Tuning adsorption capacity through ligand pre-modification in functionalized Zn-MOF analogues

The introduction of various functional groups into pores of metal-organic frameworks (MOFs) through ligand modification represents an effective strategy for turning absorption and separation performances of this novel porous material. In this work, carbonylated and sulfonylated dicarboxylic acids based on fluorene were synthesized, and a series of Zn-MOF skeletons containing mesoporous with exposed functional groups introduced by the pre-modified ligands were obtained successfully. Structural analysis revealed that the Zn-MOFs contain zinc-glycolate-carboxylate layers in which the unique hexagonal prismatic [Zn₆(O-CH₂-CH₂O)₆] clusters connected by plannar [Zn₃(OCH₂CH₂O)(COO₂)] motifs, with the supporting by the ligands between the layer, affording unique discrete channel in the structure. Attributed to these additional functional groups, these MOFs materials exhibit different adsorption capacities for carbon dioxide and methylene blue, an organic dye. Notably, all the Zn-MOFs exhibit rapid and significant adsorption capacities for cationic dyes MB in aqueous solutions. Among these, Zn-MOF 1 shows the fastest and largest adsorption properties for MB in aqueous surroundings. The underlying adsorption mechanisms were probe form both kinetic and thermodynamic considerations, revealing pseudo-second-order kinetic models and electrostatic attraction induced adsorption processes accounting for the adsorption behaviors.