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

The aim of this review is to illustrate, on one side, how much progress has been made in the last period on understanding the site structures and, on the other side, the strategies and the techniques that can be adopted to study the catalyst under working conditions. It will be shown that the methods adopted for the Cr/SiO2 system have paradigmatic character and can be extended to other catalytic systems. The next section is devoted to a discussion of the surface chemistry of the silica support, which plays a key role in the anchoring of chromium species. The abundant literature on the Phillips catalyst until 1985 will be shortly reviewed in section 3, and in this respect, we refer to several excellent review papers, most notably the McDaniel one (M. P. McDaniel, Adv. Catal. 1985, 33, 47.). In the successive part of the paper, we will try to accurately describe the answers to the two principal questions concerning the Cr/SiO2 Phillips catalyst (i.e., the structure of the active sites and the polymerization mechanism) given by the various researchers, collecting the recent experimental and theoretical results in the two fields of characterization (section 4) and catalytic activity (section 5). The final conclusion (section 6) is that the increasing use of sensitive and sophisticated surface characterization methods allows us to approach the final target and that the lesson derived from this study has general validity.