Catalytic and photocatalytic epoxidation of limonene: Using mesoporous silica nanoparticles as functional support for a Janus-like approach

Mesoporous silica nanoparticles (MSN) were used as a platform to design novel active materials for the catalytic and photocatalytic epoxidation of limonene. Binary systems comprised of TiO2 and MSN were used for the catalytic reaction when doped with manganese, and for the photocatalytic reaction when functionalised with hexadecyl chains or imidazolinyl groups. All of the MSN based systems were synthesized by condensation in emulsion. A thorough characterization of the powders has been performed by means of Inductively Coupled Plasma – Optical Emission Spectroscopy (ICP-OES), X-ray diffraction (XRD), FT-IR, Raman and EPR Spectroscopy, Fluorescence and diffuse reflectance UV–vis (DRS) Spectroscopy, gas Porosimetry, and Transmission Electron Microscopy (TEM). The obtained morphological and structural information have been related to the catalytic and photocatalytic performances for the epoxidation of limonene. MSN support finely distributes the manganese active centres in the case of the catalytic epoxidation, while acts as functional co-catalyst in the case of the photocatalytic reaction. Upon careful optimization of the experimental conditions, the catalytic process afforded conversion and selectivity values up to 90 and 84%, respectively, while the photocatalytic process provided in the best case conversions up to 80% and selectivity values of ca. 50%. Both the catalytic and photocatalytic approaches were performed under relatively mild experimental conditions and use molecular oxygen as the oxidant. Therefore, both of them represent promising green alternatives to traditional methods for the industrially relevant production of limonene epoxide. This compound, in fact, is the starting material for the production of poly(limonene carbonates), biopolymers with outstanding properties which are promising substitutes of oil derived polycarbonates.