Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
61798 | Journal of Catalysis | 2010 | 11 Pages |
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 × 10−2 s−1 unaffected by surface density, demonstrating single-site character. Calixarene-containing catalysts were up to 3× 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 ∼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.
Graphical abstractSupported calixarene–Ta catalysts are synthesized by a one-pot procedure for the epoxidation of cyclohexene. They are stable and have turnover rates insensitive to surface density. The grafted calixarene–Ta catalysts are more active and selective to direct (non-radical) epoxidation than the corresponding ligand-free case and are more selective than the analogous Ti catalysts.Figure optionsDownload full-size imageDownload high-quality image (85 K)Download as PowerPoint slide