Abstract
Supported Ta(V) oxide catalysts were prepared by grafting calixarene-Ta(V) and TaX5 complexes on SiO2 at coverages less than 0.25 Ta nm(-2). Thermogravimetric and elemental analyses and UV-visible spectroscopy indicate that catalysts consist of isolated, 1:1 Ta:ligand surface sites. Catalysts obtained by this one-pot procedure were studied in cyclohexene and cyclooctene epoxidation with H2O2. In sharp contrast with bare oxides, calixarene-containing catalysts had initial cyclohexene direct epoxidation turnover rates of 3.9 +/- 0.1 x 10(-2) s(-1) unaffected by surface density, demonstrating single-site character. Calixarene-containing catalysts were up to 3x more active than the corresponding TaCl5-based catalysts at high surface densities and were also up to 95% selective to direct (non-radical) cyclohexene epoxidation versus <65% selectivity for TaCl5-based catalysts or only similar to 20% selectivity for grafted calixarene-Ti(IV) catalysts. Capping silanols with octanol reduced epoxide hydrolysis from >50% to <30%. These catalysts demonstrate the utility of ligand-protected, supported Ta catalysts for epoxidation with H2O2 and demonstrate that a surface ligand on an otherwise traditionally prepared supported oxide can selectively direct oxidation down mechanistically distinct pathways. (C) 2010 Elsevier Inc. All rights reserved.