One-Pot Transformation of R-(+)-Limonene over Sr- and Sn-Modified Hierarchical Y Zeolite Catalysts: Toward Sustainable Conversion Routes

This study explored the use of Sr- and Sn-modified hierarchical faujasite-type zeolite Y catalysts with tailored SiO2/Al2O3 ratios to carry out the one-pot transformation of R-(+)-limonene, involving both epoxidation and isomerization steps. Unlike previously reported nonhierarchical zeolites, these catalysts couple enhanced mesoporosity, generated by controlled dealumination, with tunable Brønsted/Lewis acid site (BAS/LAS) ratios, enabling superior control over product selectivity. Comprehensive catalyst characterization (ICP/OES, XRD, N2 physisorption, pyridine-FTIR, STEM, EDX, XPS) revealed that although dealumination induced some structural changes, the overall zeolite framework was preserved in the most active catalysts. Surface area decreased and mesoporosity increased after dealumination and metal loading. The acidity decreased with aluminum removal. Catalytic testing revealed that Sr-modified zeolites, followed by Sn catalysts, exhibited the best yield toward the target products, such as dihydrocarvone and carveol (17.2% with Sr–H–Y-80-D), outperforming Cu, Ni, and Fe catalysts. The solvent choice was critical: while an acetonitrile-ethyl acetate mixture improved conversion, it also promoted side reactions that lowered the yield of the target products, making acetonitrile alone the optimal solvent. Key reaction parameters, such as limonene concentration, the oxidant type, the catalyst amount, and temperature, were found to influence the product distribution strongly. High epoxide yields were consistently associated with Sr-based catalysts exhibiting low BAS/LAS ratios and enhanced mesoporosity, confirming their potential for monoterpenes valorization through one-pot routes. The catalysts demonstrated good stability and reusability after calcination. Green metrics such as atom economy (AE = 89%), the reaction yield (ε = 17.2%), the stoichiometric factor (SF = 0.54), the material recovery parameter (MRP = 0.43), and the reaction mass efficiency (RME = 0.04) were calculated, demonstrating green credentials for Sr-hierarchical zeolite Y for the one-pot transformation of R-(+)-limonene.