New aspects of the induction period of ethene polymerization using Phillips CrOx/SiO2 catalyst probed by XPS, TPD and EPMA

In this work, an industrial Phillips CrOx/SiO2 catalyst engaged in the induction period through interaction with ethene at room temperature (RT) was studied by the combination of surface analytical methods: X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and electron probe microanalysis (EPMA). XPS method disclosed the reduction of surface chromate species (expressed as Cr(VI)O-x.surf) into surface-stabilized Cr3+ and Cr2+ species (expressed as Cr(III)O-x.surf and Cr(II)O-x.surf, respectively) by monomer during the induction period and the simultaneous formation of two kinds of surface carbon species, namely, formaldehyde and unsaturated hydrocarbons. The hydrocarbon species were supposed to be composed of not only coordinated ethene, but also adsorbed longer olefin chains. TPD measurement confirmed the formation of formaldehyde. EPMA characterization revealed that the surface Cr aggregation occurred in a low extent during the induction period resulting in a few Cr islands in sizes of several microns on catalyst surface. It was demonstrated that reduction and alkylation of Cr species as well as initiation of ethene insertion had already occurred even in the induction period. A mechanism concerning the reactions occurred during the induction period on the Phillips catalyst was proposed. The Cr(II)O-x.surf species coordinated with one formaldehyde and one ethene is postulated to be the active sites precursor for the alkylation as well as initiation of the ethene insertion. The coordinatively-adsorbed formaldehyde molecules on the catalyst are supposed to be the very reason for the hindered ethene oligomerization rather than normal polymerization, as well as for the induction of the surface Cr aggregation. (C) 2002 Elsevier Science B.V. All rights reserved.