Tuning the catalytic properties for cycloaddition of CO₂ to propylene oxide on zeolitic-imidazolate frameworks through variation of structure and chemical composition

A series of Zn- and Co containing zeolitic-imidazolate frameworks (ZIFs), such as ZIF-8(Zn)/ZIF-67(Co), MAF-5(Zn)/MAF-5(Co), and MAF-6(Zn)/MAF-6(Co), were investigated as catalysts for the synthesis of propylene carbonate (PC) from propylene oxide (PO) and CO₂. The effect of the chemical composition, textural, structural and physicochemical properties on their activity was investigated. It was found that the activity of Zn-containing materials was higher in comparison with Co-based materials. Moreover, the yield of a PC was decreased with increasing Co content in structure of mixed Zn,Co-ZIFs (ZIF-8/ZIF-67) that was related to the accessibility of active centers to reagents and the strength of their interaction with CO₂ and propylene oxide. The catalytic performance of Zn-ZIFs was found to improve with increasing the size of pore aperture due to the guest molecule facile diffusion and accessibility of the active site for reagent. In general, based on the results of the present work, the Zn-ZIFs could be considered as promising catalysts for cycloaddition of CO₂ to epoxides and thereby opens new paths for further research.