Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Redox Processes Leading to the Active Site

The detailed mechanism by which ethylene
polymerization is initiated
by the inorganic Phillips catalyst (Cr/SiO2) without recourse
to an alkylating cocatalyst remains one of the great unsolved mysteries
of heterogeneous catalysis. Generation of the active catalyst starts
with reduction of CrVI ions dispersed on silica. A lower
oxidation state, generally accepted to be CrII, is required
to activate ethylene to form an organoCr active site. In this work,
a mesoporous, optically transparent monolith of CrVI/SiO2 was prepared using sol–gel chemistry in order to monitor
the reduction process spectroscopically. Using in situ UV–vis
spectroscopy, we observed a very clean, stepwise reduction by CO of
CrVI first to CrIV, then to CrII.
Both the intermediate and final states show XANES consistent with
these oxidation state assignments, and aspects of their coordination
environments were deduced from Raman and UV–vis spectroscopies.
The intermediate CrIV sites are inactive toward ethylene
at 80 °C. The CrII sites, which have long been postulated
as the end point of CO reduction, were observed directly by high-frequency/high-field
EPR spectroscopy. They react quantitatively with ethylene to generate
the organoCrIII active sites, characterized by X-ray absorption
and UV–vis spectroscopy, which initiate polymerization.