Cycloaddition of carbon dioxide to epoxides catalysed by supported ionic liquids†
Very simple and economical SiO2 supported ionic liquid phase (SILP) materials are efficient catalysts for the addition of CO2 to epoxides, producing cyclic carbonates in high yields (up to 99%) and selectivities (up to 99%). A range of ionic liquid (IL) concentrations (5–100 wt%), SiO2-supported 1-n-butyl-3-methylimidazolium halides (SBMIm·X: X = Cl, Br and I) (1–7) and 1-ethyl-3-(3-(trimethoxysilyl)propyl)-imidazolium halides (SEPIm·X) (8, 9), were prepared and fully characterised. These hybrid materials are very active catalysts under mild reaction conditions (low temperature and atmospheric pressure or adsorbed CO2). Under the optimal reaction conditions ([S] = 3.34 mmol, [cat]/[S] = 0.50, PCO2 = 5 bar, T = 80 °C), the best SILP system yields maximum conversion in just 30 min and can be reused at least five times without a noticeable decrease in activity and selectivity. The catalytic system is also active when using a CO2 gas mixture from an industrial exhaust in both batch and continuous flow systems. A detailed structural and electronic analysis indicates that increasing the IL and water concentrations induces a solvation effect through the contact ion pair of the IL that drives the anions (Cl, Br and I) to the deeper regions of the confined space of SiO2. The catalytic performance is directly related to the presence of the nucleophilic Br anion on the outermost exposed layer of the hybrid material.