An FT-IR and reactor study of the dehydrochlorination activity of CuCl2/gamma-Al2O3-based oxychlorination catalysts

The conversion of ethylchloride into ethylene + HCl on pure and doped alumi na supports and on CuCl2-Al2O3-based oxychlorination catalysts has been inv estigated by pulse reactor and FT-IR spectroscopy. FT-IR spectra of ethylch loride adsorbed on gamma -Al2O3 show weakly molecularly adsorbed species an d ethoxy groups formed by nucleophilic substitution, Additionally, adsorbed diethylether is also observed. The analysis of the gas-phase species shows that ethoxy groups decompose, giving rise to ethylene at 523 K, Under the same conditions, gaseous HCl is also released from the surface and diethyle ther is also observed in the gas phase. Chlorination of alumina with HCl on ly partially hinders the dehydrochlorination mechanism occurring through et hoxy groups. Experiments performed on alumina doped with MgCl2 and KCI indi cate that the site reactivity scale for EtCl conversion to ethylene is the following: bare Al2O3 > CuCl2 on Al2O3 > MgCl2 on Al2O3 > KCl on Al2O3 Dopi ng with MgCl2 and in particular with KCl limits the activity of the bare su pport in the dehydrochlorination, These data strongly support the previous proposal that the exposed alumina surface catalyzes the dehydrochlorination of EDC to VCM, with a consequent loss in selectivity in the oxychlorinatio n reactor. The beneficial effect of doping with KCl and MgCl2 is due to the reduced dehydrochlorination activity. (C) 2000 Academic Press.