Ligands Make the Difference! Molecular Insights into CrVI/SiO2 Phillips Catalyst during Ethylene Polymerization.

Operando-sensitive spectroscopic techniques were employed for investigating the changes in the molecular structure of the Cr sites in the CrVI/SiO2 Phillips catalyst during ethylene polymerization. Practically, the most arduous barrier to be overcome was the separation of the chromates reduction carried out by ethylene from the subsequent polymerization. By carefully tuning the experimental parameters we succeeded in observing these two events separately. We found that the sites involved in ethylene polymerization are mainly divalent Cr ions in a 6-fold coordination, in interaction with the oxygenated byproduct (mostly methylformate, generated from the disproportionation of two formaldehyde molecules). Unreduced CrVI species are also present during ethylene polymerization as well as reduced Cr species (either CrII or CrIII) acting as spectators. Our results challenge the old vision of "naked" chromium species (i.e., low coordinated) as the active sites and attribute a fundamental role to external (and flexible) oxygenated ligands that resemble the ancillary ligands in homogeneous polymerization catalysis.

Low temperature activation and reactivity of CO2 over a Cr(II)-based heterogeneous catalyst: a spectroscopic study.

A new heterogeneous catalyst for CO(2) activation was identified in the Cr(II)/SiO(2) Phillips catalyst, one of the most important catalysts used industrially for olefin polymerization. Interestingly, it was found that Cr(II)/SiO(2) strongly activates CO(2) already at room temperature, making it available for chemicals synthesis. A preliminary attempt in this direction was done by following the reaction of CO(2) with ethylene oxide at room temperature by means of FT-IR spectroscopy, which showed the formation of ethylene carbonate. Besides non-reductive CO(2) activation, Cr(II)/SiO(2) showed good performances in catalytic reduction of CO(2) to CO, when heated under mild conditions or irradiated with UV-Vis light. Both, in situ FT-IR and UV-Vis spectroscopy, were applied to highlight the redox process occurring at the Cr centres. These results open interesting perspectives to be developed in the field of CO(2) chemical fixation.

Ethylene polymerization on a SiH4-modified Phillips catalyst: detection of in situ produced α-olefins by operando FT-IR spectroscopy.

Ethylene polymerization on a model Cr(II)/SiO(2) Phillips catalyst modified with gas phase SiH(4) leads to a waxy product containing a bimodal MW distribution of α-olefins (M(w) < 3000 g mol(-1)) and a highly branched polyethylene, LLDPE (M(w) ≈ 10(5) g mol(-1), T(m) = 123 °C), contrary to the unmodified catalyst which gives a linear and more dense PE, HDPE (M(w) = 86,000 g mol(-1) (PDI = 7), T(m) = 134 °C). Pressure and temperature resolved FT-IR spectroscopy under operando conditions (T = 130-230 K) allows us to detect α-olefins, and in particular 1-hexene and 1-butene (characteristic IR absorption bands at 3581-3574, 1638 and 1598 cm(-1)) as intermediate species before their incorporation in the polymer chains. The polymerization rate is estimated, using time resolved FT-IR spectroscopy, to be 7 times higher on the SiH(4)-modified Phillips catalyst with respect to the unmodified one.