Versatile Heterogeneous Catalytic System for the Selective Synthesis of Limonene Epoxide and Diepoxide

The products obtained from limonene epoxidation can be used as intermediates or precursors in organic and pharmaceutical synthesis. Commercial magnesium oxide was evaluated as a heterogeneous catalyst in the epoxidation of R-(+)-limonene using H2O2 in a Payne reaction system, using acetonitrile as an oxidant activator and water and acetone as solvents. The effect of temperature and the concentration of catalyst, H2O2, and limonene were evaluated; the highest yields for limonene epoxide (80%) and diepoxide (96%) were found at 50 °C and 30 min and 2 h, respectively. Fresh and spent catalysts were characterized by X-ray diffraction, thermogravimetric analysis, N2 adsorption–desorption isotherms, Fourier transform-infrared, and scanning electron microscopy-energy dispersive X-ray. The efficiency of H2O2 in the epoxidation reaction varied between 100 and 20% in the time range of 0–30 min for all tested reaction conditions. The production of peroxyacetimidic acid as an intermediate oxidant in the limonene epoxide synthesis was described using a pseudohomogeneous reversible model (E = 58.61 kJ mol–1), while the heterogeneous model describes both the decomposition of H2O2 (E = 29.06 kJ mol–1) and the epoxidation of limonene with the intermediate (E = 52.28 kJ mol–1). Leaching tests verified the heterogeneity of MgO which was used in four consecutive cycles.