The structure of active centers and the ethylene polymerization mechanism on the Cr/SiO2 catalyst:: A frontier for the characterization methods

Today, the Cr/SiO2 Phillips catalysts are responsible for the commercial production of more than one-third of all of the polyethylenes (PEs) sold worldwide. At present, three different industrial modes of operation for the Phillips polymerization process exist, and these are as follows: The solution process which uses an inert hydrocarbon to dissolve the polymer as it is formed; the slurry process known as the Phillips Particle Form process in which a liquid dispersant is used; and the gas phase process which do not require any solvent or dispersant. This review illustrates how much progress has been made in the last period on understanding the site structures and the strategies and the techniques that can be adopted to study the catalyst under working conditions. It is shown that the methods adopted for the Cr/SiO 2 system have paradigmatic character and can be extended to other catalytic systems.