FTIR study of chromium carbonyl complexes on silica–alumina: Active ethene polymerisation catalysts

IR spectra are reported of [Cr(CO) 3 (η-C 6 H 6 )] and Cr(CO) 6 adsorbed on silica–alumina. The molecules were physically adsorbed, O-ligated to two types of Lewis-acidic Al 3+ site or underwent ligand loss to give sub-carbonyl species which were active as ethene polymerisation catalysts. Ligand loss was promoted by evacuation at 25°C, thermal ageing at 30–80°C, or exposure to ethene and involved retention of incompletely coordinated Cr on the surface. Contact with ethene gave highly linear polyethene chains. The efficiency of silica–alumina as a support for active catalysts is attributed to the high population of strongly Lewis-acidic surface sites.