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

Abstract

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.

Impact of Al Activators on Structure and Catalytic Performance of Cr Catalysts in Homogeneous Ethylene Oligomerization – A Multitechnique in situ/operando Study

“…This is only possible by EXAFS discussed below. The very weak prepeak at 5986 eV was assigned to a Cr II state as well by Groppo et al . and can only be unequivocally observed for the activator MMAO.…”

“…The corresponding XANES spectra show an increased splitting between the prepeak shoulder, now around 5990.5 eV, and the main edge (Table ). Since the detection limit for a minor fraction in XANES spectra was estimated to 10 % by Groppo et al . it can thus be concluded that the prepeak shoulder at 5992.0 eV is the signature of Cr II in the present system.…”

“…Apart from the pre‐edge feature, also the edge position changes upon adding different activators due to changes in the oxidation state. As pointed out by Groppo et al., chromium valence state determination from K‐edge data is challenging, since the pre‐edge and XANES region is very sensitive to the ligand environment, it is affected by the presence of different species that are averaged in the spectral signal . It has thus to be emphasized here that the following discussion has to be considered under the basic assumption that a fraction of Cr I according to the EPR results is present, which manifest themselves in a systematic way in the pre‐edge signals.…”

CrV Bimetallic Phillips Catalyst Prepared by Citric Acid‐Assisted Impregnation on Ethylene Polymerization

“…[ 6 ] The active center is susceptible to the electronic structures and coordination environments, that a small change in the coordination environment can impact the polymerization performance and the properties of the final product. [ 7 ] Therefore, two strategies were applied in the field throughout the years. A usual method is to introduce new components on the SiO 2 support to produce a more favorable coordination environment.…”

“…For the support CrO x /SiO 2 Phillips catalyst, the molecular structure of chromate has been reported to exist in the forms of mono‐, dichromate, or even polychromate. [ 7 ] Rebenstorf and Larsson [ 14 ] determined that the catalyst surface was a bis‐chromate structure by the CO bridge bond adsorption on the Phillips catalyst at low temperature in the infrared spectrum. According to Raman spectroscopy and diffuse reflectance spectroscopy, Weckhuysen et al [ 10 ] inferred that the industrially applied Cr/Silica catalysts existed in the form of monochromates, while at higher chromium loadings, polychromates also occurred.…”