Dual hydrogen-bond donor group-containing Zn-MOF for the highly effective coupling of CO2 and epoxides under mild and solvent-free conditions

Abstract

A novel zinc(II)-based 3D metal organic framework (MOF) [Zn₃(L)₃(H₂L)·2DMF·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 Zn₃(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 Zn₃(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, 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 °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, Zn₃(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.