Tuning grafted catalysts for the selective conversion of carbon dioxide into limonene-based cyclic carbonates

The sustainable conversion of carbon dioxide into bio-based cyclic carbonates offers a promising route to green chemicals and polymers. We report the efficient synthesis of limonene-based cyclic carbonates via cycloaddition of CO2 to limonene diepoxide (LDO) under solvent-free conditions using a series of covalently grafted supported ionic liquid catalysts (SILC). Key synthesis parameters, such as ionic liquid structure, support type, grafting time, acid pretreatment, and the addition of Lewis acid sites, were systematically varied and their effect on the catalytic activity was assessed. Catalyst morphology and performance were correlated through comprehensive characterization via solid state 13C CP-MAS and 29Si NMR, TGA, Nitrogen physisorption, ion chromatography and CHNS analysis. Among the prepared systems, 4-pyrrolidino-pyridinium chloride (4PPCl) grafted on acid-treated SiO2 for 48 h showed the best performance, achieving 95 % LDO conversion with full selectivity towards limonene carbonates. Based on kinetic experiments at different temperatures the apparent activation energy was estimated to 61 kJ mol⁻¹ . Catalyst stability tests confirmed negligible ionic liquid leaching. Surface blockage by viscous residues was responsible for partial deactivation, which was mitigated by cleaning the catalyst by Soxhlet extraction. The work demonstrated how rational catalyst design, combining tailored ionic liquids with optimized supports and grafting time, can promote efficient valorization of carbon dioxide into bio-based cyclic carbonates.