Modified chromium/silica gel catalysts: impregnation of the silica gel with aluminium acetylacetonate and comparison with chromium on alumina (Alon-C) and SiO2

Abstract

Al₂O₃ supported on silica gel was prepared by impregnating silica gel with aluminium acetylacetonate/ethanol with different Al content (0.2, 0.4, 1 and 2 mmol Al/g SiO₂) and investigated in comparison with silica gel and alumina (Alon-C). It was found that the aluminium oxide is highly dispersed. However, at higher Al concentration not all aluminium is present as surface cations. Polymerization measurements with ethene showed that chromium/Al₂O₃ silica gel catalysts have, after reduction with CO, similar (0.2 and 2 mmol Al) or different catalytic activity by a factor of two (0.4 and 1 mmol Al). Owing to a small increase of the chromium(III) concentration, a faster decrease of the polymerization activity was observed. This was also seen in FTIR spectra of CO adsorbed at low temperature. From low-temperature IR spectra of CO it is concluded, that in contrast to silica gel and Al₂O₃/silica gel, where surface dichromate is stabilised in the activated catalysts, alumina (Alon-C) stabilises only surface monochromate. The surface monochromate on alumina is reduced with CO to mononuclear chromium(II) surface species, while surface dichromate on silica gel is reduced mainly to dinuclear chromium(II) surface compounds.