Facile One-Pot Synthesis of Zn/Mg-MOF-74 with Unsaturated Coordination Metal Centers for Efficient CO₂ Adsorption and Conversion to Cyclic Carbonates

Bimetallic metal-organic frameworks (MOFs) containing two different inorganic metal nodes exhibited enhanced properties in CO₂ adsorption and catalytic conversion compared with the corresponding monometallic MOFs. In this work, the novel bimetallic Zn/Mg-MOF-74 with different ratios of Zn/Mg was synthesized successfully by a facile one-pot method. Powder X-ray diffraction, Fourier transform infrared, X-ray photoelectron spectroscopy, scanning electron microscopy/transmission electron microscopy, N₂/CO₂ adsorption/desorption, and CO₂/NH₃-temperature-programmed desorption techniques thoroughly characterized the structure, morphology, and physicochemical properties of Zn/Mg-MOF-74. Besides the excellent CO₂ adsorption capacity (128.3 cm³/g at 273 K and 1 bar), Zn0.75Mg0.25-MOF-74 also showed efficient catalytic activity for the cycloaddition reaction of CO₂ and epoxides to cyclic carbonates with outstanding yield and selectivity all over 99% under solvent-free and mild conditions (60 °C, 0.8 MPa), outperforming the mechanical combination of Zn-MOF-74 and Mg-MOF-74 with the same metal contents, indicating the synergistic effect of two adjacent metals in bimetallic MOF-74. In addition, the Zn0.75Mg0.25-MOF-74 catalyst could be recycled for at least five runs and possess good versatility to various substrates. Finally, a feasible mechanism of the catalytic reaction was proposed. Thanks to the high surface area, affinity toward CO₂, and accessibility of multiple active sites of the unsaturated metal centers as active Lewis acid sites and O atoms from Zn-O and Mg-O as Lewis basic sites, efficient chemical fixation of CO₂ to cyclic carbonates was obtained over the Zn0.75Mg0.25-MOF-74 catalyst. The present facile synthesis and application of a robust bimetallic MOF catalyst offered a competitive avenue for the integration of CO₂ adsorption and CO₂ catalytic conversion.