The present invention relates to a catalytic composition which may be used in oxychlorination and to a process for the oxychlorination of ethylene using such a catalytic composition.
Oxychlorination, that is to say the chlorination of hydrocarbons by hydrogen chloride in the presence of air or of oxygen, constitutes a reaction which has been known for a long time and is usually carried out in the presence of catalysts consisting of metal salts deposited on inert supports such as aluminas, silica gels, mixed oxides or alternatively clays or other supports of natural origin. Industrially, the catalyst is most often used in a fluid bed but it may also be used in a fixed bed. The metal salts most often used are halides such as copper chloride. However, when used alone, copper chloride has the drawback of being relatively volatile, which results in a fall in catalytic activity and in the yield of the oxychlorination reaction, which is unacceptable in industrial plants.
It is well known to enhance the performance of oxychlorination catalysts consisting of supported copper chloride by addition of alkali metal chlorides, alkaline-earth metal chlorides or chlorides of rare-earth metals (lanthanides). In particular, catalytic compositions for oxychlorination which simultaneously comprise chlorides of copper, of magnesium and of alkali metals on alumina have already been proposed.
Application BP-A-0,255,156 from SOLVAY describes ternary catalytic compositions containing a mixture of chlorides of copper, of magnesium and of an alkali metal chosen from sodium or lithium, which are used in specific proportions and which enable a very good yield of 1,2-dichloroethane to be achieved in a fluid bed process for the oxychlorination of ethylene, while simultaneously reducing the corrosion of the stainless steel reactors by virtue especially of a reduction in the adhesion and lumping together of the catalyst particles.
Application EP-A-0,375,202 envisages ternary catalytic compositions based on copper chloride, magnesium chloride and potassium chloride, containing 30 to 90 g of copper, from 2 to 30 g of magnesium and from 2 to 30 g of potassium per kilo of catalytic composition, with a Cu:Mg:K atomic ratio of 1:0.1-1.0:0.1-1.0.
It has, however, been observed that most of the compositions of the prior art simultaneously comprising copper chloride, magnesium chloride and alkali metal chlorides deposited on alumina cause, in fluidized-bed type reactors for the oxychlorination of ethylene, the deposition of soiling material on the surface of the tubes of the heat exchanger located in the fluid bed. This phenomenon has been observed in particular in processes involving oxygen, in which processes the oxygen is used either in pure form or in the form of an oxygen and nitrogen mixture which is richer in oxygen than air. This behaviour of the catalytic compositions constitutes a considerable obstacle to their use. The reason for this is that an increasingly thick layer of soiling material gradually builds up on the surface of the tubes, resulting in a progressive deterioration of the heat transfer. In addition, this phenomenon may, in the long run, cause corrosion. It in consequently essential to stop the reactors regularly in order to clean the bundle of tubes of the heat exchanger.
One of the objects of the present invention is consequently to provide catalytic compositions of particularly high performance which enable, in a process for the oxychlorination of ethylene in a fluid bed, a high yield of 1,2-dichloroethane to be achieved without causing the deposition of soiling material on the surface of the tubes of the heat exchanger, especially in processes involving oxygen.
Consequently, the present invention relates to a catalytic composition comprising copper chloride, magnesium chloride and potassium chloride deposited on an alumina, which may be used in oxychlorination, characterized in that the catalytic composition contains, expressed as metal, from 30 to 90 g of copper, from 10 to 30 g of magnesium and from 0.1 to 10 g of potassium per kilo of catalytic composition. It also relates to a process for the oxychlorination of ethylene into 1,2-dichloroethane characterized in that the oxychlorination reaction is catalysed by a catalytic composition according to the invention.