Fabrication of hierarchically porous MIL-88-NH2(Fe): a highly efficient catalyst for the chemical fixation of CO2 under ambient pressure†
A hierarchically micro- and mesoporous MIL-88-NH2 metal organic framework was prepared through an easy template directed methodology. Hierarchically porous MIL-88-NH2 promoted CO2 fixation reactions with epoxides to yield cyclic carbonates at atmospheric pressure and low reaction temperature and under solventless conditions. Significant conversion of epoxides was achieved under atmospheric pressure, with excellent selectivity toward the desired five-membered cyclic carbonates. The porous nature and weak Lewis acidic–basic centers of the hierarchical MIL-88-NH2 played a crucial role in exploiting its synergism with halide nucleophiles and metal centers. The catalyst was totally recyclable up to five times without compromising the activity, and the extent of heterogeneity was also studied. On the basis of experimental inferences and our previous DFT studies, a reaction mechanism for the CO2-epoxide cycloaddition reaction has been proposed. Hierarchically porous MIL-88-NH2(Fe) tuned using a structure-directing agent demonstrated its catalytic efficacy for the first time in cyclic carbonate synthesis.