title: Molecular understanding of alumina supported single-site catalysts by a combination of experiment and theory.
abstract: The nature and structure of grafted organometallic complexes on gamma-alumina are studied from a combination of experimental data (mass balance analysis, IR, NMR) and density functional theory calculations. The chemisorptive interactions of two complexes are analyzed and compared. The reaction of [Zr(CH2tBu)4] with alumina dehydroxylated at 500 degrees C gives {[(AlsO)2Zr(CH2tBu)]+[(tBuCH2)(Als)]-}, a bisgrafted cationic complex as major surface species. The DFT calculations show that the reaction with surface hydroxyls is very exothermic and that alkyl transfer on Al atoms is favored. In contrast, [W(CtBu)(CH2tBu)3] reacts with an alumina treated under identical conditions to give selectively a monografted neutral surface complex, [(AlsO)W(CtBu)(CH2tBu)2]. This was inferred by the evolution of 1 equiv of tBuCH3 per grafted W and the presence of remaining hydroxyls. The calculations show that the reaction of [W(CtBu)(CH2tBu)3] with surface hydroxyls is in fact less exothermic and has a considerably higher activation barrier than the one of the Zr complex. Additionally, the transfer of an alkyl ligand onto an adjacent Al center is disfavored, and hence cationic species are not formed. Some ligands of this monoaluminoxy surface complex interact with remaining surface hydroxyls, which explains the complexity of the experimental NMR and IR data.
authors.affiliation: Laboratoire de Chimie (UMR 5182 CNRS/ENS), Ecole Normale Supérieure de Lyon, 46 Allée d'Italie, F-69364 Lyon Cedex 07, France.
authors.forename: Jérôme
authors.initials: J
authors.lastname: Joubert
authors.forename: Françoise
authors.initials: F
authors.lastname: Delbecq
authors.forename: Philippe
authors.initials: P
authors.lastname: Sautet
authors.forename: Erwan
authors.initials: E
authors.lastname: Le Roux
authors.forename: Mostafa
authors.initials: M
authors.lastname: Taoufik
authors.forename: Chloé
authors.initials: C
authors.lastname: Thieuleux
authors.forename: Frédéric
authors.initials: F
authors.lastname: Blanc
authors.forename: Christophe
authors.initials: C
authors.lastname: Copéret
authors.forename: Jean
authors.initials: J
authors.lastname: Thivolle-Cazat
authors.forename: Jean-Marie
authors.initials: JM
authors.lastname: Basset