Characterization of Cr/silica ethylene polymerization catalyst by TPO/TPR and FT—IR

Abstract

A Cr/silica catalyst with a Cr loading of 1 wt.% has been prepared by impregnating an aqueous solution of chromium(III) acetate onto Davison 952 silica. The characterization of the surface chromium species formed during the activation process was performed using TPO/R (temperature-programmed oxidation/reduction) and FT—IR spectroscopy. During activation with dry air, chromium(III) acetate was stabilized on the surface as a hexavalent chromium species. The average oxidation numbers of surface chromium species after activation at 823, 923, 1023, 1123, and 1223 K are found to be 6.0, 5.4, 5.1, 4.2, and 3.7, respectively. The amounts of the Cr(VI) species varied with activation temperature. The fraction of Cr(VI) atoms existing in monochromate form increased with increasing activation temperature, which was a result of the decrease in the fraction of dichromate and/or polychromate. The OH stretching bands of Cr/silica prepared by the reduction of Cr(VI)/silica with CO and the stretching bands of CO adsorbed on it indicate the existence of three distinct types of Cr(II) (Cr A(II), Cr B(II), and Cr C(II)), whose amounts are strongly dependent on the activation temperature.