Norma Jean Maraschin

Role of silanol groups in formation of supported chromocene catalysts

Abstract Deposition studies have established maximum values for the adsorption of chromocene on dehydroxylated silicas. The process of chemisorption of chromocene changes from reaction of predominantly two to one hydroxyl group as the temperature of silica increases from 100 to 800 °C. Chromocene, deposited on Cab-O-Sil type silicas heated at 200 and 400 °C, formed highly active catalysts for ethylene polymerization. Results from studies with this support are compatible with an active site model which involves reaction of chromocene with free, isolated silanol groups. Sterically hindered hydroxylic compounds such as triphenylsilanol and t-butanol react in solution with chromocene to form dimeric cyclopentadienyl chromium alkoxides. These chromium compounds do not show catalytic activity for ethylene polymerization under conditions typical for the ( C 5 H 5 ) 2 Cr SiO 2 catalyst. Furthermore, deposition of these new chromium compounds on silica did not provide, in most cases, a route to catalytic activity. However, the addition of alkylsilanes to these supported chromium compounds did lead to active catalysts. The polymerization behavior of these catalysts resembles the supported chromocene catalyst. These overall results lend support to an active site model previously described.