Sulfated alumina (AIS), a highly Brønsted acidic sulfated metal oxide, is prepared by the impregnation of γ-alumina with 1.6 M H2SO4, followed by calcination at 550°C for 3 h. 13C CPMAS NMR spectroscopy of the chemisorbed 13Cα-enriched organozirconium hydrocarbyl Cp′'2Zr(13CH3)2 (2*)/AIS (Cp′ = η5-(CH3),5C5) reveals that the chemisorption process involves M-C σ-bond protonolysis at the strong surface Brønsted acid surface sites to yield a "cation-like" highly reactive zirconocenium electrophile, Cp′2Zr13CH3+. In contrast, chemisorption of 2* on dehydroxylated alumina (DA) yields a similar cation via methide transfer to surface Lewis acid sites, while chemisorption onto dehydroxylated silica yields a μ-oxo Cp′2Zr13CH3) - OSI≡ species. Two complementary active site kinetic assays for benzene hydrogenation show that, unlike typical heterogeneous and supported organometallic catalysts, 97 ± 2% of all Cp′ZrMe3 (3)/ AIS sites are catalytically significant, demonstrating that the species identified by 13C CPMAS NMR is indeed the active species. 3/AIS mediates benzene hydrogenation with a turnover frequency of 360 h-1 at 25 °C/1.0 atm H2. Active site assays were also conducted for ethylene polymerization and reveal that 87 ± 3% of 3/AIS sites are catalytically active, again demonstrating that nearly all zirconium sites are catalytically significant. Relative rates of ethylene homopolymerization mediated by the catalysts prepared via CP2Zr(CH3)2 (1), Cp′2Zr(CH3)2 (2), Cp′Zr(CH3)3 (3), Zr(CH2TMS)4 (4), and Zr(CH2Ph)4 (5) (Cp = η5-C5H5) chemisorption on AIS are 5/AIS ≥ 4/AIS ≥ 3/AIS > 2/AIS ≥ 1/AIS for ethylene homopolymerization at 150 psi C2H4, 60°C. Under identical conditions, the polymerization rate for 3/DA is ∼1/10 that for 3/AIS.
ASJC Scopus subject areas
- Colloid and Surface Chemistry