CO2 cycloaddition over ionic liquid immobilized hybrid zeolitic imidazolate frameworks: Effect of Lewis acid/base sites

Ionic liquid (IL) immobilized zeolitic imidazolate framework (ZIF) catalysts (IL-ZIF-8) were prepared through grafting IL on a dual-ligand ZIF by a facile post-synthetic modification. The coordinately unsaturated Zn as Lewis acid sites and bromide ions as Lewis base sites were confirmed in the resulted IL-ZIF. The density of Lewis acid/base sites was tuned easily by content of mixed linkers in the framework and/or the IL loadings. The coexistence of Lewis acid/base sites remarkably improved the activity for cycloaddition of CO2 with propylene oxide, compared with parent ZIFs and IL. In the absence of any co-catalyst and solvent, the prepared IL-ZIF-8(0.3) obtained a high yield of 97% with good stability and reusability. Besides, a Lewis acid/base synergistic catalytic mechanism was proposed. The synergetic interaction of Lewis acid sites and Lewis base sites significantly reduces energy barrier of propylene oxide ring-opening to 11.5 kcal mol−1 and thus promotes the CO2 cycloaddition reaction.