Abstract
Parameters in the thermal activation of Cr/silica and Cr/silica-titania polymerization catalysts have been studied. Both the activity and the molecular weight of the polyethylene formed depended on the activation temperature, suggesting a connection with the surface hydroxyl population surrounding each active center. Hydroxyls could be removed by chemical as well as by thermal means to take advantage of this dependence, such as by calcining the unpromoted support in carbon monoxide, sulfur, or halogen, and then afterward adding the chromium anhydrously. This two-step process also provided evidence of two types of chromium centers yielding high and low MW polymer. Combining all three influences—thermal dehydroxylation, chemical dehydroxylation, and the optimum distribution of active centers—yielded catalysts of extremely high activity and melt index potential.
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