The influence of Cr precursors in the ethylene polymerization on Cr/SiO2 catalysts

Cr/SiO2 catalysts, prepared with different precursor salts, were characterized in their calcined and reduced forms and investigated in their activity for the ethylene polymerization. In the low chromium loading (I wt.% Cr), distinct fractions of Cr(VI) and Cr(III) were observed in the calcined form, depending on the precursor salt, This behavior is believed to be connected with the interaction of the chromium salt and the silica surface. After reduction with carbon monoxide, all the chromium in the hexavalent state was reduced to Cr2+, but the Cr3+ remained constant. The catalysts with more Cr6+ species in the calcined state showed better polymerization activity. The polymerization activity is associated with the distribution of Cr2+ species (Cr-A(2+), Cr-B(2+) and Cr-C(2+)). (C) 2002 Elsevier Science B.V. All rights reserved.