TY  - JOUR
T1  - Tracking the reasons for the peculiarity of Cr/Al2O3 catalyst in ethylene polymerization
AU  - Martino, Giorgia A.
AU  - Barzan, Caterina
AU  - Piovano, Alessandro
AU  - Budnyk, Andriy
AU  - Groppo, Elena
JO  - Journal of Catalysis
VL  - 357
SP  - 206
EP  - 212
PY  - 2018
DA  - 2018/01/01/
SN  - 0021-9517
DO  - https://doi.org/10.1016/j.jcat.2017.11.007
UR  - http://www.sciencedirect.com/science/article/pii/S0021951717303974
KW  - Chromium
KW  - Alumina
KW  - Ethylene polymerization
KW  - Ancillary ligands
KW  - FT-IR spectroscopy
KW  - UV–Vis-NIR spectroscopy
AB  - 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).
ER  -