Dual hydrogen-bond donor group-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) [Zn-3(L)(3)(H2L)center dot 2DMF center dot H2O] with dual hydrogen-bond donor (HBD) groups of a free carboxyl and amine, was synthesized solvothermally via 2-aminoterephthalic acid (H2L) and 3,5-pyridinedicarboxylic acid ligands. The carboxyl and amine in the prepared Zn-3(L)(3)(H2L) acting as HBD groups can promote the CO2 cycloaddition reaction with epoxides. Besides this, the contained sufficient amino groups showing Lewis base properties also facilitated Zn-3(L)(3)(H2L) to activate CO2 by forming a carbamate intermediate, while Zn(ii) centers acted as Lewis-acid sites accomplishing the epoxide activation. Therefore, Zn-3(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 degrees C, 1.0 MPa CO2 and solvent-free conditions. Corresponding carbonate yields of various epoxides could be over 95% even at room temperature by prolonging the reaction time. Moreover, Zn-3(L)(3)(H2L) showed extraordinary versatility to various epoxides and excellent recycling without any obvious loss in activity. Furthermore, a plausible mechanism was proposed for the CO2 cycloaddition to epoxides based on the structural evidence and catalytic effects.