Identification of Cr oxidation state active in ethylene polymerization on Cr/SiO2 at room temperature

High temperature treatment under vacuum or in a pure N₂ atmosphere can be used instead of a reduction step to activate Cr/SiO₂ catalysts used for ethylene polymerization. After this procedure, the Cr species active over a range of temperature has been identified as Cr(III) using CO adsorption (IR triplet bands at 2228, 2214, and 2202 cm^–1) and an ethylene polymerization activation energy of 12.1 kcal/mol. The Cr(III) species has also been shown to be present after activation using CO reduction, the more commonly used procedure. Cr(II) sites are always present when activation involves reduction with CO and become increasingly active as the temperature rises above room temperature.