Viologen-Based Cationic Metal-Organic Framework For Efficient Cr2o72- Adsorption And Dye Separation

A novel cationic metal-organic framework composed of {Cu-2(COO)(4)} paddle-wheel units and a tetracarboxylic viologen derivative, namely, {[Cu-2(bdcbp)(H2O)(2)]center dot 2NO(3)center dot 2H(2)O}(n) (Cu-CMOF, H(4)bdcbpCl(2) = 1,1'-bis(3,5-dicarboxyphenyl)-4,4'-bipyridinium dichloride), has been successfully synthesized and structurally characterized. In Cu-CMOF, the {Cu-2(COO)(4)} unit and viologen derivative both act as four-connected nodes forming an ssb-type cationic network with 4(2).8(4) topology, in which the positive charges are distributed on the organic viologen moieties. Deeper insight of the structure indicates that the 3D architecture of Cu-CMOF can be seen as packing of a 26-faceted polyhedral cage and two cuboid cages. Notably, Cu-CMOF displays a highly efficient anion exchange ability for capture and removal of anionic pollutants. UV-vis absorption spectra and digital images demonstrate that Cu-CMOF is capable of adsorbing the dichromate anion and anionic dyes effectively, such as methyl orange (MO-), Congo red (CR2-), and New Coccine (NC3-). Meaningfully, anionic dyes (MO-, CR2-, and NC3-) can be efficiently and selectively removed by Cu-CMOF in the presence of cationic dye methylene blue (MLB+). Such behaviors of anionic pollutant adsorption and dye separation are mainly caused by an ion-exchange process facilitated by the large cavity and decentralized distribution of positive charge in Cu-CMOF.