Abstract
The effect of exposure to C2H4 of different CuCl2/Al2O3 catalysts characterized by a wide copper content (0.25<Cu wt%<9.0) has been discussed. X-ray absorption near edge spectroscopy, electron spin resonance, and near infrared diffuse reflectance spectroscopy (NIR DRS) techniques and IR spectroscopy of adsorbed NO and CO, have been used to demonstrate that the key mechanism of the oxychlorination reaction is the reduction of CuCl2 to CuCl following the path 2CuCl2+C2H4→C2H4Cl2+2CuCl. This reaction holds for the cupric ions in the supported amorphous CuCl2 phase, with its activity enhanced by a very high degree of dispersion as demonstrated by chemisorption measurements (up to 72%). In contrast Cu(II) cations forming the surface copper aluminate phase are unreactive toward ethylene. A detailed analysis of the IR spectra obtained at liquid nitrogen temperature allows extraction of important information on catalyst support: (i) Cl− anions are hosted on the support surface and their presence enhances the Brønsted acidity of the surface >Al–OH groups; and (ii) >Al3+ coordinative unsaturated Lewis acidic sites are present on the catalyst. These results point out the possible determinant role of the support in side reactions and coking that are responsible for loss of selectivity and, respectively, catalyst decay during industrial runs.
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