Dual hydrogen-bond donor groups-containing Zn-MOF for the highly effective coupling of CO2 and epoxides under mild and solvent-free conditions
A novel zinc(II)-based 3D metal organic framework (MOF) [Zn3(L)3(H2L)‧2DMF‧H2O] with dual hydrogen-bond donor (HBD) groups of free carboxyl and amine, was synthesized solvothermally via 2-aminoterephthalic acid (H2L) and pydine-3,5-dicarboxylate ligands. Carboxyl and amine in the prepared Zn3(L)3(H2L) acting as HBD groups can promote the CO2 cycloaddition reaction with epoxides. Besides, the contained sufficient amino groups showing Lewis base property also facilitated Zn3(L)3(H2L) to activate CO2 by forming carbamate intermediate, while Zn(II) centers acted as the Lewis-acid sites accomplishing the epoxide activation. Therefore, Zn3(L)3(H2L) with Lewis acid-base properties and dual HBD groups exhibited an efficient heterogeneous catalysis for the coupling of epoxides and CO2. The yield of the propylene carbonate (PC) achieved 99% under 80 oC, 1.0 MPa CO2 and solvent-free conditions. The corresponding carbonate yields of various epoxides could be over 95% even at room temperature by prolonging the reaction time. Moreover, Zn3(L)3(H2L) showed extraordinary versatility to various epoxides and excellent recycling without obvious loss in activity. Furthermore, a plausible mechanism was proposed for the CO2 cycloaddition to epoxides based on the structural evidence and catalytic effects.