One-Pot Method Synthesis of Bimetallic MgCu-MOF-74 and Its CO₂ Adsorption under Visible Light

Abstract

A magnesium-based metal-organic framework (Mg-MOF-74) exhibits excellent CO₂ adsorption under ambient conditions. However, the photostability of Mg-MOF-74 for CO₂ adsorption is poor. In this study, Mg _<i>x</i> Cu_1-<i>x</i> -MOF-74 was synthesized by using a facile "one-pot" method. Furthermore, the effects of synthesis conditions on the CO₂ adsorption capacity were investigated comprehensively. X-ray diffraction, Fourier transform infrared, scanning electron microscopy, thermo gravimetric analysis, inductively coupled plasma atomic emission spectroscopy, ultraviolet-visible spectroscopy and photoluminescence spectroscopy, and CO₂ static adsorption-desorption techniques were used to characterize the structures, morphology, and physicochemical properties of Mg _<i>x</i> Cu_1-<i>x</i> -MOF-74. CO₂ uptake of Mg _<i>x</i> Cu_1-<i>x</i> -MOF-74 under visible light illumination was measured by the CO₂ static adsorption test combined with the Xe lamp. The results revealed that Mg _<i>x</i> Cu_1-<i>x</i> -MOF-74 exhibited excellent photocatalytic activity. Furthermore, the CO₂ adsorption capacity of Mg _<i>x</i> Cu_1-<i>x</i> -MOF-74 was excellent at a synthesis temperature and time of 398 K and 24 h in dimethylformamide (DMF)-EtOH-MeOH mixing solvents, respectively. Mg _<i>x</i> Cu_1-<i>x</i> -MOF-74 retained a crystal structure similar to that of the corresponding monometallic MOF-74, and its CO₂ uptake under visible light was superior to that of the corresponding monometallic MOF-74. Particularly, the CO₂ uptake of Mg_0.4Cu_0.6-MOF-74 under Xe lamp illumination for 24 h was the highest, up to 3.52 mmol·g^-1, which was 1.18 and 2.09 times higher than that of Mg- and Cu-MOF-74, respectively. The yield of the photocatalytic reduction of CO₂ to CO was 49.44 μmol·g_cat ^-1 over Mg_0.4Cu_0.6-MOF-74 under visible light for 8 h. Mg²⁺ and Cu²⁺ functioned as open alkali metal that could adsorb and activate CO₂. The synergistic effect between Mg and Cu metal strengthened Mg _<i>x</i> Cu_1-<i>x</i> -MOF-74 photostability for CO₂ adsorption and broadened the scope of its photocatalytic application. The "bimetallic" strategy exhibits considerable potential for use in MOF-based semiconductor composites and provides a feasible method for catalyst design with remarkable CO₂ adsorption capacity and photocatalytic activity.