title: Proton transfers are key elementary steps in ethylene polymerization on isolated chromium(III) silicates.
abstract: Mononuclear Cr(III) surface sites were synthesized from grafting [Cr(OSi(O(t)Bu)3)3(tetrahydrofurano)2] on silica partially dehydroxylated at 700 °C, followed by a thermal treatment under vacuum, and characterized by infrared, ultraviolet-visible, electron paramagnetic resonance (EPR), and X-ray absorption spectroscopy (XAS). These sites are highly active in ethylene polymerization to yield polyethylene with a broad molecular weight distribution, similar to that typically obtained from the Phillips catalyst. CO binding, EPR spectroscopy, and poisoning studies indicate that two different types of Cr(III) sites are present on the surface, one of which is active in polymerization. Density functional theory (DFT) calculations using cluster models show that active sites are tricoordinated Cr(III) centers and that the presence of an additional siloxane bridge coordinated to Cr leads to inactive species. From IR spectroscopy and DFT calculations, these tricoordinated Cr(III) sites initiate and regulate the polymer chain length via unique proton transfer steps in polymerization catalysis.
authors.affiliation: Department of Chemistry and Applied Biosciences, Eidgenössische Technische Hochschule Zürich, CH-8093 Zürich, Switzerland;
authors.forename: Murielle F
authors.initials: MF
authors.lastname: Delley
authors.affiliation: Department of Chemistry and Applied Biosciences, Eidgenössische Technische Hochschule Zürich, CH-8093 Zürich, Switzerland;
authors.forename: Francisco
authors.initials: F
authors.lastname: Núñez-Zarur
authors.affiliation: Department of Chemistry and Applied Biosciences, Eidgenössische Technische Hochschule Zürich, CH-8093 Zürich, Switzerland;
authors.forename: Matthew P
authors.initials: MP
authors.lastname: Conley
authors.affiliation: Department of Chemistry and Applied Biosciences, Eidgenössische Technische Hochschule Zürich, CH-8093 Zürich, Switzerland;
authors.forename: Aleix
authors.initials: A
authors.lastname: Comas-Vives
authors.affiliation: Department of Chemistry and Applied Biosciences, Eidgenössische Technische Hochschule Zürich, CH-8093 Zürich, Switzerland;
authors.forename: Georges
authors.initials: G
authors.lastname: Siddiqi
authors.affiliation: Laboratoire de Chimie Catalyse Polymères et Procédés, Unité Mixte de Recherche 5265, Université de Lyon, 69616 Villeurbanne, France; and.
authors.forename: Sébastien
authors.initials: S
authors.lastname: Norsic
authors.affiliation: Laboratoire de Chimie Catalyse Polymères et Procédés, Unité Mixte de Recherche 5265, Université de Lyon, 69616 Villeurbanne, France; and.
authors.forename: Vincent
authors.initials: V
authors.lastname: Monteil
authors.affiliation: General Energy Research Department, Paul Scherrer Institute, CH-5232 Villigen, Switzerland.
authors.forename: Olga V
authors.initials: OV
authors.lastname: Safonova
authors.affiliation: Department of Chemistry and Applied Biosciences, Eidgenössische Technische Hochschule Zürich, CH-8093 Zürich, Switzerland; ccoperet@ethz.ch.
authors.forename: Christophe
authors.initials: C
authors.lastname: Copéret
keywords: C–H activation; heterogeneous catalysis