New cobalt-based metal–organic frameworks for catalysing sulfite in the degradation of reactive brilliant red

Advanced oxidation processes utilizing metal-organic frameworks (MOFs) to enhance the catalytic degradation efficiency of synthetic dyes in wastewater require a thorough understanding of the activation mechanism. Herein, four new MOFs, named [Co(Hchtc)(bpe)] (1), [Co(Hchtc)(bpy)]·H₂O (2), [Co_1.5(Hcptc)(bpy)_0.5(H₂O)₂]·2H₂O (3) and [Co₂(cptc)(bpe)₂(H₂O)]·H₂O (4) (H₃chtc = cyclohexane-tricarboxylic acid, H₄cptc = cyclopentane-tetracarboxylic acid, bpe = <i>trans</i>-1,2-(bis(4-pyridyl)ethene), bpy = 4,4'-bipyridine), were exploited as catalysts in the degradation of X3B by the activation of sulfites. Complexes 1 and 2, the first MOFs constructed with H₃chtc, are similar two-dimensional layered structures composed of double organic linkers and dinuclear units in which the coordination numbers of cobalt ions are five and six, respectively; complex 3 is also a layered framework assembled with six-coordinated trinuclear cobalt units and bpy, whereas complex 4 is a three-dimensional framework including both 5- and 6-coordinated cobalt ions. Interestingly, catalysts 1/SO₃^2- and 4/SO₃^2- demonstrate a superior degradation efficiency on X3B, while 2/SO₃^2- and 3/SO₃^2- exhibit moderate degradation efficacy on X3B. Further investigations suggest that both the adsorption capacity of the dye and the coordination vacancy on the metal ions contribute to the degradation process, with the latter exerting an activation effect of sulfite ions.