Three novel Co(ii)-based MOFs: syntheses, structural diversity, and adsorption properties

Abstract

In this paper, three new cobalt-based metal organic frameworks (MOFs) with different topologies, namely, {[Co(HL)(tib)(H₂O)]·2H₂O}_n (1), [Co₃(L)₂(bibp)₄(H₂O)₂]_n (2) and [Co₂(L)(bip)(μ₃-OH)]_n (3) (H₃L = 3-(3,5-dicarboxylphenoxy)-6-carboxylpyridine, tib = 1,3,5-tirs(1-imidazolyl)benzene, bibp = 4,4′-bis(imidazolyl)biphenyl, bip = 3,5-bis(imidazoly-1-yl)toluene) synthesized by a hydrothermal method, were investigated. Different auxiliary ligands led to the diversity of the three MOFs in terms of structure and properties. Hereafter, the adsorption properties of the MOFs on different dyes were investigated; of interest is that the adsorption performance of MOF 3 on Congo red dye is particularly outstanding, including excellent performance in terms of removal rates, selectivity and recyclability. Its adsorption kinetics and adsorption isotherms were also investigated. Finally, its possible adsorption mechanism was analyzed by zeta potential measurements, infrared spectroscopy and N₂ adsorption–desorption isotherms. It was hypothesized that the exquisite confined microenvironments consisting of electrostatic interactions, a suitable pore volume, and complicated van der Waals forces of hydrogen bonding and π–π stacking played a critical role during the adsorption process. It was shown that MOF 3 can be used as an effective adsorbent for the adsorption of Congo red dye in wastewater.