On the first stages of the ethylene polymerization on Cr2+/SiO2 Phillips catalyst:: time and temperature resolved IR studies

IR spectroscopy, even in the time resolved mode, failed up to now in the identification of the species formed during the initiation of the ethylene polymerization step on the Cr2+/SiO2 Phillips catalyst. To overcome this problem in this investigation we have designed and performed new experiments allowing to collect IR spectra at lower T or/and in presence of a poison (CO), which are both experimental conditions where the polymerization rate is so highly depressed to hopefully allow the detection of the species formed in the initiation stage. Time, pressure and temperature dependent IR spectra collected in the 173-300K range on the Cr2+/SiO2 catalyst in CO/C2H4 atmosphere show, at the lowest temperatures, the formation of mixed Cr2+...(CO)(CH4) physisorbed complexes and, upon increasing the temperature the switching on of the C2H4 polymerization. A 5-10% fraction of Cr2+ sites forming the mixed Cr2+...(CO)(C2H4) is sufficient to greatly inhibit the polymerization activity. This suggest that the Cr2+ sites active at 300 K represent a small fraction of the total number of Cr2+ sites present on the surface of the catalyst. (C) 2003 Elsevier Science B.V. All rights reserved.