The polymerization of ethylene on chromium oxide catalysts I. Kinetic studies

Abstract

The rates of polymerization of ethylene on a supported chromium (VI) oxide Phillips catalysts have been measured. Catalysts were calcined in air at 460 °C and activated by pretreatment with ethylene at 300 °C. With increasing pretreatment times the activity of the catalyst increased to a maximum, after which over-reduction occurred and the activity fell. The products of the pretreatment process were water, carbon dioxide and a trace of butenes. Rates of polymerization were first order in ethylene pressure over the temperature range studied ( – 95 to 150 °C). The first order rate constant was sensitive to the initial pressure of ethylene added to the catalyst at the temperature at which the polymerization reaction was carried out. The results are explicable in terms of the production of active centres in the catalyst surface during contact with ethylene at 300 °C. Subsequent evacuation at 300 °C produced from some of these centres sites which had to be reactivated by adsorption of ethylene at low temperatures. The extent of re-activation increased with increasing ethylene pressure. The variation of first order rate constant with temperature showed a maximum at ca . – 23 °C and an apparent activation energy of 0.8 kJ mol -1 for the range –95 to – 23 °C. At temperatures above 227 °C the rate of polymerization was extremely slow.