Molecular understanding of alumina supported single-site catalysts by a combination of experiment and theory

The nature and structure of grafted organometallic complexes on γ-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(CH₂Su)₄] with alumina dehydroxylated at 500 °C gives {[(Al_sO)₂Zr(CH₂tBu)] ⁺[(tBuCH₂)(Al_s)]^-}, 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, [(Al_sO)W(≡CtBu)(CH ₂tBu)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.