Solvent-free chromium catalyzed aerobic oxidation of biomass-based alkenes as a route to valuable fragrance compounds

Chromium containing mesoporous molecular sieves MCM-41 were shown to be an efficient heterogeneous catalyst for the liquid-phase aerobic oxidation of various monoterpenic alkenes under mild solvent-free conditions. The material was prepared through a direct hydrothermal method and characterized by ICP-AES, N2 adsorption–desorption, TEM, XRD, SAXS, and H2-TPR techniques. Characterizations suggest that chromium introduced in MCM-41 is essentially incorporated in the silica framework, with no extraframework chromium oxides being detected. Various oxygenated monoterpenoids important for the flavor and fragrance industry were obtained with high combined selectivities (75–92%) at 30–40% substrate conversions. The oxidation of β-pinene led almost exclusively to allylic mono-oxygenated derivatives, whereas limonene and α-pinene gave both epoxides and allylic oxidation products. The catalyst undergoes no metal leaching and can be easily recovered and re-used. A silica-included chromium catalyst prepared through a conventional sol–gel method showed activity comparable with that of Cr-MCM-41; however, selectivity was much lower.

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► Cr-MCM-41 was prepared by a direct hydrothermal method using a surfactant.
► Cr–SiO2/sol–gel was prepared by a conventional sol–gel method.
► The oxidation of limonene and α-pinene gives epoxides and allylic oxidation products.
► β-Pinene gives allylic mono-oxygenated derivatives in up to 92% combined selectivity.
► Cr-MCM-41 showed much higher selectivity than Cr–SiO2/sol–gel in these reactions.