The Phillips CrOx/SiO2 catalyst is one of the most commonly used system in
the industrial production of high-density polyethylene (HDPE). This system has
been extensively studied since decades, mainly by means of experimental techniques.
However, the progress in determination of the nature of the chromium sites
and understanding of surface reactions occurring has been rather not satisfying
and many issues are still under debate. Among others, structure of the chromium
oxide species on the surface of the reduced catalyst is not well defined. Likewise, the
mechanism of the catalyst reduction and active site formation is not well established.
Many experimental studies indicate that chromium exists on the surface of
amorphous silica in a wide variety of different forms including monomeric, dimeric
and polymeric species. Different oxidation states are also possible, as Cr(VI), Cr(V),
Cr(IV), Cr(III), Cr(II) were detected. There is a general consensuses that Cr(VI)
dominates at the surface after the catalyst preparation, while Cr(II) and Cr(III) are
mainly formed after contact with a reducing agent, like ethylene or CO. Nevertheless,
the detailed structure of these oxide species, as well as the active sites, is still
strongly discussed. As experimental studies do not show a clear picture about the
nature of silica-supported chromium catalyst, very helpful can be computational
approach which can provide complementary information, not accessible by experimental
techniques.
In this short review we summarize the recent progress in the field of the Phillips
(CrOx/SiO2) catalyst focusing on the most relevant theoretical papers that were published
within last couple of years. We also highlight the need of applying advanced
models if realistic theoretical description of the CrOx/SiO2 system is to be achieved.
Additionally, different computational approaches in modeling of heterogenous catalysts
are discussed.
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