Pre-reduction of the Phillips CrVI/SiO2 catalyst by cyclohexene: A model for the induction period of ethylene polymerization

The pre-reduction of the industrially relevant Cr-VI/SiO2 Phillips catalyst by cyclohexene at room temperature was investigated by means of operando XANES, Diffuse Reflectance UV-Vis and transmission FT-IR spectroscopies. It was found that cyclohexene efficiently reduces the surface chromates sites (Cr-VI) at room temperature to give mainly divalent chromium sites and aldehyde by-products, which are rapidly converted in ester species. These by-products remain in strong interaction with the reduced chromium sites, defining a complex and tunable ligand sphere, which controls the entrance of incoming molecules, including ethylene. Unlike the Cr-VI/SiO2 catalyst, the cyclohexene-reduced Cr/SiO2 catalyst polymerizes ethylene already at room temperature with no induction time. The reactivity of cyclohexene with the Cr-VI species mimics that of ethylene during the induction period on Cr-VI/SiO2 catalyst, simplified by the absence of the further polymerization step. Hence, the results discussed in this work are potentially useful to understand what happens during the induction period in the presence of ethylene