Cr/silica catalyst with Cr loading of 1 wt% has been prepared by impregnating an aqueous solution of chromium(III) acetate and $CrO_3$ onto various silicas. The characterization of surface chromium species formed during the activation process was performed with TPO/R (Temperature Programmed Oxidation/ Reduction) and FTIR spectroscopy. During activation with dry air chromium(III) acetate was stabilized on 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 amount of Cr(VI) species were varied with activation temperature. The fraction of the Cr(VI) atoms, existed as a monochromate, increased as the activation temperature increased arising from 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), $Cr_C$(II)) of which amounts are strongly dependent on the activation temperature.
The Cr(VI) catalyst activated with dry air and Cr(II) catalyst prereduced with CO at 623 K showed a different polymerization rate profile. In case of Cr(II) catalyst, there was no induction time and the shape of polyethylene particles resembled the catalyst particles. As first ethylene was polymerized mostly on the exterior surface of Cr(II) catalyst. In the case of Cr(VI) catalyst, however, there was induction time and the polyethylene particles were fractured severely. This fragmentation is dependent on the process of active site formation. The molecular weight of polyethylene polymerized over Cr(II) catalyst was higher than that of Cr(VI) catalyst.
The hydroxyl groups were dehydroxylated due to the water gas shift reaction during the CO reduction above 670 K. $Cr_A$(II) site was disappeared during the reduction with CO at 1123 K. This can be attributed to the incorporation of $Cr_A$(II) species with unstable siloxane formed during the dehydroxylation with CO at 1123 K. Monochromate was formed during the reoxidation of deactivated catalyst.
The effect of physical properties of catalyst support was also investigated. The catalyst having small pores less than 30Å in radius can not polymerize the ethylene. The pore size distribution of catalyst influenced the polymerization rate profile profoundly. Once the pore size is larger than 30Å in radius, the extent of pore volume did not affect the kinetic profile. However, the extent of surface area corresponding this pore size significantly affected the polymerization rate profile.
In-situ FTIR study of ethylene polymerization at low pressure was investigated. The hydroxyl group did not participate in the initiation reaction of ethylene polymerization over Cr(II) prereduced with CO at 623 K. Ethylene polymerization over Cr(II)catalyst in which hydroxyl group was exchanged with OD group did not show any ir band of C-D bond. Methyl group could not observed after polymerization through FTIR spectroscopy. These results imply that the initiation reaction might be carried out by a metallacyclobutane intermediate.
