Microkinetic Modeling of Homogeneous and Gold Nanoparticle-Catalyzed Oxidation of Cyclooctene

Small gold nanoparticles (AuNPs) have recently shown potential to act as a catalyst for oxidation reactions mediated by free radicals, with their role postulated to be facilitating hydrogen abstraction by gold superoxo species and/or activation of hydroperoxides. Cyclooctene oxidation using molecular oxygen as the oxidant at 373 K showed high selectivity to the epoxide product, cyclooctene oxide, with either tert-butyl hydroperoxide as an initiator or in the presence of small AuNPs (5-8 atoms). While previous studies have investigated the mechanism leading to high epoxide selectivity using density functional theory, a full microkinetic model was developed in this work using automated network generation to determine the relative contributions of elementary reactions and the role of AuNPs. A cycle of radical addition of peroxy and alkoxy radicals with subsequent epoxidation reactions can justify the observed activity and selectivity to epoxide at multiple temperatures and initiator concentrations. The alcohol, ketone, and hydroperoxide minor products are formed when the peroxy and alkoxy radicals perform hydrogen abstraction or β-scission. The overall selectivity is determined by the competition between the addition and hydrogen abstraction reactions. With the underlying homogeneous reaction model validated, the effect of the AuNPs was determined through an expanded model that includes cycles for both hydrogen abstraction by superoxo gold species and hydroperoxide decomposition via species derived from AuNPs. The model suggests that hydrogen abstraction by the superoxo gold species dominates at short times compared to homogeneous initiation, increasing the activity during the induction period and reducing the time before radical chain oxidation commences. On the time scale of an hour, hydroperoxide decomposition is significantly catalyzed by the presence of gold species, increasing the overall rate of chain propagation.