Grafted Ta–calixarenes: Tunable, selective catalysts for direct olefin epoxidation with aqueous hydrogen peroxide

Abstract

Supported Ta(V) oxide catalysts were prepared by grafting calixarene–Ta(V) and TaX 5 complexes on SiO 2 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 H 2O 2. 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 TaCl 5-based catalysts at high surface densities and were also up to 95% selective to direct (non-radical) cyclohexene epoxidation versus <65% selectivity for TaCl 5-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 H 2O 2 and demonstrate that a surface ligand on an otherwise traditionally prepared supported oxide can selectively direct oxidation down mechanistically distinct pathways.