Rapid Microwave-Assisted Synthesis of Fe/Co Bimetallic Metal-Organic Frameworks and Evaluating the Role of Coordinatively Unsaturated Sites for Selective CO2 Capture

In this study, a novel CO2 adsorbent was synthesized using a rapid microwave-assisted process, combining iron and cobalt metal ion precursors with terephthalic acid. A spindle-shaped bimetallic MOF sorbent with a crystal structure similar to MIL-88B was observed. The properties of the bimetallic MOFs were studied in comparison with their monometallic equivalents, and the mixed-metal MOFs showed a higher surface area and greater CO2 adsorption capabilities. X-ray photoelectron spectroscopy studies revealed the presence of both bi- and trivalent ions, although only Fe3+ and Co2+ were used in the synthesis. Furthermore, the bimetallic MOFs exhibited coordinatively unsaturated metal sites, as evidenced by the acid-base titrations. Structural defects from incomplete coordination due to rapid synthesis and mixed-valence metal ion substitutions led to strong acidity. As a result, the bimetallic adsorbent exhibited a CO2 adsorption capacity of 1 mmol g-1 with a 40-fold selectivity of CO2 over N2 under ambient conditions. The synthesis of adsorbents via the microwave process provides an energy-efficient pathway to produce MOFs with abundant active sites for selective CO2 capture. HIGHLIGHTS Rapid microwave-assisted synthesis of bimetallic MOF resulted in MIL-88B topology Abundant unsaturated metal sites due to incomplete coordination at metal nodes CO2 adsorption of bimetallic MOF encompasses the monometallic MOFs Higher selectivity of ~40 times for CO2 over N2 at ambient conditions Acidity due to charge imbalance & unsaturated metal sites influenced CO2 capture GRAPHICAL ABSTRACT