Design and synthesis of functionalized defective MOFs for catalytic conversion of CO2 to cyclic carbonates under green conditions

In this paper, the functionalized ligand 2-(imidazole-1-yl)terephthalic acid was obtained by grafting an imidazole group onto 2-bromoterephthalic acid. Then, defective MOFs were synthesized with Hf ions and the ligand in the presence of monocarboxylic acid acting as a modulator, and NHC (nitrogen heterocyclic carbene)-CO2 adducts were grafted onto the Hf-based MOFs to catalyze the cycloaddition reaction of epoxides with CO2 to produce cyclic carbonates under green conditions of ambient pressure and being solvent- and cocatalyst-free. The mesopore defects created by the modulator and DMC (dimethyl carbonate) solvothermal etching, the enhanced Lewis acidity of the Hf clusters and the grafted NHC-CO2 adducts can improve synergistically the catalytic performance of the MOFs via decreasing activation energy, promoting mass transfer and providing more active catalytic sites. The results show that the highest conversion rate reaches 81.64% at 100 degrees C and 10 h for the cycloaddition reaction of CO2 with epichlorohydrin. Finally, a possible reaction mechanism was proposed based on characterization and catalytic measurement.