Characterization of Cr2+ and ethylene polymerization on Cr/SiO2 catalysts

Cr/SiO2 catalysts were prepared by impregnation of chromium acetate (1 and 3 wt.% Cr) and bis(triphenylsilyl)chromate (0.5 wt.% Cr) over commercial si lica. These samples were analyzed by temperature programmed reduction (TPR) and infrared spectroscopy (FT-IR) of CO and C2H4 adsorbed in order to char acterize the chromium species, mainly Cr2+ species distribution (Cr-A(2+), Cr-B(2+) and Cr-C(2+)) and polymerization aspects. The catalysts prepared w ith chromium acetate presented only Cr6+ after calcination, while the catal yst prepared with bis(triphenylsilyl)chromate presented Cr6+ and a fraction of Cr3+. After reduction with CO, the catalysts prepared with chromium ace tate showed more Cr-B(2+) than Cr-A(2+) species. The distribution of Cr2+ s pecies influenced the catalytic activity. The catalyst with 3 wt.% Cr from acetate presented the greatest amount of species Cr-A(2+) and Cr-B(2+) and the best polymerization activity. The behavior of isolated hydroxyls band d uring polymerization tests, characterized by FT-IR, suggests its participat ion in the reaction mechanism. (C) 2001 Elsevier Science B.V. All rights re served.