Green synthesis of nanoscale cobalt(ii)-based MOFs: highly efficient photo-induced green catalysts for the degradation of industrially used dyes

Two new metal-organic frameworks (MOFs) based on cobalt metal, namely, [{Co-0.5(5N(3)-IPA)(0.5)(1,2-bpe)(1.25)}](infinity) (1) and [{Co(5NH(2)-IPA)(1,2-bpe)}](infinity) (2) (5N(3)-IPA = 5-azidoisophthalic acid, 5NH(2)-IPA = 5-aminoisophthalic acid, 1,2-bpe = 1,2-bis(4-pyridyl)ethylene) were synthesized under a solvothermal condition and characterized. MOF 1 has a rare three-dimensional structure with 3-fold interpenetration, while MOF 2 has a 2D pillar-layered structure. The particle size of the as-synthesized MOFs was reduced to produce nanoscale MOFs (NMOFs) by a hand grinding technique while retaining the composition and structural regularity of the as-synthesized MOFs. SEM studies revealed that NMOFs 1 and 2 were hexagonal and square shaped, respectively. Since the band gap energy for MOFs 1 and 2 (2.81 eV and 2.65 eV, respectively) falls in the visible region, heterogeneous catalytic activities of NMOFs 1 and 2 in aqueous medium towards Reactive Yellow HE4R (RY84), Reactive Black HFGR (RBH), Rhodamine B (RhB) and methylene blue (MEB) were investigated under visible light irradiation. The introduction of hydrogen peroxide was observed to significantly increase the photocatalytic efficiency via dye degradation. Hydroxyl radical (OH) is supposed to act as the main active species during oxidation, which was further verified by performing the degradation experiment in the presence of tert-butyl alcohol, which can act as a radical trapper for hydroxide radicals in the reaction.