Elucidating the Role of Water on Limonene Oxidation with H₂O₂ over γ-Al₂O₃

Limonene oxide, which is produced from limonene epoxidation, is a valuable molecule that can be applied in flavor, fragrance, and renewable polymer applications. A catalytic reaction system using H₂O₂ with γ-Al₂O₃ and ethyl acetate (EtOAc) as the solvent has been explored as an effective system for this reaction. In these previous studies, a number of postulates have been proposed as to how water affects the reaction; therefore, the focus of this work is to elucidate the role of water in limonene epoxidation. While not impacting the selectivity to limonene oxide, the amount of water in the reaction system is shown to significantly impact the limonene reactivity. Furthermore, through both addition of excess water and removal of water with a Dean-Stark apparatus, the control of the H₂O₂/H₂O ratio is demonstrated to be the primary factor controlling reactivity. In contrast, changes in limonene concentrations for a specific H₂O₂/H₂O ratio are shown to have little impact on the reaction rate. This study shows that the competitive adsorption of H₂O₂ and water on the catalyst surface is key in explaining the water impact on the reaction performance.