Hydrogenation of olefins and polymerization of ethene over chromium oxide/silica catalysts: V. In situ Infrared measurements and investigation of the polymer

Abstract

The mechanism of the polymerization of ethene over chromium oxide/silica catalysts was investigated by in situ infrared measurements of the catalyst after D 2 activation, CO adsorption, or polymerization of ethene. After D 2 treatment a broad band at 2550–2760 cm −1 showed the presence of OD groups, caused by a spillover of deuterons via chromium ions toward the silica surface. After polymerization on such a catalyst an indication was obtained for the presence of CD bonds, while after polymerization on a normally reduced catalyst (CO, 470 °C) the absorption band of a CH 3 group was observed. Both facts as well as results presented earlier point to an initiation reaction of ethene with H (or D) atoms from the silica surface and a Cr 2+ ion to form an ethyl group bonded to a chromium ion. Propagation then proceeds by cis insertion of σπ-adsorbed monomer. Evidence for this was obtained from the CO adsorption experiments. The produced polymer was isolated from the catalyst and its molecular weight was determined. From these measurements a number of active sites of 0.3% of all chromium atoms could be deduced. Such a small number would not have allowed an observation by ir of CH 3 groups. The discrepancy was explained by taking into account the porous structure of the catalyst; only polymer chains at the outer ends of the pores can grow to full extent while chains located at the interior part will soon become inaccessible for ethene molecules because the pores become clogged by the polymer formed at their outer ends.