AN INFRARED STUDY OF CO/C2H4 COADSORPTION AND REACTION ON SILICA-SUPPORTED CR(II) IONS

Ethylene and CO interact at low pressure on a reduced Phillips catalyst to yield CO/C2H4 mixed complexes involving the most exposed type of Cr(II) (A type). Cr centres responsible for ethylene polymerization form the most stable mixed complexes. At high pressure (greater-than-or-equal-to 40 torr), Cr(A)(CO)C2H4 complexes react with a second ethylene molecule to yield a metallacyclohexanone species. After reaction, polymerization is prevented and oligomerization of ethylene is only observed on less reactive sites. A hierarchy of ions Cr(A) may be established in this way. The implications for the polymerization mechanism are discussed.