Influence of Defects and Linker Exchange on Removal of Phosphate Using MOFs with the Node Structure <i>M</i>₆(OH)₄(O)₄ for <i>M</i> = Hf, Zr, or Ce

Abstract

Metal–organic frameworks (MOFs) have demonstrated great potential as high-capacity, tunable, and readily synthesizable sorbents for a variety of contaminants in aqueous solutions. In particular, MOFs with the M6(OH)4(O)4 node structure have attracted much attention for these applications due to their remarkable stability in water. However, they often contain structurally uncoordinated sites because of either the MOF’s topology or the introduction of defects during synthesis. When considering the removal of oxoanions from solution, the concentration of such defects has been linked to different adsorption characteristics. The coordinating metal atom in the node and defect concentration together dictate oxophilicity and binding strengths with guest molecules and terminal ligands. In this work, we employ density functional theory calculations to investigate the influence of defects and the choice of metal centers on the binding characteristics of phosphate species to M6(OH)4(O)4 nodes, with M being Hf, Zr, or Ce. We focus on several binding modes arising from linker exchange at two defect sites during adsorption of phosphate anions and compare them to the pristine site binding. We find clear preference in binding strength for the bidentate binding mode replacing both terminal ligands, followed by the replacement of the charged ligand (OH– or COO–), H2O replacement, and finally binding at the pristine site. These results also suggest a trend in the binding strength of phosphate anions to the metal node of Hf > Zr > Ce. Our theoretical investigations elucidate the adsorption mechanisms of inorganic phosphate species on the M6(OH)4(O)4 node, which is needed to advance the design of new MOFs for the removal of phosphate and other oxoanions to mitigate the negative effects of water eutrophication and other corresponding environmental concerns.

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