Abstract
The initial activity of Cr(III) SiO 2 catalysts for ethylene polymerization was found to be a strong function of the activation temperature and the metal loading up to a level of 0.1 wt% Cr. Carbon monoxide poisoning of 0.11 wt% Cr(III) SiO 2 catalysts, activated at 400 and 900 °C, indicated that the active chromium is only 4 to 7% of the total chromium. Reduction of Cr(III) SiO 2 with CO at 400 °C resulted in a loss in activity at 25 °C which was mirrored by an increase in Cr(II) concentration, as determined by chemiluminescence measurements. Increasing the activation temperature from 400 to 900 °C resulted in a 10-fold increase in activity with not more than a 2-fold increase in the number of sites. At low temperatures (< 50 °C) the active site for polymerization is believed to be a surface-isolated Cr(III) ion which is coordinatively unsaturated. Dehydroxylation at elevated temperatures increases the number of coordination sites available for the substrate molecule and the growing polymer chain.
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