Alumina-supported copper chloride 3. Effect of exposure to ethylene

The effect of exposure to C2H4 of different CuCl2/Al2O3 catalysts character ized by a wide copper content (0.25 < Cu wt% < 9.0) has been discussed. X-r ay absorption near edge spectroscopy, electron spin resonance, and near inf rared diffuse reflectance spectroscopy (NIR DRS) techniques and IR spectros copy of adsorbed NO and CO, have been used to demonstrate that the key mech anism of the oxychlorination reaction is the reduction of CuCl2 to CuCl fol lowing the path 2CuCl(2) + C2H4 + C2H4Cl2 + 2CuCl. This reaction holds for the cupric ions in the supported amorphous CuCl2 phase, with its activity e nhanced by a very high degree of dispersion as demonstrated by chemisorptio n 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 Bronsted acidity of th e surface > Al-OH groups; and (ii) > Al3+ coordinative unsaturated Lewis ac idic sites are present on the catalyst. These results point out the possibl e determinant role of the support in side reactions and coking that are res ponsible for loss of selectivity and, respectively, catalyst decay during i ndustrial runs. (C) 2001 Academic Press.