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Silica-supported chromium catalysts for ethylene polymerization: The active oxidation states of chromium

1986; Elsevier BV; Volume: 99; Issue: 1 Linguagem: Inglês

10.1016/0021-9517(86)90207-1

1090-2694

David L. Myers,

biodegradable polymer synthesis and properties

The reduction of Cr(III)SiO2 to Cr(II)SiO2 with CO led to a change in the kinetically significant site for the polymerization of ethylene at low temperatures and pressures. Cr(III)SiO2 displayed higher activity than Cr(II)SiO2 over the same range of reaction temperatures. Cr(II)SiO2 catalysts were characterized by a sharp falloff in polymerization activity at temperatures <40 °C. Activation energies for polymerization over Cr(III)SiO2 and Cr(II)SiO2 were 10.6 and 17.1 kcal mole−1, respectively. Hydrogen sulfide poisoning of Cr(III)SiO2 and Cr(II)SiO2 indicated that the active fraction of Cr was 13 and 34% of the total chromium, respectively. Infrared spectra of adsorbed CO and NO suggest that the active sites on Cr(II) catalysts are C-type Cr(II) ions with at least two vacant coordination positions available for the growing polymer chain and the monomer. Thermal activation of Cr(II)SiO2 catalysts at 700 °C (in vacuo) resulted in both an increase in the concentration of C-type Cr(II) ions and polymerization activity. A decrease in the activation energy of ethylene polymerization for Cr(II)SiO2(700) and a blue shift in the infrared band associated with the nitrosyl complex of this species suggest that the thermal treatment modified the surface coordination of these ions. Most importantly, thermal activation of Cr(II)SiO2 produced a Cr(II) catalyst which is active for ethylene polymerization at 25 °C.