Advanced oxidation process using cobalt-based metal-organic frameworks via operating Co(II)/Co(III) redox cycling for efficient organic contaminant degradations

Abstract

Cobalt-terephthalate metal-organic frameworks (Co-BDC MOFs), is a well-known-designed inorganic-organic hybrid polymeric material. Because of its oxo-cluster, it has a durable and potent capability for cyclic-redox applications particularly when synthesis gives cobalt in mixed valency conditions, Co(II) and Co(III). The mixed valency condition offers a fast redox cycling progression due to the intimate localization of both Co(II) and Co(III) within the Co-BDC structure (the oxo-cluster) and consequently should accelerate H2O2 decomposition. Hence, in this work, Co-BDC MOF has been synthesized by the typical solvothermal method and has been characterized via SEM/EDX, XRD, FTIR, and XPS analyses. The most important result of characterizations is that cobalt presents pristinely with mixed valency states Co(II)/Co(III). To assess its heterogenous catalysis performance, Co-BDC has been applied in combination with H2O2 for an advanced oxidation process (AOP) against a test dye methylene blue (MB). According to kinetic studies, H2O2 decomposition was found to obey the regular second-order model with rate constant = 0.3649 mM−1 min−1, while, MB degradation obeyed the pseudo-first-order model with rate constant ranges from 0.1116 to 0.0716 min−1 depending on MB initial concentration. Hence, it has been concluded that MB degrades by relatively highly abundant oxidative radicals for the applied H2O2 concentration. Also, a catalytic system where the effects of initial MB concentration (2, 5, and 10 ppm), and dosage of 10, 20, 50, and 100 mg of Co-BDC were investigated. According to the results, MB degrades well at different doses of Co-BDC. Because Co(II) and Co(III) coexist in the Co-BDC matrix, a single-catalyst synergistic catalysis mechanism may be proposed.