Highly stable Ni8-pyrazolate metal–organic frameworks for adsorption of methylene blue from water

Abstract

Three Ni₈-pyrazolate metal–organic frameworks, namely Ni₈–NiL₁, Ni₈–CuL₁ and Ni₈–CuL₂, were prepared using metallosalen-based ligands (NiL₁, CuL₁, and CuL₂). The octanuclear cubic clusters [Ni₈(OH)₄(H₂O)₂] render these materials with sufficient robustness, enabling their reusability in water for the removal of methylene blue (MB) over three cycles. Because of π–π stacking interactions, these title materials can effectively adsorb MB and demonstrate an exceptional adsorption capacity (up to 1011.8 mg g⁻¹ for Ni₈–CuL₂). Meanwhile, these materials show an adsorption preference for MB instead of methyl orange and eosin Y. The kinetic studies reveal that Ni₈–NiL₁ and Ni₈–CuL₁ conform to the pseudo second-order model, while Ni₈–CuL₂ fits to the pseudo first-order model. The adsorption isotherms of the three materials were also determined to obey the Langmuir model. These results indicate that these title materials are promising adsorbents for removing MB from water.