution display a good activity and generally form little carbon dioxide and ethyl acetate. A further feature of these catalysts is that only small amounts of high boilers are formed when using these catalysts. Although these amounts are only small, they represent a problem in ecological and process engineering terms. Such high boilers are, for example, ethylidene diacetate, ethylene glycol and its acetates or diacetoxy ethylenes.
U.S. Pat. No. 3,775,342 discloses a process for producing catalysts comprising palladium, potassium and gold by impregnation of a support with a solution of palladium and gold salts, subsequent treatment with an alkaline solution which leads to water-insoluble palladium and gold compounds being deposited on the support and subsequent reduction of the metal compounds to the corresponding noble metals. Treatment of the support material with an alkali metal acetate solution can be carried out before or after the reduction step.
U.S. Pat. No. 4,048,096 teaches a process for producing catalysts comprising palladium, potassium and gold, in which process the support material is firstly impregnated with an aqueous solution containing a mixture of palladium and gold salts. Here, the volume of the impregnation solution corresponds to the pore volume of the support material. The moist support is subsequently completely covered with an aqueous alkaline solution, e.g. an aqueous sodium metasilicate solution, and left to stand at room temperature for 12 hours. In this way, the metal salts are converted into water-insoluble compounds and thus fixed to the support material. The palladium and gold compounds are reduced to the corresponding metals by subsequent treatment with a reducing agent. For this purpose, for example, an aqueous hydrazine solution is added with gentle agitation and the mixture is left to stand for 4 hours after the addition. After washing and drying, the support material laden with palladium and gold is treated with an alkali metal acetate solution and dried again. The catalyst obtained has a shell structure in which the palladium and gold are distributed over the surface of the support material in a shell thickness of about 0.5 millimeters.
In the process disclosed in U.S. Pat. No. 5,332,710 for producing a coated catalyst comprising palladium, gold and potassium, the support which has been impregnated with an aqueous palladium and gold salt solution is dipped into an aqueous fixing solution containing sodium hydroxide or potassium hydroxide and is agitated therein for at least 0.5 hour. In the fixing technique disclosed, the support which is completely covered by the fixing solution is agitated by rotation as from the commencement of the treatment with the fixing solution.
It has now surprisingly been found that the addition of vanadium and/or its compounds significantly improves the high-boiler selectivity of the catalyst. For the purposes of the present invention, the high boiler selectivity is the ratio of the amount of high boilers formed in the vinyl acetate synthesis to the amount of ethylene reacted. High boilers are, inter alia, the abovementioned compounds.
The invention accordingly provides a process for preparing vinyl acetate in the gas phase from ethylene, acetic acid and oxygen or oxygen-containing gases over a catalyst comprising palladium and/or its compounds, gold and/or its compounds and alkali metal compounds on a support, wherein the catalyst further comprises vanadium and/or its compounds.
The invention further provides a catalyst comprising palladium and/or its compounds, gold and/or its compounds and alkali metal compounds on a support, wherein the catalyst further comprises vanadium and/or its compounds.
The catalysts of the invention are preferably produced by:
(1) impregnating the support with a soluble vanadium compound and subsequently drying it;
(2) impregnating the pretreated support with soluble palladium and gold compounds;
(3) converting the soluble palladium and gold compounds on the support into insoluble compounds by means of an alkaline solution;
(4) reducing the insoluble palladium and gold compounds on the support by means of a reducing agent;
(5) washing the support and subsequently drying it;
(6) impregnating the support with a soluble alkali metal compound; and
(7) finally drying the support at not more than 150° C.
The steps (2) to (7) are known, for example, from U.S. Pat. Nos. 3,775,342; 4,048,096 and 5,332,710.
Apart from impregnation of the support with the soluble vanadium, palladium and gold compounds and also alkali metal compounds, it is also possible to employ other techniques known to those skilled in the art for applying the catalytically active substances to the support, for example multiple vapor deposition, spraying or dipping, if appropriate with use of ultrasound.
It is likewise possible to exchange steps (1) and (2), i.e. firstly to impregnate the support with a solution comprising palladium and gold compounds and, after drying, to apply a vanadium compound to the treated support.
Suitable supports are the known inert support materials such as silica, aluminum oxide, aluminosilicates, silicates, titanium oxide, zirconium oxide, titanates, silicon carbide and carbon. Particularly suitable supports of this type are those having a specific surface area of from 40 to 350 m
2
/g (measured by the BET method) and a mean pore radius of from 50 to 2000 Å (Ångström) (measured using mercury porosymmetry), especially silica (SiO
2
) and SiO
2
—Al
2
O
3
mixtures. The supports used can have any shape, e.g. spheres, pellets, rings, stars or particles of other shapes, and their diameter or their length and thickness is generally in the range from 3 to 9 mm.
As support, it is possible to choose, for example, aerogenic SiO
2
or an aerogenic SiO
2
—Al
2
O
3
mixture which can be prepared, for example, by flame hydrolysis of silicon tetrachloride or a silicon tetrachloride/aluminum trichloride mixture in a hydrogen/oxygen flame (U.S. Pat. No. 3,939,199).
The vanadium compound to be applied in step (1) is preferably a vanadium salt such as a vanadyl salt, a vanadate or an isopolyvanadate. Particular preference is given to using vanadyl salts such as chlorides, sulfates, oxalates, acetates and acetylacetonates. It is also possible to apply a plurality of vanadium salts and/or vanadyl salts, but just one vanadium salt or vanadyl salt is generally applied.
The elements palladium and gold to be applied in step (2) are preferably applied in the form of salt solutions, either individually in any order or together. Preference is given to using a single solution in which these elements to be applied are present in the form of salts. Particular preference is given to using a single solution in which just one salt of each of these elements to be applied is present.
In the case of interfering anions, e.g. chlorides, it has to be ensured that these anions are largely removed before the catalyst is used. This is achieved by washing the doped support, for example with water, after the palladium and gold applied, for example, as chloride have been converted into an insoluble form, for instance by fixing

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