Silica-alumina catalysts, active sites ethylene polymerization

In addition to the differences in catalytic activity, there are other fundamental differences between the two catalysts. With alumina, surface hydrogen participates both in the adsorption (on sites II) and in the polymerization of ethylene. With silica-alumina this is not the case, at least not in the adsorption of ethylene. Related results have been obtained by Ozaki and Kimura 22) who have recently studied the isomerization of w-butenes on deuterated acid catalysts and in the presence of deuteropropylene on nondeuterated acid catalysts. In the initial stages of the reaction scarcely any deuterium was found in the reactant butene on silica-alumina while it appeared extensively in the reactant and the product on alumina. 

Phillips Chromox Catalyst. Impregnation of chromium oxide into porous, amorphous silica-alumina followed by calcination in dry air at 400-800°C produces a precatalyst that presumably is reduced by ethylene during an induction period to form an active polymerization catalyst (47). Other supports such as silica, alumina, and titanium-modified silicas can be used and together with physical factors such as calcination temperature will control polymer properties such as molecular weight. The precatalyst can be reduced by CO to an active state. The percent of metal sites active for polymerization, their oxidation state, and their structure are the subject of debate. These so-called chromox catalysts are highly active and have been licensed extensively by Phillips for use in a slurry loop process (Fig. 14). While most commonly used to make HDPE, they can incorporate a-olefins to make LLDPE. The molecular weight distributions of the polymers are very broad with PDI > 10. The catalysts are very sensitive to air, moisture, and polar impurities. 

---