Series of M-MOF-184 (M = Mg, Co, Ni, Zn, Cu, Fe) Metal–Organic Frameworks for Catalysis Cycloaddition of CO₂

Abstract

In light of the chemical exploitation of CO₂, new reusable materials for efficiently catalyzing the cycloaddition of CO₂ and epoxides under moderate conditions are needed. Herein, a new series of isostructural metal-organic frameworks (MOFs) M₂(EDOB) [EDOB^4- = 4,4'-(ethyne-1,2-diyl)bis(2-oxidobenzoate), M = Mg, Ni, Co, Zn, Cu, Fe], known as M-MOF-184, analogous to a well-studied MOF-74 structure, were synthesized and fully characterized. The M-MOF-184 (M = Mg, Co, Ni, Zn) frameworks exhibit accessible mesopore channels (24 Å) and high porosity. Among them, Mg-MOF-184 demonstrated the most upper surface area (>4000 m² g^-1) in any reported MOF-74-type frameworks. Furthermore, Co-MOF-184 revealed the highest CO₂ uptake (73 cm³ g^-1, at 298 K), and Zn-MOF-184 showed the highest catalytic activity upon the cycloaddition of CO₂ (96% conversion, 86% selectivity, and 82% yield) under mild conditions (1 atm CO₂, 80 °C, 6 h, and solvent-free). Notably, the catalytic performance of Zn-MOF-184 outperformed that of the original M-MOF-74 (M = Mg, Co, Zn) materials and various Zn-based MOFs. To evaluate the acidity and basicity of a series of M-MOF-184 (M = Mg, Co, Ni, Zn) frameworks, the interaction of these MOFs with acetonitrile vapor was investigated by vapor adsorption and ATR-FTIR spectroscopy measurements. As such, Zn-MOF-184 showed the strongest Lewis acidity derived by Zn cations, which was correlated to the highest catalytic activity upon the cycloaddition of CO₂. Interestingly, the 2-oxidobenzoate anions from Co-MOF-184 showed the strongest basicity among the series, which was associated with the highest saturated acetonitrile uptake (544 cm³ g^-1 at 298 K). Our findings suggest that the integration of Lewis acidic and basic sites, high surface area, and large accessible pores into the framework can facilitate the CO₂ fixation reaction.