(521z) CO2 cycloaddition with Epoxide into Cyclic Carbonates Using Zeolitic Imidazolate Frameworks: Effects of Defect Centres, Surface Sites and Ionic Liquids

Zeolitic imidazole frameworks (ZIFs) are effective materials for CO₂ capture and show noteworthy catalytic activity owing to the presence of Lewis acidic metal centres and different functional groups on the linkers. They are green catalysts for cycloaddition of CO₂ with epoxides to form cyclic carbonates which is an atom efficient route for utilization of CO₂. Herein, we report periodic density functional theory (DFT) based analysis of ZIF-8, ZIF-67 and ZIF-90 and their ionic liquid immobilized counterparts for CO₂ cycloaddition with propylene oxide to show the effect of different metal centres (Zn and Co), linker functionalities (polar and non-polar), ionic liquids, and defect centres for the cycloaddition reaction. On the surface, the dangling linker defect formed due to interaction with water forms Metal-OH most favourably in ZIF-67 while in the ZIF pores, the missing linker defect is predominant and is most favourably formed in ZIF-90. In the Lewis acid mechanism, the ring-opening of epoxide is facilitated by the Lewis acid-base pair (M-OH) while in the Brønsted acid mechanism, it is facilitated by the N-H of the linker. On the ZIF surface, the higher Lewis acidity of the Co²⁺ than Zn²⁺ makes the reaction more favourable on ZIF-67 than ZIF-8 while the Brønsted acid mechanism shows similar energy trend due to the same linker. The reactions are more favourable on ZIF-90 due to the presence of the polar linker (CHO functionality) which additionally enhances the Lewis acidity of the Zn²⁺ centre. Inside the ZIF pores, the ring-closing step is unfavourable in all the ZIFs while it is favoured on surface due to the lower steric hindrance on surface. This DFT study suggests the cyclic carbonate selectivity on the ZIFs to be in the order ZIF-90 > ZIF-67 > ZIF-8, which is in line with experimental results with other epoxides.