Chromia/silica–titania cogel catalysts for ethene polymerisation. Polymerisation kinetics

Abstract

The kinetics of ethene polymerisation over a series of chromia/silica–titania cogels have been studied. The catalysts were activated by reduction in CO at 623 K. Rates of polymerisation were first order in ethane pressure below 373 K. Above 373 K the rates tended towards second-order kinetics. Arrhenius plots were non-linear and exhibited a maximum, the nature of which was dependent upon the reduction time and the catalyst composition. For polymerisation at 273 K the activity of the catalysts passed through a maximum with increasing reduction time. Catalysts which were inactive at 273 K due to extensive reduction were active at 343 K, and in some cases gave bimodal activity curves. Active catalysts were deactivated by evacuation or reduction in CO at 873 K. Rates of polymerisation at 343 K for catalysts deactivated in vacuo at 873 K showed an initial decrease in activity which was followed by a small increase in activity before further deactivation. The results are consistent with a Langmuir–Hinshelwood polymerisation mechanism. Three types of active site have been identified.