Viologen-Based Cationic Metal–Organic Framework for Efficient Cr2O72– Adsorption and Dye Separation

A novel cationic metal–organic framework composed of {Cu2(COO)4} paddle-wheel units and a tetracarboxylic viologen derivative, namely, {[Cu2(bdcbp)(H2O)2]·2NO3·2H2O}n (Cu-CMOF, H4bdcbpCl2 = 1,1′-bis(3,5-dicarboxyphenyl)-4,4′-bipyridinium dichloride), has been successfully synthesized and structurally characterized. In Cu-CMOF, the {Cu2(COO)4} unit and viologen derivative both act as four-connected nodes forming an ssb-type cationic network with 42.84 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.