Evidence for supramolecular catalysis in the synthesis of cyclic organic carbonates over zeolite Y impregnated with metal iodides

Abstract

• Styrene carbonate was produced over zeolite Y impregnated with KI. • The conversion and selectivity increase with the loading of impregnated KI. • Calculations showed the epoxide attack by iodide as the rate-determining step. • Calculations indicated a perfect fit of the transition state inside the zeolite cage. • Results were consistent with an enzyme-like behavior in accelerating the reaction. The reaction of epoxides with CO 2 to afford cyclic organic carbonates was studied over zeolite Y impregnated with metal iodides. The experimental results indicated that formation of styrene carbonate is significantly higher on zeolite Y impregnated with KI compared to KI alone. The conversion and selectivity increase with the loading of impregnated KI on the zeolite, although the epoxide/iodide molar ratio were in the range of 86 to 220, among the highest ever reported for this reaction. A periodical DFT study indicated that metal halides are stabilized upon embedding inside the zeolite Y cavity and interact with the framework counter cations. The theoretical study of the reaction pathway, considering the CO 2 cycloaddition to ethylene oxide as model, indicated that the opening of the epoxide ring is the rate-determining step, in agreement with the experimental results that showed a correlation between conversion and the KI loading. The energy of activation is lower on the zeolite/metal halide systems, explaining the higher activity of the metal halides when embedded within the zeolite cavity. The calculations indicated that the reaction occurs on two opposite framework aluminum sites having the embedded metal halide between them. Whereas the epoxide interacts with the framework counter cation of one site, the metal iodide interacts with the counter cation of an opposite site, providing the perfect alignment for the nucleophilic attack and leading to the preferred geometry for the transition state, which resembles a S N 2 type mechanism, with nucleophilic attack of the iodide occurring on the opposite side of the ring oxygen atom. This remarkable behavior is characteristic of enzymatic reactions and stresses the importance of host-guest supramolecular catalysis occurring on zeolitic systems. Graphical Abstract .