Zeolites LiY, NaY, KY and ZnY were impregnated with small amounts of the corresponding metal halides (chloride, bromide and iodide) and tested as catalysts for the carboxylation of styrene oxide with CO2. The KI/KY system was tested at 100 °C and 50 bar and yielded 100% of styrene carbonate, by 1H NMR, within 6 h, whereas KI alone required 15 h to achieve the same yield. The reaction occurs inside the zeolite pores, with the metal cations acting as Lewis acidic centers and the impregnated halides as nucleophiles. Theoretical calculations indicated that chlorides are better stabilized within the zeolite cage than iodides, explaining the reactivity order and suggesting a solvent-like behavior of the zeolite structure.
