Kinetic study of cyclooctene epoxidation with aqueous hydrogen peroxide over silica-supported calixarene-Ta(V)

Replacing traditional TaCl₅ precursors, calixarene-Ta(V) complexes are grafted on SiO₂ at self-limiting loadings of 0.2 mmol·g^-1 (0.25 Ta·nm^-2) to synthesize an active and selective catalyst for epoxidation with aqueous H₂O ₂. The catalyst is synthesized in a one-pot procedure and has turnover rates that are a very weak function of Ta surface density, in contrast with catalysts that are subsequently calcined, following traditional practice. Cyclooctene epoxidation turnover rates exceed 240 h^-1 and epoxide hydrolysis to the cyclooctanediol is extremely low, <2%, and does not increase with conversion. Calcination decreases turnover rates by 30%, and undesired selectivity to the cyclooctanediol is 5% and increases with conversion. The background rate of unproductive H₂O₂ decomposition is also decreased by 50% when using the calixarene capping ligand as compared to the bare oxide. A kinetic study of this system indicates a near first order dependence of the epoxidation rate on Ta content and H ₂O₂ concentrations; cyclooctene is weak positive order indicating some inhibition. H₂O, product epoxide, and co-product cyclooctanediol are inhibitors, with cyclooctanediol being the strongest inhibitor on a molar basis, underscoring the importance of the reduced selectivity towards this species for the capped catalyst. A kinetic expression is proposed and describes initial rates and epoxide yields with time in good agreement with experimental data. The proposed catalytic cycle includes equilibrated formation of six-coordinate Ta(OH)X surface species as the most abundant intermediates. The rate limiting step is proposed to be the reaction between an activated Ta-hydroperoxide and an alkene in free solution, involving an external nucleophilic attack of the pi-system of the olefin on the relatively electropositive oxygen of the Ta-hydroperoxide species. The bulky and hydrophobic phenolate ligands pay a role in maintaining high Ta oxide dispersion, decreasing inhibition by polar species, and creating intrinsically more Lewis acidic sites, leading to increased epoxidation rates and decreased rates of undesired H₂O₂ decomposition and epoxide hydrolysis. This new class of ligand-capped, supported Ta oxide catalyst is not only more active and selective than its bare oxide analogue, but also provides a well-behaved catalyst for kinetic modeling.