Oxychlorination catalysis. Ethylene adsorption and conductivity studies on copper chlorides

Abstract

The adsorption of ethylene on CuCl, CuCl₂ and on a physical mixture of the two has been studied using temperature-programmed desorption (t.p.d.) and gas-adsorption chromatography (g.a.c.). The number of chemisorptive centres on CuCl estimated by both of these techniques (4 × 10¹² centre cm^–2) correlates well with the estimate of the Cu²⁺ impurity in the CuCl evaluated by conductivity and e.s.r. measurements. The adsorption site is therefore taken to be the Cu²⁺ ion in CuCl. The heat of chemisorption of ethylene on CuCl was found to 43 kJ mol^–1 and the desorption activation energy was between 59 and 67 kJ mol^–1, from which the adsorption is seen to be weakly activated. G.a.c. measurements have also allowed evaluation of the heat of physisorption of ethylene on CuCl; it is 10 kJ mol^–1.

The number of chemisorptive sites on CuCl₂ is estimated by t.p.d. to be lower than on CuCl, having a value of 1 × 10¹² site cm^–2. These sites are thought to be chloride-ion vacancies in the surface of the CuCl₂. This surface is, however, energetically heterogeneous for ethylene chemisorption having desorption energies of 73 and 83 kJ mol^–1, the latter being only ca. 10% of the total number of sites.