Alumina as heterogeneous catalyst for the regioselective epoxidation of terpenic diolefins with hydrogen peroxide

The epoxidation of several terpenic diolefins (carvone, limonene, terpinolene, α-terpinene and γ-terpinene) in anhydrous conditions using alumina as a heterogeneous catalyst and hydrogen peroxide as the oxidant agent is studied in this work. Several kinds of chromatographic aluminas have been employed (acidic, basic and neutral), in order to study the effect of the surface acidity on the reaction. The reusability of the catalyst has also been addressed. Basic alumina was the catalyst that gave better activity and selectivity. The reactivity of the substrate increases with the nucleophilic character of its double bonds, and the regioselectivity to some monoepoxides is also related to this nucleophilic character. The epoxidation rate of carvone, limonene and terpinolene increased with the alumina/terpene molar ratio. The selectivity to epoxides increased for carvone, and it passed through a maximum for limonene and terpinolene with this variable. From the comparison between experiments performed removing the catalyst, without removing it, and without catalyst, it was deduced that alumina is responsible for the high activity and selectivity obtained when it is added to the reaction medium, and that a heterogeneous epoxidation occurs on alumina surface. The study of the reusability of alumina in the epoxidation of limonene and terpinolene shows negligible changes on selectivity after several cycles, whereas the decrease of activity in successive cycles is significant for the long reaction times used in limonene epoxidation.

Graphical abstractThe epoxidation of several terpenic diolefins (carvone, limonene, terpinolene, α-terpinene and γ-terpinene) in anhydrous conditions using alumina as a heterogeneous catalyst and hydrogen peroxide as oxidant agent is studied in this work. Several kinds of chromatographic aluminas have been employed (acidic, basic and neutral). The reusability of the catalyst has also been addressed. Figure optionsDownload full-size imageDownload as PowerPoint slide