Process for the preparation of trichloroethylene

The invention relates to a process for the preparation of trichloroethylene from perchloroethylene and hydrogen by means of a copper/rhodium catalyst impregnated with a water-soluble phosphonium halide.

A process for preparing trichloroethylene from perchloroethylene and hydrogen in which a catalyst consisting of an active charcoal support impregnated with copper in elemental or chemically bonded form and palladium, rhodium or ruthenium, in each case in elemental or chemically bonded form, is employed, is known from DE-A 2,819,209 (EP-B 5,263). The high temperatures which are required to obtain satisfactory conversion rates are a disadvantage of this procedure.

To increase the catalyst activity at a lower reaction temperature, DE-A 3,804,265 proposes the use of copper/rhodium catalysts impregnated with phosphines or phosphites. The fact that, in an expensive two-stage process for impregnation of the catalyst support, the aqueous solution of the copper compound and rhodium compound first has to be applied, and in the following step the phosphines or phosphites dissolved in organic solvents have to be applied is a disadvantage of this procedure. Above all, however, the increase in catalyst activity is limited with respect to time, so that the drop in activity after operating times of less than only 1000 hours must be counteracted by an increase in the operating temperature.

The object of the present invention was thus to provide a process for the preparation of trichloroethylene from perchloroethylene and hydrogen, with which high conversions can be achieved even at a relatively low operating temperature, even over long operating times.

The invention relates to a process for the preparation of trichloroethylene from perchloroethylene and hydrogen by means of a supported catalyst consisting of active charcoal having a BET surface area of more than 500 m 2 /g, 0.5 to 20% by weight of copper in elemental or chemically bonded form and 0.01 to 1.0% by weight of rhodium or palladium in elemental or chemically bonded form at a temperature of 150° to 250° C. and a hydrogen pressure of 1 to 10 bar absolute, wherein the supported catalyst is impregnated with 0.1 to 10.0% by weight of a water-soluble phosphonium halide.

Active charcoal, preferably in granular form, having a BET surface area of more than 500 m 2 /g and a particle size of 2 to 10 mm is employed as the catalyst support.

The copper is applied to the support in elemental or chemically bonded form in amount of 0.5 to 20.0% by weight, preferably 5.0 to 15.0% by weight, based on the total weight of catalyst support and active components. Water-soluble copper salts, in particular CuCl 2 , are particularly preferred.

Rhodium is employed in elemental or chemically bonded form in an amount of 0.01 to 1.0% by weight, preferably 0.02 to 0.2% by weight, based on the total weight of catalyst support and active components. Water-soluble rhodium compounds, in particular complex salts of rhodium(III) chloride, are particularly preferred. Instead of rhodium, it is also possible to employ palladium in elemental or chemically bonded form and in the amounts just stated for rhodium. Water-soluble palladium compounds, such as, for example, PdCl 2 , are also particularly preferred here.

According to the invention, the catalyst support is also impregnated with 0.1 to 10.0% by weight, preferably 3.0 to 7.0% by weight, based on the total weight of catalyst support and active components, of a water-soluble phosphonium halide. Water-soluble phosphonium halides of the general formula (Ph 3 PR)X are preferably employed; wherein Ph represents a phenyl radical.

R represents hydrogen or substituted or unsubstituted alkyl or aryl radicals. Examples of these are the methyl, ethyl, propyl, n-butyl, iso-butyl, pentyl, hexyl, heptyl, octyl, nonyl, benzyl, p-chlorobenzyl, p-tert.butylbenzyl, allyl, 2-methyallyl, chloromethyl, dichloromethyl, iodomethyl, ethoxycarbonylmethyl and acetonyl radicals.

Chloride, iodide or bromide is preferably employed as the halide X.

Preferred phosphonium halides are methyltriphenylphosphonium chloride, methyltriphenylphosphonium bromide, ethyltriphenylphosphonium chloride, ethyltriphenylphosphonium bromide, n-propyltriphenylphosphonium chloride, n-propyltriphenylphosphonium bromide, allyltriphenylphosphonium chloride, allytriphenylphosphonium bromide, n-butyltriphenylphosphonium chloride and n-butyltriphenylphosphonium bromide.

Methyltriphenylphosphonium chloride, methyltriphenylphosphonium bromide, ethyltriphenylphosphonium chloride and ethyltriphenylphosphonium bromide are particularly preferred.

The phosphonium halides can be prepared in a manner which is known per se. For example, the triphenylphosphonium salts are obtained by the reaction of triphenylphosphine with the corresponding hydrogen halide compounds. If the optionally substituted alkyl halides are employed, the alkyl-triphenylphosphonium salts are accessible. Further procedures for the preparation of phosphonium halides are described in the following publications: J. Buddrus, Chem. Ber. 107, 2062 (1974); G. Wittig, U. Schollkopf, Chem. Ber. 87, 1318 (1954); D. Denney, L. Smith, J. Org. Chem. 45, 3404 (1962); G. Wittig, M. Schlosser, Chem. Ber. 94, 1373 (1961); G. Aksnes, A. Eide, Phosphorus 4 (3), 209 (1974); Houben-Weyl, Methoden der organischen Chemie (Methods of organic chemistry) 12/1, 79 (1963); and Ullmann, Encyclopadie der technischem Chemie (Encyclopedia of industrial chemistry) 18, 380 (1979).

For the impregnation, the individual components, that is to say copper (salt), rhodium (salt) or palladium (salt) and phosphonium compound, preferably in aqueous solution, are applied to the active charcoal separately or as a mixture, for example by steeping. The catalyst support impregnated in this way is then dried.

For reaction of the perchloroethylene, the catalyst is introduced in poured form into a reaction tube. The reaction is carried out at a temperature of 150° to 250° C. and under a pressure of 1 to 10 bar absolute. The perchloroethylene is preferably reacted in amounts of 0.5 to 5.0 mol per hour and per liter of contact mass (catalyst volume) together with 0.1 to 1 times the molar amount per hour of hydrogen.

The following examples serve to further illustrate the invention: