Microwave-Assisted Rapid Synthesis of Well-Shaped MOF-74 (Ni) for CO₂ Efficient Capture

In the present work, a series of MOF-74 (Ni) materials with narrow micropore channels and abundant unsaturated metal sites was respectively prepared via hydrothermal (HT), condensation reflux (CE), and microwave-assisted (MW) methods. The physicochemical properties of synthesized materials were characterized by powder X-ray diffraction, N₂-sorption, field-emission scanning electron microscopy, Fourier-transform infrared (FTIR), thermogravimetric (TG)/TG-FTIR, X-ray photoelectron spectroscopy, UV-vis-near infrared, NH₃/CO₂-temperature programmed desorption, and in situ diffuse reflectance infrared Fourier transform spectroscopy. Their CO₂/N₂ adsorption performances were evaluated by isotherm adsorption and dynamic adsorption experiments. We found that the MW is a rapid and facile protocol for the synthesis of MOF-74 (Ni) materials with highly efficient CO₂ capture capacity. The well-shaped MW-140 adsorbent with superior CO₂ adsorption capacity of 5.22 mmol/g at 25 °C can be obtained within 60 min by the MW process, almost 6 times higher than that of the commercial activated carbon (0.89 mmol/g). Results of dynamic adsorption experiments showed that the MW-140 material possesses the highest CO₂ adsorption capacity of 3.37 mmol/g under humid conditions (RH = 90%). Importantly, MW-140 has excellent adsorption stability and recyclability, superior CO₂ capture selectivity (CO₂/N₂ = 31), and appropriate isosteric heat in CO₂ adsorption (21-38 kJ/mol), making it a promising and potential material for industrial CO₂ capture. Characterization results demonstrated that the high capture capability of MOF-74 (Ni) materials can be attributed to the synergistic effect of abundant narrow micropore channels and rich five-coordinated Ni²⁺ open metal sites which are beneficial for the trapping of CO₂ molecules.