Links Between Single-Site Heterogeneous and Homogeneous Catalysis. DFT Analysis of Pathways for Organozirconium Catalyst Chemisorptive Activation and Olefin Polymerization on γ-Alumina

This contribution focuses on the catalytic properties of the organozirconium precatalyst Cp2Zr(CH3)2 chemisorbed on dehydroxylated gamma-alumina (Al2O3), as analyzed via density functional theory. The interactions of the catalytically active cationic Cp2ZrCH3+ adsorbate species are scrutinized at two possible model Al2O3 (110) surface sites, namely mu2-O and mu3-O, representing the principal reactive species on the alumina surface. It is found that zirconocenium coordination occurs via two different geometries (dioxo-bridged and oxo-bridged) at both the mu-O and mu2-O surface sites. This process is compared to that for forming the related homogeneous phase Cp2ZrCH3+H3CB(CeFs)3- ion pair structure. It is found that the interaction of the Cp2ZrCH3 adsorbate species with the Pr2-O sites is far stronger than that with the B3-O sites due to the greater unsaturation of the former. Furthermore, the interaction with the mu3-O sites is weaker than that in the parent homogeneous ion pair. The catalytic activity of the chemisorbed Cp2ZrCH3+ systems for ethylene polymerization is investigated at both mu2-O and mu3-O sites and compared with the analogous Cp2ZrCH3+H3CB(C6F5)3- -mediated process in solution. A Cossee enchainment mechanism proceeds via ethylene pi-complex formation and an alpha-agostic assisted transition state to yield gamma- and beta-agostic insertion products. The overall kinetics of enchainment are closely correlated with the energetics of pi-complex formation, and it is suggested that the differing kinetic behaviors of the surface-bound Cp2ZrR+ species on the various Al2O3 coordination sites and the analogous homogeneous species reflect differences in the olefin pi-complex stabilization energies. These computational results agree well with the experimental data which indicate that only fractions of the surface bound species are catalytically significant but that these are far more catalytically active than the homogeneous analogues.