The Active Sites in the Phillips Catalysts: Origins of a Lively Debate and a Vision for the Future

In this work, we summarize and critically compare some of the experimental results recently published on the Phillips catalyst, in the attempt to make the point on a few particularly debated questions that have recently animated the specialized literature; in particular, we discuss the structure of the active chromium sites and how ethylene polymerization initiates on them. The data collected in this article unequivocally demonstrate that the structural and electronic properties of the chromium sites strongly depend on the strain of the silica surface, which in turns is affected by both the activation treatment and the chromium loading. This explains, at least partially, the differences of results obtained in different research groups. Another fundamental message is the need of applying the largest possible set of characterization methods, including theoretical calculation on large and flexible models. Our final purpose (and hope) is to promote a positive and constructing discussion on this catalyst, as a premise to create a solid scientific base useful to both the young researchers approaching this field and the industrial researchers who daily work with it.