p-Directed Incorporation of Phosphonates into MOF-808 via Ligand Exchange: Stability and Adsorption Properties for Uranium.

We report a class of p-directed, precise incorporation of phosphonate ligands into a zirconium-based metal-organic framework (Zr-MOF), MOF-808, via ligand exchange. By replacing of formate ligands with methylphosphonic acid (MPA), ethanephosphonic acid (EPA), and vinylphosphonic acid (VPA), whose p values are slightly higher than that of the benzenetricarboxylic acid (BTC) linker in MOF-808, daughter MOFs can be synthesized without controlling the stoichiometric amounts of added MPA. The methylphosphonate MOFs (808-MPAs) demonstrate high porosities, with only small changes in the pore diameter and specific surface area when compared with the parent MOF-808. PXRD patterns and structure refinements indicate the expansion of the lattice for all MOFs after decorating with methylphosphonate ligands. The XPS spectra reveal a charge redistribution of the Zr node after ligand exchange. FTIR and P MAS NMR spectra, combined with DFT calculation, suggest that the methylphosphonate ligand is connected to the Zr node as CHP(O)(OZr)(OH) species with an accessible acidic P-OH group. Besides, 808-MPAs demonstrate excellent chemical stability in concentrated HCl, concentrated HNO, hot water, and 0.2 mol/L trifluoroacetic acid solutions. Impressively, 808-MPAs show ultrafast adsorption performance for uranyl ions using the ion-exchange property of P-OH sites in their cavity environment, with an equilibrium time of 10 min, much quicker than the previous adsorbents. The present study demonstrates a series of important proof-of-concept examples of the p-directed Zr-MOFs with tunable phosphonate-terminated ligands, which can extend to other phosphonate-functionalized Zr-based framework platforms in the near future.