Zn-EDTA Complexes As Easy-to-handle and Stable Heterogeneous Catalysts for Cycloaddition of Carbon Dioxide to Epoxides

Bi-functional Zn-EDTA complexes that are non-toxic and stable can be conveniently synthesized using water as solvent. The complexes are efficient heterogeneous catalysts for the synthesis of cyclic carbonates from CO2 and epoxides without the need of a co-catalyst or solvent. Among the complexes, ZnBr2- EDTA-2Na is the most effective, showing a turnover frequency of 96.9 h-1 (cat./PO = 0.25 mol%, 150 °C, 2.25 MPa, 4 h). The catalyst can be easily recovered and reused without significant loss of activity across a test of five runs. We studied the effects of reaction temperature, pressure, and time as well as catalyst amount on the reaction. It was found that the synergetic actions of metal centers (Lewis acidic) and halide anions (nucleophilic) promote the cycloaddition reaction. A possible mechanism is proposed.