Acid Stability of MIV (M = Zr, Hf, Ce, Th) UiO-66 MOFs from a Solution Perspective

Abstract

UiO-66 is one of the most studied metal-organic frameworks (MOFs), comprised of hexamer nodes M^IV₆O₄(OH)₄ (M₆; M = Zr, Ce, Hf, Th) bridged by benzenedicarboxylate (BDC) linkers. UiO-66 is well-known for its stability, based on the characterization of solids after treatment with mineral acids or more rarely, carboxylic acids to engineer defects. Characterization of the solutions to quantify and determine the speciation of dissolved UiO-66 has been largely ignored, regardless of the acid type. However, this information could yield better control over intentional defect engineering, the design of colloidal MOFs, and assembly of new materials from MOF building blocks. We show that Zr, Hf, and Ce-UiO-66 are wholly disassembled into soluble hexamer nodes and precipitated BDC linkers in 2-10 M acetic and formic acid. In contrast, Th-UiO-66 nodes erode completely to monomers and crystallize as acetate-linked Th-monomer chains. The Zr, Hf, and Ce formate solutions crystallize hexamer compounds, affirming the presence of M₆ in solution, as initially observed by small-angle X-ray scattering (SAXS). Two new crystal structures are reported: (1) [Ce₆O₄(OH)₄(H₂O)(HCOO)₁₂]·4.8H₂O, a 1-dimensional hexamer framework and (2) [Hf₆O₄(OH)₄(H₂O)₄(HCOO)₁₂]Cl₂·8H₂O, a 2-dimensional hexamer framework. SAXS of colloids from 0.5 M formic acid-treated UiO-66 (Zr, Hf) shows that the 100-200 nm aggregates retain the long-range order of the MOF and coexist in solution with dissolved linked hexamers, providing an understanding of the dissolution process. Quantification of metal dissolved from UiO-66 into 2 M formic acid indicates ∼0.2 M Zr₆/Hf₆ solubility limit in these conditions. We also show leaching of 4-5% Hf, 16% Zr, and 57% Ce from their respective UiO-66 frameworks into two mineral acids, where the high solubility of cerium is attributed to its facile Ce^III/IV redox activity. The higher solubility of Zr compared to Hf in mineral acids is attributed to greater Zr-ligand lability, an emerging and consistent trend that defines and differentiates Zr/Hf oxocluster behavior.

Keywords: UiO-66; hafnium; hexamer clusters; metal−organic frameworks; small-angle X-ray scattering; zirconium.