Zeolite Y-based catalysts for efficient epoxidation of R-(+)-Limonene: Insights into the structure-activity relationship

Parent, hierarchical, and metal-modified hierarchical zeolite Y were investigated as heterogeneous catalysts in the R-(+)-limonene epoxidation, a catalytic route for synthesizing precursors of bio-polycarbonates, an alternative to isocyanate polyurethanes. The fresh catalysts underwent detailed characterization using XRD, N ₂ physisorption, TEM, SEM-EDX, pyridine-FTIR, NH ₃-TPD, CO ₂-TPD, UV–Vis-DRS, and solid-state NMR. Spent materials were investigated by TPO-MS and TGA, confirming low coke formation on the catalytic surface. The most active material was K–Sn-modified dealuminated zeolite Y, reflected in a high turnover frequency (TOF) of 96 h ⁻¹. This material exhibited the lowest Brønsted to Lewis acidity ratio (0.1), the highest mesoporosity fraction (43%), and the lowest total surface area (465 m ² g ⁻¹). Aprotic polar solvents with high polarity and medium donor capacity appeared suitable for limonene epoxidation. Limonene conversion of ca. 97% was reached at 70 °C, H ₂O ₂: limonene molar ratio = 7, and acetonitrile as a solvent, while selectivity to total monoepoxides exhibited values up to 96% under different reaction conditions. Hydration of internal epoxides to limonene diol was favored at high temperatures and high H ₂O ₂/limonene molar ratios. The efficiency of H ₂O ₂ reached maximum values of about 85% at low H ₂O ₂ amounts, while no significant influence was observed for temperature, catalyst amount, and the initial concentration of limonene. A plausible reaction mechanism was proposed for the R-(+)-limonene epoxidation with H ₂O ₂ based on the experimental findings.