Tracking the reasons for the peculiarity of Cr/Al2O3 catalyst in ethylene polymerization

[Display omitted] •A heterogeneity of Cr sites are formed when Cr6+/Al2O3 is reduced in CO or in H2.•They differ in oxidation state, local structure and ancillary ligands.•Only the Cr2+4c sites are involved in ethylene polymerization.•CO-reduced Cr/Al2O3 is 15 times faster in ethylene polymerization...

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Published in:Journal of catalysis 2018-01, Vol.357, p.206-212
Main Authors: Martino, Giorgia A., Barzan, Caterina, Piovano, Alessandro, Budnyk, Andriy, Groppo, Elena
Format: Article
Language:English
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Summary:[Display omitted] •A heterogeneity of Cr sites are formed when Cr6+/Al2O3 is reduced in CO or in H2.•They differ in oxidation state, local structure and ancillary ligands.•Only the Cr2+4c sites are involved in ethylene polymerization.•CO-reduced Cr/Al2O3 is 15 times faster in ethylene polymerization than the Cr2+/SiO2 catalyst.•The key is the higher ionicity of the CrOAl bond and the nature of ancillary ligands. Looking to the past, heading to the future. In this contribution we explain the reasons why the Cr/Al2O3 Phillips catalysts exhibit a faster kinetics profile in ethylene polymerization reaction with respect to Cr/SiO2. Diffuse reflectance UV–Vis and FT-IR spectroscopies unequivocally demonstrate that, albeit several types of reduced Cr sites are stabilized by the Al2O3 support, only the 4-fold coordinated Cr2+ sites are active precursors in ethylene polymerization, as for Cr2+/SiO2. Nevertheless, kinetic experiments indicate that ethylene polymerization is 15 times faster on CO-reduced Cr/Al2O3 than on CO-reduced Cr/SiO2. The difference is even more striking (two order of magnitude) when the reaction rates per active Cr sites are compared. Our experimental results suggest two reasons behind the faster polymerization kinetic of Cr/Al2O3: (1) the higher ionic character of the CrOAl bond with respect to the CrOSi one; (2) the nature of the ancillary ligands in the coordination sphere of the Cr active sites (which are mainly carbonates for CO-reduced Cr/Al2O3 and siloxane bridges for CO-reduced Cr/SiO2).
ISSN:0021-9517
1090-2694
DOI:10.1016/j.jcat.2017.11.007