An anionic metal–organic framework: metathesis of zinc(ii) with copper(ii) for efficient C3/C2 hydrocarbon and organic dye separation

Abstract

A novel anionic metal–organic framework (NEM-7-Zn) with constricted pores, which was formed by narrow channels and blocks of functional groups, was constructed under solvothermal conditions. Due to the low stability of the Zn-paddlewheel upon termination of the solvation, NEM-7-Zn possesses a very low surface area. For reinforcing the framework stability, metal-ion metathesis in a single-crystal-to-single-crystal fashion on NEM-7-Zn was performed, and then a new Cu^II MOF (termed NEM-7-Cu) with an identical framework was produced. Notably, modified NEM-7-Cu possesses constricted pores and is decorated with Lewis basic pyridyl sites and uncoordinated –COO⁻ groups, resulting in high selectivities for C₃H₆/C₂H₄ (8.61), C₃H₆/C₂H₂ (6.75), C₃H₈/C₂H₂ (7.1), and C₃H₈/C₂H₄ (8.82) under ambient conditions. Moreover, NEM-7-Cu shows highly selective adsorption of organic dye molecules (MB⁺ over CV⁺, RB⁺, MO⁻, and SD⁰) and the adsorbed MB⁺ can be released in a NaCl-containing CH₃OH solution. Such a post-metal-ion metathesis method can be used to synthesize new and desired frameworks that could not be obtained by direct synthesis.