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

Abstract

Two new metal–organic frameworks (MOFs) based on cobalt metal, namely, [{Co_0.5(5N₃-IPA)_0.5(1,2-bpe)_1.25}]_∞ (1) and [{Co(5NH₂-IPA)(1,2-bpe)}]_∞ (2) (5N₃-IPA = 5-azidoisophthalic acid, 5NH₂-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.