Bimetallic Fe:Co metal–organic framework (MOF) with unsaturated metal sites for efficient Fenton-like catalytic degradation of oxytetracycline (OTC) antibiotics

Fe:Co MIL-88B nanorods with abundant unsaturated metal centers have been synthesized via a rapid microwave-assisted solvothermal process and used as a Fenton-like catalyst for oxytetracycline (OTC) degradation. The bimetallic Fe:Co MOFs outperformed their monometallic counterparts in catalytic degradation in both alkaline and acidic environments, with the highest catalytic activity in the alkaline pH region. The synthesis time, temperature, and bimetal ratios were essential in creating MOFs with high catalytic activity. Acid-base titration clearly revealed the resulting MOFs to possess high acidity, although no acid solution was used in the synthesis. The acidity arises probably from the charge imbalance in the mixed substitutions of Fe(III) and Co(II) in the MIL-88B topology under rapid synthesis, which leads to the metal centers having incomplete coordination and thus are very active. These highly active metal centers can catalyze hydrogen peroxide into reactive radicals to degrade OTC efficiently with 50 mg/L OTC being degraded completely within 90 min at pH 8, and 0.125 g/L catalyst loading. The obtained catalysts also displayed high stability and reusability. The metal ions leaching of the catalysts are less than 1.0 ppm in the fresh use and decrease to less than 0.1 ppm after 4 cycles, while still sustaining the catalytic activity over 90 % of its initial efficiency. This work demonstrates the use of mixed metals in synthesizing MOFs to achieve highly active, broad pH, Fenton-like catalysts that may be useful for advanced oxidation and environmental remediation.