Synthesis, spectroscopy, and catalytic properties of cationic organozirconium adsorbates on "super acidic" sulfated alumina. "Single-site" heterogeneous catalysts with virtually 100 active sites.
Sulfated alumina (AlS), a highly Brønsted acidic sulfated metal oxide, is prepared by the impregnation of gamma-alumina with 1.6 M H(2)SO(4), followed by calcination at 550 degrees C for 3 h. (13)C CPMAS NMR spectroscopy of the chemisorbed (13)C(alpha)-enriched organozirconium hydrocarbyl Cp'(2)Zr((13)CH(3))(2) (2)/AlS (Cp' = eta(5)-(CH(3))(5)C(5)) reveals that the chemisorption process involves M[bond]C sigma-bond protonolysis at the strong surface Brønsted acid surface sites to yield a "cation-like" highly reactive zirconocenium electrophile, Cp'(2)Zr(13)CH(3)(+). 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 mu-oxo Cp'(2)Zr((13)CH(3))-OSi[triple bond] species. Two complementary active site kinetic assays for benzene hydrogenation show that, unlike typical heterogeneous and supported organometallic catalysts, 97 +/- 2% of all Cp'ZrMe(3) (3)/AlS sites are catalytically significant, demonstrating that the species identified by (13)C CPMAS NMR is indeed the active species. 3/AlS mediates benzene hydrogenation with a turnover frequency of 360 h(-1) at 25 degrees C/1.0 atm H(2). Active site assays were also conducted for ethylene polymerization and reveal that 87 +/- 3% of 3/AlS 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 Cp(2)Zr(CH(3))(2) (1), Cp'(2)Zr(CH(3))(2) (2), Cp'Zr(CH(3))(3) (3), Zr(CH(2)TMS)(4) (4), and Zr(CH(2)Ph)(4) (5) (Cp = eta(5)-C(5)H(5)) chemisorption on AlS are 5/AlS > or = 4/AlS > or = 3/AlS > 2/AlS > or = 1/AlS for ethylene homopolymerization at 150 psi C(2)H(4), 60 degrees C. Under identical conditions, the polymerization rate for 3/DA is approximately 1/10th that for 3/AlS.
Nicholas
Christopher P
CP
Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA.
Ahn
Hongsang
H
Marks
Tobin J
TJ